Sex roles and sexual selection: lessons from a dynamic model system

Sex roles and sexual selection: lessons from a dynamic model system Our understanding of sexual selection has greatly improved during the last decades. The focus is no longer solely on males, but also on how female competition and male mate choice shape orna- mentation and other sexually selected traits in females. At the same time, the focus has shifted from documenting sexual selection to exploring variation and spatiotemporal dynamics of sexual selection, and their evolutionary consequences. Here, I review insights from a model system with exceptionally dynamic sexual selection, the two-spotted goby fish Gobiusculus flavescens. The species displays a complete reversal of sex roles over a 3-month breeding season. The reversal is driven by a dramatic change in the operational sex ratio, which is heavily male-biased at the start of the season and heavily female-biased late in the season. Early in the season, breeding-ready males outnumber mature females, causing males to be highly competitive, and leading to sexual selection on males. Late in the season, mating-ready females are in excess, engage more in court- ship and aggression than males, and rarely reject mating opportunities. With typically many females simultaneously courting available males late in the season, males become selective and prefer more colorful females. This variable sexual selection regime likely explains why both male and female G. flavescens have ornamental colors. The G. flavescens model system reveals that sexual behavior and sexual selection can be astonishingly dynamic in response to short-term fluc- tuations in mating competition. Future work should explore whether sexual selection is equally dy- namic on a spatial scale, and related spatiotemporal dynamics. Key words: adult sex ratio, female ornament, Gobiusculus flavescens, male ornament, mate choice, mate search, mating compe- tition, operational sex ratio, OSR, two-spotted goby Introduction social systems (e.g., Amundsen 2003). That being said, certain organisms have proven particularly useful for exploring fundamen- Model organisms have proven highly valuable in understanding fun- tal principles of behavior, including sexual selection. Among fishes, damental questions in biology. This has particularly been the case in influential models include guppies Poecilia reticulata and related neurobiology, developmental biology, genetics, molecular biology, poecilids, three-spined sticklebacks Gasterosteus aculeatus, pipe- and to a certain extent evolution. Important model organisms in- fishes (Syngnathidae), and cichlids (Cichlidae) (Amundsen 2003), clude fruit flies Drosophila melanogaster, house mice Mus muscu- but several other taxa have also provided model organisms highly lus, Norway rats Rattus norvegicus, zebra fish Danio rerio, and suitable for exploring specific research areas in behavior and evolu- thale cress Arabidopsis thaliana. By contrast, model organisms have not been equally central to animal behavior and evolutionary ecol- tion. If we are to understand nature’s diversity, we need to draw ogy, due to the diversity of life histories, ecological adaptations, and insights from a diversity of model organisms. V C The Author(s) (2018). Published by Oxford University Press. 363 This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact journals.permissions@oup.com Downloaded from https://academic.oup.com/cz/article-abstract/64/3/363/4986908 by Ed 'DeepDyve' Gillespie user on 21 June 2018 364 Current Zoology, 2018, Vol. 64, No. 3 The aim of this article is to provide an overview of insights from and swift acclimation to laboratory conditions, allow analyses of a model system that has proven unusually dynamic, and hence ex- male care dynamics of relevance to many other fish families (e.g., Blenniidae, Centrarchidae, Cichlidae, Gasterosteidae, and ceptionally suitable for analyzing the regulation of sex roles and sex- Pomacentridae). ual selection: the small marine goby fish Gobiusculus flavescens In sexual selection research, the most widely used model organ- (Figure 1). I place the G. flavescens work in a context of theoretical isms have historically been birds (Darwin 1871; Andersson 1994; (and some empirical) work for each of the topics covered. These in- Amundsen 2003). However, most birds (and mammals) do not ac- clude animal sex roles, operational sex ratio (OSR) dynamics, sexual climate easily to laboratory conditions, and only few birds mate and selection theory, ornamentation and signaling in males and females, breed in captivity. By contrast, many gobies (and members of some mate choice, mating competition and mate search, environmental other fish families) are easily kept in small aquaria and display their effects on sexual competition and sexual selection, and alternative natural behavioral repertoire, including courtship, mating competi- reproductive tactics. Given the breadth of topics, however, it is be- tion, mate choice, and breeding, in captivity. Such species are ideally yond the scope of the article to provide a comprehensive discussion suited for experimental tests of sexual behaviors and how these are of the vast literature that exists on each topic. affected by variation in the social and physical environment. Accordingly, work on several species of temperate gobies, many of Gobies as Model Organisms them close relatives of G. flavescens, have provided insights of wide- ranging relevance on mate choice, mating competition, and sexual Gobies (Gobiidae) are mostly small, substrate-brooding fishes that selection. The most extensively used models are sand gobies occur in both marine and freshwater environments world-wide Pomatoschistus minutus (e.g., Forsgren et al. 1996b; Lindstro ¨m (Patzner et al. 2011). Gobiidae is one of the most speciose fish fami- 2001; Svensson and Kvarnemo 2003) and common gobies P. lies, with about 2,000 species described (e.g., Agorreta et al. 2013). microps (e.g., Magnhagen 1994; Svensson et al. 1998; Heubel et al. Recent molecular analyses have revealed that Gobiidae consists of 2 2008). Important contributions to mating dynamics and sexual se- distinct sub-clades which separated about 54 million years ago, in lection have also been made on several other species, including the the early Eocene (Thacker 2015). There is an ongoing discussion as closely related painted gobies P. pictus (e.g., Amorim and Neves to whether the sub-clades should be considered separate families or 2008; Amorim et al. 2013), marbled gobies P. marmoratus remain within Gobiidae (Thacker 2009, 2013; Pezold 2011; (Locatello et al. 2016) and lagoon gobies Knipowitschia panizzae Thacker and Roje 2011; Agorreta et al. 2013; Tornabene et al. (e.g., Mazzoldi et al. 2003; Pizzolon et al. 2008), all of which belong 2013). The “European sand gobies,” including the model organism to the gobionelline Pomatoschistus lineage (Gobionellidae sensu of this article, cluster within the gobionelline-like gobies (sensu Thacker 2009, 2015). These species all have a mainly European dis- Agorreta et al. 2013) and would thus be part of a potential new tribution (Thacker 2015). In Australia, the desert goby Gobionellidae family (sensu Thacker 2009, 2013) representing the Chlamydogobius eremius, a member of the gobionelline less speciose sub-clade (ca. 650 species, Thacker 2015). Mugiogobius lineage, has recently become an important model for Whether gobies constitute 1 or more phylogenetic families, they sexual selection research (e.g., Svensson et al. 2010; Lehtonen et al. share many characteristics with respect to morphology and biology. 2016). Goby sexual selection models of the gobiine Gobius lineage Many species, including those of the “sand goby group” (Huyse (Gobiidae sensu Thacker 2009; 2015) include black gobies Gobius et al. 2004; Thacker 2013), are small and occur at high densities in niger (e.g., Rasotto and Mazzoldi 2002; Scaggiante et al. 2005), the wild. Gobies have paternal care of eggs, making them suitable grass gobies Zosterisessor ophiocephalus (e.g., Mazzoldi et al. 2000; models for testing theories regarding costs of reproduction, resource Scaggiante et al. 2005), and round gobies Neogobius melanostomus allocation, and parent–offspring conflict. The paternal care (e.g., Marentette et al. 2009; Bleeker et al. 2017). In tropical envi- employed by gobies is the most common form of care in teleost ronments, research on coral gobies (Gobiodon spp., e.g., Munday fishes (Clutton-Brock 1991; Balshine 2012), having evolved inde- 2002, Paragobiodon xanthosomus, e.g., Wong et al. 2008) and pendently in at least 22 evolutionary fish lineages (Mank et al. blue-banded gobies Lythrypnus dalli (e.g., Lorenzi et al. 2009) have 2005). Thus, gobies, being often easy to study due to their small size been instrumental in understanding mechanisms and function of sex change and social dynamics. Gobies are generally considered to have conventional sex roles, but sex role reversal occurs late in the breeding season in G. flavescens (Forsgren et al. 2004) and has also been reported in the American tidewater goby Eucyclogobius new- berryi (Swenson 1997). The two-spotted Goby G. flavescens: A Model for Sex Role Dynamics The two-spotted goby G. flavescens belongs to the mostly European Pomatoschistus lineage of gobies (Agorreta et al. 2013) and is simi- lar to the much-studied P. minutus and P. microps in many respects, including size, morphology, and breeding biology. Therefore, studies on G. flavescens can, together with work on these and related gobies, reveal joint patterns of reproductive dynamics. However, G. flavescens differs from these and most other extensively studied goby species in life-style and habitat. Most other members of the Figure 1. The model organism Gobiusculus flavescens (two-spotted goby) in mutual courtship display. The female in front. Photo: V Nils Aukan. “sand goby group” (Huyse et al. 2004; Agorreta et al. 2013) are Downloaded from https://academic.oup.com/cz/article-abstract/64/3/363/4986908 by Ed 'DeepDyve' Gillespie user on 21 June 2018 Amundsen  Gobies as models in sexual selection 365 Figure 2. Study sites, habitats, and nest substrates of G. flavescens in Scandinavia. (A–D) Study locations in West Sweden (A), West Norway (B), mid-Norway (C) and South Finland (D). (E–H) Diversity of kelp and seaweed habitats, dominated by Saccharina latissima (E), Laminaria hyperborea (F), Fucus serratus (G), and filamentous algae (H), respectively. (I–L) Diversity of nesting subtrates: blue mussel M. edulis (I), base of S. latissima (J), atop dead bryozoans on L. digitata (K), V C and acetate sheet inside artificial PVC nest (L). All photos: Trond Amundsen. benthic, inhabiting shallow bays with substrates ranging from gravel males and socially grouped females is unique among closely related to silt, and with species partly distributed in accordance with sub- gobies, and possibly among gobies in general. strate characteristics. These species spend the majority of their time The reason why G. flavescens is such a powerful model for on, or partly immersed in, the substrate. By contrast, G. flavescens understanding the dynamics of sex roles and sexual selection is the is semi-pelagic and inhabits kelp forests and seaweed beds (Figure 2) species’ exceptionally variable adult and operational sex ratio (OSR) along the rocky shores of Western Europe (Figure 3). The preference (Forsgren et al. 2004). This variation has allowed extensive investi- for macro-algal habitats, which is unique to G. flavescens among gations on how mating competition regimes affect sexual behaviors European gobies, makes it extremely abundant over much of its dis- and consequent sexual selection. It should, however, be pointed out tribution: for instance, it is by far the most abundant fish species of that the G. flavescens model system is not the only fish (or other) near-shore shallow waters in Norway. Being semi-pelagic means model system that displays variation in OSR and mating competi- that individuals shift between residing among the macro-algal vege- tion. Such variation is widespread, not the least in fishes, but usually tation and foraging in the nearby water column (up to a few meters within the bounds of either conventional (male competition) or from shore), reflecting a trade-off between foraging and predator reversed (female competition) sex roles. What is near-unique about avoidance (Utne et al. 1993; Utne and Aksnes 1994). Individuals our study population of G. flavescens is the documented extent of rarely rest on the substrate except during spawning and, in the case variation, involving a complete shift from conventional to reversed of males, during parental care. However, despite swimming, they sex roles within a single breeding season (Forsgren et al. 2004; usually “stay put” within a few meters range (usually less) most of Myhre et al. 2012). When we started exploring sex role dynamics in the time. Unlike its close Pomatoschistus relatives, G. flavescens G. flavescens, no similarly dynamic system had been described in assembles in loose foraging shoals that range from less than ten to any vertebrate species (Forsgren et al. 2004). The conspicuous fe- several hundred individuals, or even more in the case of juveniles male ornamentation, different from that of the male, makes G. fla- (Svensson et al. 2000, personal observation). During the breeding vescens an especially suitable model for analyses of female season, however, most males defend territories in the kelp forest, ornamentation (Amundsen and Forsgren 2001). The female orna- and are thus often solitary (Forsgren et al. 2004). Males that do not mentation of G. flavescens is unique among closely related members breed usually join the female-dominated shoals, but sexual interac- of the sand goby clade (Svensson et al. 2009a). tions are exceedingly rare in the shoals. During mate search and Besides its unusually dynamic breeding biology, G. flavescens when ready to spawn, females occur in smaller unisexual shoals or also stands out as a uniquely suitable model system for logistic sometimes solitarily (Myhre et al. 2012). The situation with solitary reasons. Because G. flavescens lives and breeds in shallow (mostly Downloaded from https://academic.oup.com/cz/article-abstract/64/3/363/4986908 by Ed 'DeepDyve' Gillespie user on 21 June 2018 366 Current Zoology, 2018, Vol. 64, No. 3 Figure 3. Geographic distribution of G. flavescens. Reprinted from International Union for Conservation of Nature (IUCN) 2014. Gobiusculus flavescens. The IUCN Red List of Threatened Species. Version 2017-1. 0–3 m) and mostly clear coastal waters, the species’ social, sexual, work has been based at the Sven Love ´ n Centre for Marine Sciences and reproductive behaviors can be easily observed and quantified by in Fiskeba ¨ ckskil, situated at the mouth of the Gullmar Fjord 0 00  0 00 snorkelers (Forsgren et al. 2004; Myhre et al. 2012). The species is (58 14 60 N, 11 26 44 E, Figure 4). Field work has been con- unusually tolerant to disturbance, and can therefore be observed at ducted in the archipelago nearby the research station; experiments close range (<1 m) while performing its natural repertoire of sexual in aquaria or mesocosm tanks have been conducted at the station. and reproductive behaviors both in the field (e.g., Forsgren et al. Additionally, studies (especially on alternative reproductive tactics 2004; Myhre et al. 2012) and in the laboratory (e.g., Amundsen and and parental care; e.g., Skolbekken and Utne-Palm 2001; Utne-Palm Forsgren 2003; Borg et al. 2006; Myhre et al. 2013; Wacker et al. et al. 2015; Monroe et al. 2016) have been made on a population on 2013). Gobiusculus flavescens also readily breeds in captivity (e.g., the West coast of Norway, from a base at Espeland Marine 0 00  0 00 Bjelvenmark and Forsgren 2003; Svensson et al. 2006). The species Biological Station (60 16 11 N, 5 13 19 E, Figure 4). is extremely abundant along Scandinavian (and other East Atlantic) rocky shores (e.g., Fossa ˚ 1991), and easy to catch in large numbers for population studies (e.g., Wacker et al. 2014; Utne-Palm et al. Biology of the Model Organism 2015) or laboratory experiments. Population samples are typically Male and female size collected by beach seine (Utne-Palm et al. 2015), whereas fish to be In the W Sweden study population, adults of both sexes are mostly used in behavioral experiments are typically caught individually by 35–55 mm long (total length), with the majority of individuals being dip nets while snorkeling (e.g., Wacker and Amundsen 2014). Due 40–50 mm (Wacker et al. 2014, T. Amundsen et al., unpublished to its abundance and shallow breeding habitat, both natural and data). In that and most other populations studied, the species is weak- artificial nests in the field can be easily inspected (e.g., Forsgren ly sexually size dimorphic, with males being slightly larger than et al. 2004) or collected (e.g., Mobley et al. 2009; Monroe et al. 2016), for instance for quantification of reproductive success, egg females (T. Amundsen et al., unpublished data). The W Norway population, however, has reversed sexual size dimorphism with parameters, and parentage. Taken together, the species is ideally suited for analyses of sexual and reproductive dynamics. females being on average larger than males, due to an abundance of The majority of published studies of mating dynamics in very small males in this population (Utne-Palm et al. 2015). Body size varies significantly between years (Wacker et al. G. flavescens, including those discussed in the present article, have been conducted on a population on the West coast of Sweden. The 2014), and geographically (T. Amundsen et al., unpublished data). Downloaded from https://academic.oup.com/cz/article-abstract/64/3/363/4986908 by Ed 'DeepDyve' Gillespie user on 21 June 2018 Amundsen  Gobies as models in sexual selection 367 Figure 4. Locations of study sites for G. flavescens. The majority of work referred to in this article was made in the archipelago around and at the Sven Love´n Centre for Marine Science at Kristineberg (research station; red circle in right panel), situated at the mouth of the Gullmar Fjord in West Sweden. Some studies were also carried out in West Norway (blue square in left panel). Red arrows: locations for studying sex role reversal (Forsgren et al. 2004), yellow arrows: loca- tions for the mate sampling study (Myhre et al. 2012), green arrows: locations for studying sexual selection in the wild (Wacker et al. 2014), blue arrow: location for parentage study (Mobley et al. 2009). Remaining studies were made in laboratories at the Kristineberg Research Station. In all years and all populations studied, variation in size is greater in several species of mussel (Wacker et al. 2014, personal observation). males than in females (T. Amundsen et al., unpublished data). A typical male territory includes many potential nesting sites, espe- cially because G. flavescens often breeds on algae. It is not always obvious whether a male primarily defends an area (with several nest- Ecology ing opportunities), or a specific nesting structure (e.g., a cavity on a Gobiusculus flavescens occurs along rocky shores from N Norway to kelp), prior to mating. The species readily breed in artificial nests Portugal (Miller 1986; Borges et al. 2007), including parts of the made of PVC tubing (Figure 2), both in the laboratory and in the Baltic Sea (Figure 3). In the Nordic study populations, it mainly field (e.g., Forsgren et al. 2004; Wacker et al. 2013; Monroe et al. occurs at 0–5 m depth during the breeding season, with nests often 2016). just 1–2 m below the low tide mark. The species inhabits both shel- Male G. flavescens compete for ownership of favorable nest sub- tered and semi-exposed shores, but appears to be absent or less abun- strates by visual displays and physical aggression, and attract dant at the most exposed locations. Due to its very high abundance in females to their nests with elaborate courtship, involving lateral dis- rocky shores kelp forests (Fossa˚1991; Utne-Palm et al. 2015), and be- cause most of the Nordic coastlines are rocky shores (Figure 2), plays with erected fins (Figure 1) and undulating lead swims toward G. flavescens is a keystone species in coastal ecosystems (Fossa˚1991; the nest (Amundsen and Forsgren 2001; Forsgren et al. 2004). Males also produce sounds close to the nest just prior to mating, Giske et al. 1991; Nordeide and Salvanes 1991; Hop et al. 1992). In and in the nest during spawning (de Jong et al. 2018). Spawning Norway, G. flavescens hasbeenreportedtobe the main prey of first- females attach each individual egg to the substrate, which may take and second-year codfish in studied fjord systems (Fossa˚1991; Nordeide and Salvanes 1991) and has been central in models of fjord 1–2 h for a clutch of usually 500–2,000 eggs (Pe ´ labon et al. 2003; ecosystem productivity (Giske et al. 1991; Salvanes et al. 1992). Svensson et al. 2006; Forsgren et al. 2013). Males are typically ei- ther unsuccessful in mating, or mate with several females in succes- sion. Thus, successful males in Norwegian and Swedish study Breeding populations mate with a median of 4–5 females (Figure 5a; Mobley The species is mostly annual, with both males and females usually et al. 2009; Monroe et al. 2016). The total brood size in a male’s having only 1 reproductive season (Johnsen 1945). In the Nordic nest can therefore be very large (Figure 5), at the extreme >10,000 countries, breeding commences in April–May and usually ends in eggs (Gordon 1983, personal observation). Consecutive clutches are late July (Forsgren et al. 2004; Myhre et al. 2012; Wacker et al. often of similar age, suggesting that they are spawned in quick suc- 2014), yet with some variation seemingly related to latitude and cli- cession, but significant age differences among clutches within a mate (personal observation). In more southerly locations, breeding brood may occur (personal observation). Once the nest is full, the may start earlier and/or end later (Collins 1981; Miller 1986, male is “out of mating competition” until the brood hatches. The A.M.S. Faria, personal communication). Gobiusculus flavescens is a eggs are usually laid in a single layer (Figure 2i–l), and hatch after a substrate brooder, with males defending nests in which one or usual- period of 1–3 weeks, depending on sea temperature (Skolbekken ly more females deposit clutches of eggs (Mobley et al. 2009; and Utne-Palm 2001; Bjelvenmark and Forsgren 2003; Svensson Wacker et al. 2014; Monroe et al. 2016). Breeding occurs in natural 2006). During this period, the brood is cared for by the male, by fan- crevices, with no nest building or modification of the nesting sub- strate (as is common in benthic gobies inhabiting more sheltered ning and cleaning the eggs (Skolbekken and Utne-Palm 2001; locations) (Figure 2). Common nest substrates include empty mus- Bjelvenmark and Forsgren 2003), and by defending them against sels (e.g., Mytilus edulis, Mobley et al. 2009; Wacker et al. 2014), predators (e.g., conspecific or hetero-specific fishes or small shore crabs Carcinus maenas). Once the brood hatches, the male may en- which appear to be a favored substrate, natural crevices in the algal gage in attracting females for a new brood. In the laboratory, the re- vegetation (e.g., at the base of kelp leaves and in their holdfasts, Gordon 1983, personal observation), and under stones. Gobiusculus cess time between hatching and engagement in courtship can be flavescens appears opportunistic in choice of breeding substrate, negligible (Eriksen 2007). Unless disturbed, caring males usually with nests found on a range of kelp and seaweed species and in spend >50% of their time in the nest, during which they cannot Downloaded from https://academic.oup.com/cz/article-abstract/64/3/363/4986908 by Ed 'DeepDyve' Gillespie user on 21 June 2018 368 Current Zoology, 2018, Vol. 64, No. 3 Figure 5. Relationships between mating success and reproductive success (A), and between nest size and reproductive success (B), in G. flavescens. Mating and reproductive success were quantified from parentage analyses using microsatellites. Mussels in (B) are all blue mussels Mytilus edulis. Reproduced from (A) Figure 2 and (B) Figure 1 in Mobley et al. (2009), BMC Evolutionary Biology 9:6. forage (Skolbekken and Utne-Palm 2001; Bjelvenmark and Forsgren digital scale, and coloration of fish, gonads, eggs, and biopsies by 2003). Assuming a recess time of a few days between successive standardized photographic methods. Condition is usually quantified broods, and a normal climatic succession, a male can theoretically as residuals from length–mass correlations, except when small and care for about 6 broods over the course of a southern Nordic breed- large males are compared, in which case condition factor is used. ing season. Further north, lower sea temperatures allow for fewer Fish size (body mass and total length) is usually recorded at a re- breeding cycles; for instance, a maximum of about 3 successive search station laboratory, but occasionally using portable devices in broods in mid-Norway (T. Amundsen, unpublished data). Mortality the field in cases where fish are to be returned swiftly to their natural of males is high during the breeding season in the W Sweden main habitat (and nests). study population (Forsgren et al. 2004), and costs of reproduction may prevent males that are still alive from realizing their potential Unusually Dynamic Sex Roles number of breeding events. Like males, females can reproduce re- peatedly over the course of the breeding season, with reproductive Sex roles and sex ratios: definitions, dynamics, and rate affected by temperature. In P. minutus, temperature affects the theories of regulation reproductive rate more in males than in females (Kvarnemo 1994); In many animal species, male reproductive success is limited by ac- this is likely also the case for G. flavescens. cess to mates (females), whereas female reproductive success is lim- ited by resources required to produce and care for offspring Male and female ornamentation (Bateman 1948; Trivers 1972). In such species, mating competition Both male and female G. flavescens are extravagantly ornamented and consequent sexual selection are expected to be stronger in males (Figure 1; Amundsen and Forsgren 2001). Males have a series of iri- (Darwin 1871; Andersson 1994). The icon of such conventional (or descent blue lateral spots and two larger dark spots, one at the base traditional) sex roles is the peafowl (e.g., Petrie et al. 1991). of the tail and one at the base of the pectoral fin. They also sport an However, already Darwin (1871) was aware that species exist in enlarged and colorful dorsal fin, with alternating lines of iridescent which females are the more mate-limited sex, resulting in sexual se- blue and orange–red coloration. The anal fin of males is uniformly lection for large body size and conspicuous coloration in females. gray in color, and is displayed during exaggerated aggressive Examples include several waterbirds (Colwell and Oring 1988; encounters, during which the whole body may turn darker. Females Emlen and Wrege 2004) and pipefishes (Berglund and Rosenqvist have only traces of iridescent spots along the sides, and lack signifi- 2003). Such cases are today described to have reversed sex roles cant fin pigmentation (Figures 1 and 8). However, gravid females (Berglund et al. 1986b; reviewed in Eens and Pinxten 2000). In such display conspicuously orange-colored bellies, which they actively species, females compete for access to males. display to males during courtship (Figure 1), by bending their bodies The term “sex roles” is used with a multitude of meanings in for maximal exposure (Amundsen and Forsgren 2001; Sko ¨ ld et al. human society, and is, unfortunately, also used in several meanings 2008). Female belly coloration is mainly caused by variably yellow in evolutionary science (see, e.g., Vincent et al. 1992; Forsgren et al. to orange eggs that are visible through the semi-transparent skin, 2004; Ah-King and Ahnesjo ¨ 2013). This warrants a clear definition but also by red pigment cells (erythrophores) in the belly skin of the term as used in the present article. It also serves as a warning (Figure 6a–c; Svensson et al. 2005; Sko ¨ ld et al. 2008). that the scientific discourse about “sex roles” is sometimes muddled by different uses of the term. Building on seminal works by Williams General procedures (1975) and Emlen and Oring (1977), Vincent et al. (1992) and later Our work on G. flavescens is based on a range of approaches, Kvarnemo and Ahnesjo ¨ (1996) defined sex roles to solely describe which sex faces the strongest mating competition: conventional including (1) observational and experimental work in the field, (2) experimental work in aquaria and mesocosm tanks in the labora- when strongest in males, reversed when strongest in females. This is tory, (3) analyses of egg quality, color, and its chemical basis and the meaning of sex roles employed in this article – and in all our regulation in whole-fish, gonads, and skin, and (4) population sam- work on G. flavescens (e.g., Forsgren et al. 2004). However, the pling. Body length (total length, to the nearest 0.5 mm) is recorded term sex roles has also been used (i) to encompass competition by using a measuring board, body mass to the accuracy of 0.01 g on a courtship only, describing which sex is most active in courtship Downloaded from https://academic.oup.com/cz/article-abstract/64/3/363/4986908 by Ed 'DeepDyve' Gillespie user on 21 June 2018 Amundsen  Gobies as models in sexual selection 369 Figure 6. Variation in OSR and mating competition over the breeding season. The figure shows within breeding-season trajectories of operational sex ratio (OSR) (A), propensity for aggressive behavior (B), and propensity to court (C) in a study of sex role dynamics in G. flavescens. The open circles in (A) and white-to-black bars in (B) and (C) represent 4 recording sessions distributed over the course of the breeding season. The OSR changes from male- to female-biased (A), with a concerted decrease in male mating competition (by male–male aggression and courtship efforts), and a simultaneous increase in female mating competition by the same means (B, C). Propensities to behave by aggression or courtship represent the likelihood that the actual behavior takes place at a given encounter be- tween same- or opposite-sex individuals. Reproduced (A) from Figure 1 and (B–C) from Figure 2 in Forsgren et al. (2004), Nature 429:551–554. while adopting other terms for agonistic mating competition (male– Apart from mating competition, sexual selection is driven by mate male or female–female) (Saraiva et al. 2012). We suggest that vari- choice, post-mating sperm competition and cryptic choice, and sev- ation in courtship only is better termed courtship roles, as sometimes eral other processes (Andersson 1994; Eberhard 1996; Birkhead and done (e.g., Gwynne and Simmons 1990; Borg et al. 2002). Møller 1998). These processes may work in concert (i.e., be addi- Moreover, and more commonly, the sex role term has been used (ii) tive). However, they could also select for different traits, for instance in the broader meaning of encompassing both mating competition if traits that make males superior in competition are not the same as and mate choice. The basis for such a broader concept is the theory those important in female mate choice (Qvarnstro ¨ m and Forsgren that the two are usually inversely related: when one sex is the more 1998; Wong and Candolin 2005). They could also work in opposite competitive, the other sex will be the more-choosy (Trivers 1972). directions on the same traits (Hunt et al. 2009), for instance if large That need not always be the case, however, for instance if quality males are more successful in competition but females prefer small variation is much greater in the less-competitive sex (Owens and males (Petrie 1983). In such instances of conflicting selection pres- Thompson 1994) or if competition and choice interact (Berglund sures from competition, choice, and other mechanisms, increasingly et al. 2005). Another and more commonplace practice is to (iii) in- strong mating competition in a sex need not necessarily imply stron- clude parental care when defining sex roles: conventional sex roles ger overall sexual selection on that sex. In most cases, however, then encompasses predominant male–male competition and female traits promoting success in competition are likely to be selected. care; reversed sex roles predominant female–female competition and The OSR describes the relative abundance of males and females male care (e.g., Liker et al. 2013; Janicke et al. 2016). This is in line “on the mating market”—whether there are more individuals of one with Darwin’s (1871) bird-based reasoning: in several avian taxa, or the other sex that are ready to mate at any point of time (Emlen the extent of care and competition are inversely related and the and Oring 1977; Kvarnemo and Ahnesjo ¨ 1996). The OSR can either broader definition therefore largely “works” (Liker et al. 2013). be male-biased (more mating-ready males than females, the more However, as emphasized by Vincent et al. (1992), predominant commonplace situation among animals), or female-biased (more male mating competition occurs in many species with male parental mating-ready females than males). It can also be relatively even, as care. This is particularly often the case in fishes, in which uniparen- in socially monogamous seabirds with limited extra-pair sex. OSR tal male care is the more common form of care (Gross and Sargent theory predicts that the sex facing a shortage of potential mates (i.e., 1985; Clutton-Brock 1991). In the majority of fishes with paternal toward which the OSR is biased) should show stronger mating com- care, including three-spined sticklebacks G. aculeatus (Bakker 1994) petition (Vincent et al. 1992; Kvarnemo and Ahnesjo ¨ 1996, 2002). and several species of gobies (e.g., Lindstro ¨ m 1988; Borg et al. Such competition could be manifested in agonistic interactions with 2002), mating competition is clearly stronger in males than in same-sex competitors (by displays or physical aggression), in efforts females under most circumstances. For these reasons, we (e.g., to attract the other sex by courtship, or both. Forsgren et al. 2004; Myhre et al. 2012, this article) follow Vincent Fundamentally, the OSR is determined by variation in adult sex et al. (1992) and Kvarnemo and Ahnesjo ¨ (1996) in using the sex role ratio (ASR) and potential reproductive rate (Parker and Simmons term in its simplest and most fundamental form: to describe which 1996; Kvarnemo and Ahnesjo ¨ 2002). When the ASR varies little sex experiences the strongest mating competition. This definition is from unity, the potential reproductive rate is the main factor deter- applicable to all sexual species. mining the OSR. However, strong ASR biases toward males or females can override the effect of sex differences in potential repro- ductive rate. Mating competition as a driver of sexual selection Mating competition is one of the major processes that drive sexual Recently, Szekely et al. (2014b) have argued that the ASR regu- selection, and the “direction” and strength of mating competition is lates recource competition whereas the OSR regulates mating com- therefore expected to affect sexual selection (Kokko and Monaghan petition. It has also been suggested that sex differences in cost of 2001). This may be the reason why OSR effects on mating competi- reproduction rather than OSR are the ultimate determinants of mat- tion are often taken to imply effects on sexual selection. However, ing competition (sex roles) (Kokko and Monaghan 2001; Kokko mating competition should not be equated with sexual selection. and Johnstone 2002). There is an ongoing theoretical debate about Downloaded from https://academic.oup.com/cz/article-abstract/64/3/363/4986908 by Ed 'DeepDyve' Gillespie user on 21 June 2018 370 Current Zoology, 2018, Vol. 64, No. 3 the role of OSR in shaping mating competition, sex roles, and sexual season. By contrast, gravid females appeared to be highly abundant, selection (e.g., Kokko and Jennions 2008; Klug et al. 2010a; Kokko often actively courting the few males present. This situation et al. 2012; Fromhage and Jennions 2016; Clutton-Brock 2017; appeared very different from that experienced early in the breeding Jennions and Fromhage 2017). However, neither our empirical season. Realizing that we might be faced with a rather unique sys- work with the G. flavescens model system nor this article aim to ad- tem of sex role reversal over the breeding season, during the follow- dress all issues raised in that debate. Instead, the aim of our work ing breeding season we conducted an extensive field study to test has been to empirically explore the role of the OSR as a driver of whether our impressions reflected reality. The aims of the study mating competition and sexual selection. were to test whether the OSR changed from male-biased to female- Prior to our work, empirical studies on other model systems had biased over the course of the season and, if so, whether mating com- established that variation in OSR was associated with variation in petition changed accordingly, from conventional sex roles early in the strength of mating competition in several species (see Weir et al. the season to reversed sex roles later on. Swimming transects (18– 2011; de Jong et al. 2012), yet mostly within the bounds of either 33 m) along 10 stretches of coastline by a total of 6 different islands, conventional (e.g., Kvarnemo et al. 1995) or reversed sex roles (e.g., we quantified numbers of males and females in each transect 4 times Vincent et al. 1994). Complete sex role reversals in response to OSR over the breeding season, from late May until mid-July. In line with variation had, previous to our work, only been found in 2 species of our hypothesis, we found a drastic decline in the number of males katydid insects, Anabrus simplex (Gwynne 1993) and Kawanaphila observed, with only about 10% as many males in mid-July as in nartee (Gwynne and Simmons 1990; Simmons and Bailey 1990; May. For females, the reduction in numbers over the season was far Gwynne et al. 1998), regulated by food supply. In these species, the less pronounced (Figure 1 in Forsgren et al. 2004). Thus, G. flaves- change is mainly in courtship roles (which sex is most actively court- cens experienced a more dramatic change in the ASR (including ing). In sticklebacks G. aculeatus, female courtship had been found individuals ready and not ready to breed) than reported in any verte- to increase dramatically over the breeding season (Kynard 1978). brate species before, as far as we know. The cause of this change is almost certainly male mortality, as a result of increased predation on solitary and displaying males, costs from repeated cycles of care Adult sex ratio (Smith and Wootton 1995), or both (Forsgren et al. 2004; Wacker Our work on G. flavescens has investigated ASR because it, together et al. 2013). A higher male than female mortality by the end of the with variation in potential reproductive rate (Clutton-Brock and breeding has also been found in sticklebacks G. aculeatus (Kynard Parker 1992), drives OSR variation (e.g., Ahnesjo ¨ et al. 2008). ASRs 1978). In G. flavescens, the temporal change in ASR was of a magni- can vary substantially in animals, both naturally and as a conse- tude clearly overriding any change in male and female reproductive quence of sex-biased harvesting regimes (e.g., Adams et al. 2000; rates over the season. Temporal changes in ASR are not uncommon Forsgren et al. 2002; Szekely et al. 2014a, 2014b). Strongly biased (e.g., Ancona et al. 2017) but were exceptionally extreme in G. sex ratios are particularly prevalent in species without chromosomal flavescens. sex determination, like in many fishes (Charnov and Bull 1989). For example, sex-changing fishes almost always have strongly female- biased sex ratios (e.g., Wacker et al. 2016), with extreme cases Operational sex ratio including haremic species like anthiases (Fam. Serranidae) having The OSR of an organism is usually different from the ASR because only 10–20% males (Molloy et al. 2007). By contrast, the XY and only a fraction of males and females are ready to mate at any point ZW chromosomal sex determination of mammals and birds con- of time (e.g., Kvarnemo and Ahnesjo ¨ 2002). The degree of difference strains sex ratio variation even if significant deviations from unity between the 2 measures is variable among and within species (e.g., are still common (Liker et al. 2013). Such deviations could result Szekely et al. 2014b). At the extreme, ASR and OSR may show op- from minor biases in primary sex ratio, but more commonly from posite temporal dynamics (Carmona-Isunza et al. 2017). sex differences in mortality (Trivers 1972; Szekely et al. 2014a). In One way to express the distinction between adult and OSR is to humans, modestly biased ASRs are common, either female-biased as estimate which individuals are “out” of mating competition (caring a result of high male early-life mortality (Pouget 2017), male-biased or maturing eggs, or excluded from breeding due to competition) as a consequence of infanticide and sex-differential care (Brooks and which are “in” (ready to mate, Parker and Simmons 1996; 2012), or locally fluctuating (Kramer et al. 2017). Such biases may Ahnesjo ¨ et al. 2001; Kvarnemo and Ahnesjo ¨ 2002). In repeated have significant impacts on human society, behavior, and well-being spawners like G. flavescens, females need time to mature a new (e.g., Brooks 2012; Schacht and Smith 2017; Zhou and Hesketh clutch after spawning, and only those with mature eggs are part of 2017). the “mating pool” (Parker and Simmons 1996). Recently spawned Measuring adult (and operational) sex ratios in the wild is diffi- females are slim whereas maturing egg-batches cause females to dis- cult in many organisms (e.g., Ancona et al. 2017; Kappeler 2017). If play more or less distended bellies (e.g., Svensson et al. 2009b). G. flavescens, however, recording ASR is relatively straightforward, Only those that are ready or near-ready to spawn should, by defin- as males and females inhabit the same shallow-water habitat and be- ition, be included in OSR estimates. In G. flavescens, female matur- cause the species occurs at very high densities, is relatively station- ity can be judged visually from belly extension and coloration (spent ary, easy to observe at close range, and easy to sex. Males and females are mostly drab). We recorded female “roundness” on a 3- females are usually easily distinguishable in the species, based on graded scale, and included the upper 2 roundness classes in our OSR coloration, body form, and behavior (Figure 1). estimate. Gobiusculus flavescens is not a sex-changer but a strongly biased Males of G. flavescens are usually either stationary or roaming. ASR, often with more females than males, is commonplace Males with a nest, or else ready to breed, are territorial and station- (T. Amundsen et al., unpublished data). During our initial work on ary. Thus, we excluded roaming males when estimating OSR male mate choice in the model system (Amundsen and Forsgren (Forsgren et al. 2004). In G. flavescens, like in many other substrate- 2001, 2003), we experienced increasing difficulties in finding males brooding fishes, successful males can simultaneously care for mul- for our experiments in mid-July, toward the end of the breeding tiple clutches sequentially spawned by several females. With Downloaded from https://academic.oup.com/cz/article-abstract/64/3/363/4986908 by Ed 'DeepDyve' Gillespie user on 21 June 2018 Amundsen  Gobies as models in sexual selection 371 spawning often lasting 1–2 h for each female, a male can theoretical- late July (Figure 6a). In May, the OSR was heavily male-biased ly mate with >10 females during a single day. Thus, at a given point (OSR> 0.8) whereas it was even more heavily female-biased in late in time, a male represents a breeding opportunity for >1 female, July (OSR< 0.1), with a near-linear change over the 2 intermediate provided that he has space for >1 additional clutch in his nest. How recording periods (Figure 6a; Forsgren et al. 2004). A similarly ex- many females can spawn in his nest is thus a function of nest size treme change in OSR over a single breeding season has, to our and the amount of eggs already in the nest (the percent nest area al- knowledge, not been reported in any other vertebrate. However, ready occupied). In G. flavescens, typically 4–5 females spawn in a that is not to say that such changes do not occur in other animals, as male’s nest (Figure 5a). Mussel nests are often full or near-full with temporal changes in OSR have been relatively little studied (but see such numbers of clutches (Mobley et al. 2009; Monroe et al. 2016). Kynard 1978; Wootton et al. 1995). Knowing typical clutch size (Pe ´ labon et al. 2003; Svensson et al. 2006; Forsgren et al. 2013) and area covered by such a clutch A reversal of sex roles (Bjelvenmark and Forsgren 2003) from laboratory studies, we could With OSR changing from strongly male- to strongly female-biased calculate, for each nest, how many more clutches could be accom- over the course of the breeding season, we predicted stronger mating modated in the nest. For instance, a male with an empty mussel nest competition in males early in the season and stronger mating compe- of average size would represent a mating opportunity for about 4 tition in females late in the season (Forsgren et al. 2004), expressed females (Forsgren et al. 2004). If the nest was half-full, he could ac- by efforts to entice opposite-sex individuals to mate (courtship) or commodate 2 more clutches. In estimating OSR of the population, by intra-sexual aggression (visual displays or physical aggression we included recorded nest fullness in the calculation, multiplying the toward competitors). Thus, we recorded courtship and intra-sexual number of territorial (stationary) males with the average number of aggression by males and females at each stage of the season (details further clutches a nest could accommodate at that time (Table 1 in of behaviors: see Forsgren et al. 2004). This is most easily done by Forsgren et al. 2004). When males can care for clutches from mul- counting the number of occurrences of each behavior for each sex tiple females, it is not the number of males per se that defines mating and time of season (the frequency of competitive behaviors; de Jong opportunities for females, but the number of further clutches these et al. 2012). However, such frequencies are essentially the products males can accommodate and care for at any point of time (Parker of (i) the number of encounters between opposite- or same-sex indi- and Simmons 1996). viduals and (ii) their propensity to compete by courtship and aggres- sion at a given encounter (Figure 7; de Jong et al. 2012). Because a Seasonal trajectory of OSR change in OSR entails a change in density of one or both sexes, it in- Based on the criteria described above, we estimated the trajectory of evitably affects the frequency of encounters (e.g., Vincent et al. OSR over the better part of the breeding season, from late May until 1994) and could lead to OSR effects on the frequency of courtship Figure 7. How to measure mating competition: by frequencies of behaviors or propensities to behave? If OSR effects on courtship or aggression are measured by how often an act happens under various sex ratios, changes in encounter rate with opposite or same sex individuals will cause changes in numbers of courtship or agonistic acts even in the absence of any effect of sex ratio on individual behavior (the propensity for courtship and aggression at encounters). In this figure, the term competitor-to-resource ratio (CRR) is used instead of OSR in order to make the logic independent of sex of the actor [see de Jong et al. (2012) for further detail]. (A–D) Effects on courtship. With an increasing CRR (i.e., fewer potential mates), there will be fewer mate encounters (thin dashed lines). Even if this causes an increased propensity to court (A–C, thin lines), the frequency of courtship (bold lines) will decrease over the whole (A) or part (B, C) of the CRR range. In (D), we assume no effect of CRR on the propensity to court, in which case courtship frequency will decrease due to fewer encounters. (E–H) Effects of CRR on aggres- sion (agonistic behavior). With increasing CRR (i.e., more competitors), frequencies of same-sex encounters (thin dashed lines) will increase. Depending on how this affects the propensity to behave aggressively upon encounters, the result will be smaller or greater differences between trajectories for aggression propen- sity (thin lines) and frequency of aggression (bold lines). Trajectories could be qualitatively different over certain ranges of CCR (E–G), or over the full CRR range (H). Reprinted (A–D) from Figure 1 and (E–H) from Figure 2 in de Jong et al. (2012), Behavioral Ecology 23:1170–1177, by permission of Oxford University Press. Downloaded from https://academic.oup.com/cz/article-abstract/64/3/363/4986908 by Ed 'DeepDyve' Gillespie user on 21 June 2018 372 Current Zoology, 2018, Vol. 64, No. 3 or aggression without any true change in the propensity to court or Petroscirtes breviceps, Shibata and Kohda (2006) found sex roles to to be aggressive at encounters (Figure 7; de Jong et al. 2012). change from conventional at the start of the season to reversed at However, it is the propensity to act by courtship or aggression peak season and then back again to conventional in late season, (Figure 7) that is predicted to change with a changing OSR, and which they interpreted as a response to nest site limitation. In pea- which reflects mating competition. We therefore analyzed data on cock blennies Salaria pavo, Saraiva et al. (2012) found spatial vari- courtship and aggression by calculating the likelihood of these ation in courtship roles, with males more active in courtship than behaviors to happen at a given observed encounter (Forsgren et al. females in the Gulf of Trieste, and females more active in Southern 2004). Such a propensity-based approach to mating competition has Portugal. Male aggression toward females showed the same spatial obvious strengths but had only rarely before been used in analyses pattern, whereas intra-sexual aggression was far more prevalent in of OSR effects (but see Berglund 1994; Borg et al. 2002). males than females in both populations (Saraiva et al. 2012). Male– In both sexes, we found a dramatic change in the propensity to female aggression can be interpreted as a terminal form of courtship compete over the breeding season, in accordance with predictions when females are not responsive, as is commonly observed among from OSR theory (Figure 6b, c). The change was, as predicted, op- fishes. Saraiva et al. (2012) attributed the different courtship roles to posite in the 2 sexes, for both courtship and aggression. Males population differences in nest density, nest competition, and in par- showed a strong propensity to behave aggressively to other males, ticular to a higher prevalence of sneaker males in S Portugal. These and to court females, early in the season, but with a dramatic decline findings suggest that insights gained from the G. flavescens model for both behaviors as the OSR became more female-biased over the system may have wide-ranging relevance. We believe that this rele- season (Figure 6b, c). Females, on the other hand, were very little vance extends well beyond fishes, and may apply widely across ani- engaged in courtship and very rarely aggressive at the start of the mal taxa. season, but were eager to court and often behaved agonistically to other females when the OSR was female-biased toward the end of the breeding season (Figure 6b, c). In result, males were much more Sex Role Reversal: Just Season or a Causal eager to court and compete than females early in the season, where- Relationship with OSR? as females were much more eager to court and compete than males By their very nature, field studies of OSR variation and related late in the season. Thus, as predicted from OSR theory (Emlen and changes in mating competition (Forsgren et al. 2004; Myhre et al. Oring 1977; Kvarnemo and Ahnesjo ¨ 1996), our study of G. flaves- 2012) are correlational: what these studies established was a con- cens revealed a complete change in sex roles during a 3-month certed change in OSR and mating competition over the breeding sea- breeding season. The change was from conventional sex roles (pre- son. Exploring natural variation in the wild, such studies cannot dominant male mating competition) when the OSR was male-biased strictly establish conclusive causation even if observed patterns are at the start of the season, to reversed (predominant female mating highly suggestive of a causal relationship. Hypothetically, the competition) as the OSR became female-biased later in the season change in mating competition (courtship and aggression) over the (Figure 6; Forsgren et al. 2004). season could result from some other factor co-varying with time of Analyzing personality traits of G. flavescens in a later study season even if there are no obvious candidates for co-variates caus- (Magnhagen et al. 2014), we found that males studied late in the ing such effects. season behaved less boldly (in standardized personality tests) than those tested earlier in the season (Magnhagen et al. 2014). The re- An aquarium experiment that failed and what to learn duction in male boldness matches the near-absence of male–male from it competition late in the season. In substrate brooders like G. flaves- We performed 2 different experiments to test whether OSR per se cens, territory and nest defence may render males more vulnerable affects courtship and competition, first in small aquaria (de Jong to predators, and parental care may entail costs (energetically or by et al. 2009; Wacker et al. 2012) and later in bigger mesocosm tanks compromised immune-competence) that are either fatal or leave them out of the mating pool due to poor condition (Forsgren et al. (Wacker et al. 2013). Both experiments focused on effects of sex 2004; Wacker et al. 2014). ratio on competition behavior, as expressed by courtship and intra- sexual aggression. In the aquarium experiment, we compared a male-biased and a Setting the stage for the model system: OSR and female-biased OSR, at 2 densities (Table 1 in de Jong et al. 2009), dynamics of sexual selection by providing all males with a nest and using only ready-to-spawn The documented temporal dynamics of sex roles linked to OSR vari- females. The experiment was conducted in relatively small (60 L) ation entailed a unique potential of the model system for exploring aquaria with the males housed in the larger (40 L) part toward one sexual dynamics. This set the stage for much of our later work with end and females in the smaller (20 L) part at the other end, separated G. flavescens, in the field and in the laboratory. As it is becoming in- by a transparent acrylic divider. Recording the frequency of court- creasingly clear that sexual selection varies in time and space in ship by males to females, by females to males, and all instances of many species, understanding the underlying dynamics becomes in- intra-sexual interactions, we found overall little effect of sex ratio creasingly important. With mating competition today found to vary (or density) on frequencies of courtship or aggression by males or in relation to OSR in a range of species and taxa (Weir et al. 2011; females (Figures 1 and 3 in de Jong et al. 2009). However, the male de Jong et al. 2012), it is not unlikely that other vertebrates (and courtship frequency was significantly higher when the sex ratio was other animals) may have similarly dynamic sex roles as G. flavescens female-biased, seemingly opposite to expectation from OSR theory even if documented examples are few. Any system where, for one or another reason, the OSR changes dramatically over the season—or (Figure 1b in de Jong et al. 2009). between breeding seasons—may potentially display similar dynam- While these results at first sight contradict sex ratio regulation of ics. Reversals of courtship or sex roles have now been documented mating competition, in retrospect we realized limitations to the set- in at least 2 more fish species, both of the Blenniidae family. In up that we believe contributed to the negative results. First, the Downloaded from https://academic.oup.com/cz/article-abstract/64/3/363/4986908 by Ed 'DeepDyve' Gillespie user on 21 June 2018 Amundsen  Gobies as models in sexual selection 373 density of fish was clearly higher than occurring in mating situations (Figure 2 in Wacker et al. 2013). Notably, an even sex ratio of in the wild (personal observation), with unknown consequences for ready-to-mate individuals in this set-up implied a male-biased OSR, mating behavior. This is not unique to our study but is often the because each male could accommodate clutches from multiple case in laboratory studies of fish behavior. The high density may be females (as outlined above, see Forsgren et al. 2004). why only about 60% of the males took up a nest, despite there being However, even this set-up precluded a clear distinction between equally many nests as males (Wacker et al. 2012). Moreover, at very individual encounters, as the environment was relatively open in high densities, male aggression may be reduced, due to the cost of order to facilitate observation, with the consequence that fishes frequent aggressive encounters (Emlen and Oring 1977; Grant et al. could often see more than one other fish at a time. We were there- 1995; Weir et al. 2011). In the high-density situation of the test fore limited to recording frequencies of behaviors also for this de- aquaria, both males and females were continuously and simultan- sign. However, as the number of males was kept constant across eously exposed to multiple opposite-sex individuals (potential treatments, with OSR variation caused by varying the number of mates) and same-sex competitors, at both sex ratios and densities. females, male–male encounter rates would not be affected by OSR. Second, by the nature of the set-up, males and females were not In consequence, frequencies of male–male aggression would reflect allowed to complete interactive courtship and mate. This was inten- propensities to behave aggressively in this specific design. As pre- tional, but may have been experienced as constant mating rejection, dicted from OSR theory, we found a strong effect of OSR on the with unknown effects on the propensity to court. Finally, and per- male propensity to behave aggressively. Thus, the mesocosm experi- haps most importantly (de Jong et al. 2009, 2012), the set-up pre- ment provided support for a causal effect of OSR on sex role vented us from recording propensities to court and compete at a variation. given encounter. This was technically because we were unable to separate individual encounters as multiple individuals of each sex Female Ornamentation: Male Preference, Causes, were always in visual contact across the acrylic divider, and is a problem shared with most other aquarium tests of mating competi- and Dynamics tion (de Jong et al. 2012). More fundamentally, it was because sep- Until about 20 years ago, ornamentation (including coloration) in arate encounters with individual other fish do not occur in this type females had been little studied and largely overlooked, with the pre- of set-up: the fish fundamentally experienced one continuous en- dominant view being that conspicuous female traits were due to gen- counter with multiple con- and hetero-sexuals. Thus, the set-up only etic correlation (see Amundsen 2000a, 2000b). During recent allowed recording of frequencies of behaviors. As outlined above, decades, however, there has been an increasing recognition that fe- frequencies of competitive behaviors (courtship, aggression) by male extravaganza can be a result of male mate choice, female–fe- individuals may not always reflect their propensity to behave com- male competition, or other selection pressures (reviewed in, e.g., petitively when encountering a potential mate (courtship) or intra- Amundsen and Pa ¨ rn 2006; Kraaijeveld et al. 2007; Clutton-Brock sexual competitor (aggression) (Figure 7; de Jong et al. 2012). This 2009). Critical studies have, however, been hampered by female or- is particularly the case for courtship, for which frequencies of behav- namentation often being identical to or a “paler version” of male or- ior and propensities to behave would have different, and often namentation (e.g., Hill 1993; Amundsen et al. 1997). This has opposite, trajectories of response to OSR variation (Figure 7a–d; de rendered it impossible to entirely rule out genetic correlation which Jong et al. 2012). Reviewing experiments on OSR effects on court- has historically been the dominant interpretation of conspicuous ship in various species, we indeed found a marked difference traits in females (Darwin 1871; Lande 1981). However, female or- between frequency-based and propensity-based studies: frequency- namentation that differs from that of males of the same species based studies tend to produce results that at first sight appear oppos- occurs in several taxa. For example, sex-changing fishes often dis- ite to expectations from OSR theory; propensity-based studies tend play conspicuous coloration both as females and males, yet in very to support OSR theory (Figure 3 in de Jong et al. 2012). different ways (e.g., Michael 2001). Ornamental traits that are While one should always be cautious in discarding findings that unique to females offer the best test cases for sexual (and other) do not fit expectation, we retrospectively believe the set-up of this functions of showy female traits but have, unfortunately, been very experiment illustrates that seemingly well-designed experiments little studied. Thus, the ornamentation of G. flavescens, with both may fail to match in-the-wild-reality to an extent that leaves them sexes showy yet in very different ways (Figure 1, and Figure 1 in less informative. Amundsen and Forsgren 2001), offers an excellent opportunity for testing female ornament function (Amundsen and Forsgren 2001). Support for a causal effect of OSR on sex roles In order to perform a more realistic test for a causal relationship be- Male choice for female coloration tween OSR and mating competition, we manipulated sex ratio (and Today it is well established that male mate choice can occur under a density) in a mesocosm experiment, using large tanks of 500 and range of social and ecological conditions (Edward and Chapman 2000 L (Wacker et al. 2013). The density treatment was achieved by 2011). Prior to our work, several studies of birds had demonstrated having equally many fish in tanks of the 2 sizes; the sex ratio treat- a male preference for female ornamentation (Amundsen 2000b). In ment by keeping the number of males constant and varying the num- fishes, male mate choice for more fecund females has been docu- ber of females, resulting in a 2 densities  3 sex ratios design. The mented in a range of species (Sargent et al. 1986). Males of several main benefits of the design were that densities better mimicked the fishes also display strong preferences for conspecifics over heterospe- situation in the wild, and that, like in the wild, males and females cifics, which contributes in maintaining reproductive isolation (e.g., could engage in unconstrained mating behavior and could also Schlupp 2010; West and Kodric-Brown 2015; Roberts and mate. This experiment confirmed a causal effect of sex ratio (but not of density) on mating competition behavior: male engagement in Mendelson 2017). In sex-role reversed pipefishes, several studies courtship and aggression was higher with an even sex ratio than have found males to be choosier than females, preferring more orna- when the sex ratio was female-biased (moderately or strongly) mented females (e.g., Berglund et al. 1986a; Berglund and Downloaded from https://academic.oup.com/cz/article-abstract/64/3/363/4986908 by Ed 'DeepDyve' Gillespie user on 21 June 2018 374 Current Zoology, 2018, Vol. 64, No. 3 Figure 8. Female ornamentation in G. flavescens.(A) Female in “normal body coloration,” with colorful gonads (insert) visible through the skin. (B) Female aggre- gating dorsal and lateral melanophores to become near-transparent during courtship. (C) Belly skin biopsies (lateral to lateral) showing melanophore (brown– black) and erythrophore (orange–red) pigment cells maximally dilated (left) and maximally aggregated (right). (D) Belly coloration correlates strongly with gonad carotenoid content. Solid circles indicate females visually judged as “colorful”; open circles indicate females judged as “drab.” (E) Gonads have higher caroten- oid content late than early in the season. (F) Male preference for the more colorful female when given a choice between 2 females that differed in experimentally manipulated belly coloration, in terms of percent of time spent near the more colorful female (upper) and percent of displays directed at the more colorful female (lower). (A, B) Photos by T. Amundsen, gonad insert by P. A. Svensson. (C) Adapted, with permission, from Figure 2 in Svensson et al. (2005), Journal of Experimental Biology 208:4391–4397. (D) Reproduced from Figure 2 in Svensson et al. (2006), Functional Ecology 20:689–698, by permission of John Wiley and Sons. (E) Reproduced from Figure 3C in Svensson et al. (2009), Behavioral Ecology 20:346–353, by permission of Oxford University Press. (F) Reproduced from Figure 4 in Amundsen and Forsgren (2001), PNAS 98:13155–13160, copyright National Academy of Sciences. Rosenqvist 2001). In fishes with conventional sex roles, a male pref- coloration by experimental manipulation (Figure 8f; Amundsen and erence for female temporary colors that signal readiness to spawn Forsgren 2001). The latter experiment was important because it have been found in some species (e.g., Rowland et al. 1991; ruled out any confounding covariates and thus provided conclusive McLennan 1995) whereas little has been known about whether such evidence that males responded to female coloration as such. The preferences exist when coloration varies among mature females (but results indicated that the bright orange belly of G. flavescens could, see Beeching et al. 1998). at least in part, be due to sexual selection by male choice. In order to test if female coloration was subject to selection by In the years following our study, there has been an increased, if males, we conducted a dyadic aquarium test in which males were not extensive, interest in male choice in relation to female coloration offered a choice between 2 females that differed in belly coloration in fishes. Extant studies have mostly revealed a male preference for (more or less brightly orange) but that were closely matched in size female coloration (sockeye salmon Onchorhynchus nerka, Foote (Amundsen and Forsgren 2001, 2003). As is usual in such tests, the et al. 2004; lagoon gobies K. panizzae, Pizzolon et al. 2008; the cich- respondent male could see the females but not get into physical con- lid Pelvicachromis taeniatus, Baldauf et al. 2011). However, a study tact, in order to avoid a bias caused by the stimulus fish. The test of female coloration (red pelvic spines) in three-spined sticklebacks was performed late in the breeding season, but only for logistical G. aculeatus (Nordeide 2002) found males to prefer drab rather reasons, as we at that point were unaware of the seasonal dynamics than colorful females. In a population of the same species in which of sex roles and sexual selection in the species. We found a very females display red throat coloration, Wright et al. (2015) found no strong preference for the more colorful female, both in terms of time male preference for female throat coloration. Clearly, more studies in association and courtship displays (Figure 8f; Amundsen and are needed before we can conclude whether female coloration in Forsgren 2001). The same strong effect was found when giving fishes is generally subject to selection by male choice. Across taxa, males a choice between 2 females that differed in natural coloration, however, there is increasing evidence that male choice plays a part and when letting them choose between 2 females that differed in in female ornament evolution (Clutton-Brock 2007, 2009). Downloaded from https://academic.oup.com/cz/article-abstract/64/3/363/4986908 by Ed 'DeepDyve' Gillespie user on 21 June 2018 Amundsen  Gobies as models in sexual selection 375 Causes of female coloration belly color or egg carotenoids on egg development and hatching suc- The orange belly coloration of G. flavescens females results from 2 cess. We found, however, near-significant effects of belly coloration sources, pigmented eggs and epidermal red pigment cells (Figure 8a– and egg carotenoid content on the length of newly hatched larva c; Svensson et al. 2009a). The external visibility of the gonads is (Tables 2 and 3 in Svensson et al. 2006). Carotenoid content showed regulated by modulation of melanophore pigment cells: when the a marginally significant negative relationship with time until spawn- ing for the females (Svensson et al. 2006). Overall, these results sug- pigment is aggregated, the skin becomes more transparent and the gest some positive effect of belly color and egg carotenoid content gonads highly conspicuous. The fact that gonads are part of the sig- on reproduction, but the results should be treated with caution due nal makes the dynamics and signaling potential different from to the many tests performed. At the same time, the overall high de- “ordinary” ornaments that are usually external, without any physio- velopmental and hatching success of eggs (Svensson et al. 2006), logical or reproductive function. In G. flavescens, like in other ani- with consequent limited individual variation, may have given tests mals, the obvious candidate to cause egg pigmentation was on these 2 parameters low power in detecting effects with the sample carotenoids. Using HPLC, we documented a high content of astax- sizes of the study. anthin, a carotenoid typical of marine fishes, in the eggs (Svensson Across species, the evidence for female ornaments to signal off- 2006). We also found significant concentrations of idoxanthin and adonixanthin, which are both metabolites of astaxanthin (Svensson spring quality is equivocal (Amundsen and Pa ¨ rn 2006; Nordeide 2006). The total egg carotenoid concentration correlated strongly et al. 2013), and suggest complex and variable relationships between with female belly coloration as quantified from digital images, using female ornamentation and offspring production. In G. aculeatus, the CIE L*a*b* color system (Figure 8d; Svensson et al. 2006). females with redder pelvic spines had less carotenoids in their eggs, Importantly, image analyses confirmed that females visually judged suggesting a trade-off between ornaments and offspring (Nordeide to be colorful and drab, respectively, differed very significantly in et al. 2006). A similar negative correlation between skin redness and measured belly coloration (Table 1 in Svensson et al. 2006) and also egg carotenoid content has also been found in trout Salmo trutta in carotenoid content (Table 2 in Svensson et al. 2006). Thus, these (Wilkins et al. 2017), and female ornamentation has been found to analyses validated the visual judgment of coloration applied in the decrease offspring viability in Arctic charr Salvelinus alpinus mate choice experiments (Figure 8f; Amundsen and Forsgren 2001, (Janhunen et al. 2011). Nordeide et al. (2013) have suggested that 2003). red female spines in G. aculeatus are due to genetic correlation, with Gobiusculus flavescens, like other animals, cannot synthesize antagonistic selection (Arnqvist and Rowe 2002) on the use of caro- carotenoids but get them from prey organisms, in the case of tenoids by the 2 sexes. Indeed, a recent genetic study has revealed G. flavescens mostly from calanoid copepods (Berg 1979). that loci coding for red coloration in G. aculeatus are located at the Astaxanthin is an antioxidant of potential value during the fragile same place in the genome in males and females (Yong et al. 2016). phase of pre- and post-hatching larval development (e.g., Blount Such conflicting selection on males and females is, however, not et al. 2000). Carotenoids may also positively affect immune function relevant for G. flavescens, where carotenoid-based ornamentation is and thus health [Blount et al. (2003), see Blount (2004) and a uniquely female trait. Svensson and Wong (2011) for reviews on carotenoid function]. At the same time, astaxanthin could act as an antioxidant in the adult Dynamics and regulation of female coloration female, and be used to form red pigment cells in the skin. This makes In animals where coloration stems from dermal pigment cells (e.g., for a complex trade-off in the allocation of carotenoids, between fishes, amphibians, and decapods), individuals can modify their color own use as an antioxidant, deposition in developing eggs for anti- by dilation or aggregation of chromatophore pigments (e.g., oxidant or immune function, or deposition in skin pigment cells Aspengren et al. 2009; Stuart-Fox and Moussalli 2008, 2009; Sko ¨ld (Svensson 2006; Svensson and Wong 2011). Compared with other et al. 2013, 2016). Such color change can have signaling as well as species studied for male choice in relation to female coloration, G. protective functions (e.g., Stuart-Fox and Moussalli 2008; Stuart-Fox flavescens is unique in directly displaying its egg quality during et al. 2008; Olsson et al. 2017). Gobiusculus flavescens females have courtship, while at the same time modulating belly coloration by high densities of dorsal and lateral melanophores that account for means of skin pigmentation and chromatophore regulation their baseline brownish body coloration. Melanophore density is, (Svensson et al. 2005; Sko ¨ ld et al. 2008). however, much less in the belly region, which is therefore more trans- parent (Figure 8c). By contrast, the belly has red erythrophore pig- Benefits of female coloration ment cells that are absent from other body parts (Figure 8c). Thus, G. From a male perspective, mating with a “more orange” female may flavescens has the potential to modulate the “darkness” and transpar- provide direct benefits in terms of fertilizing high-quality eggs ency of their body as well as their degree of redness by means of pig- (Blount et al. 2000) and indirect benefits if egg and skin carotenoids ment cell regulation (Svensson et al. 2005; Sko ¨ ld et al. 2008). signal a high genetic quality. Because of the inability of animals to Given the strong female mating competition late in the breeding synthesize carotenoids, carotenoid-based ornaments have been sug- season (Figure 6; Forsgren et al. 2004) and the male preference for gested to act as honest quality indicators (Hill 1991; Lozano 1994). colorful females when female competition is at its strongest This idea has gained considerable empirical support, but the evi- (Figure 8f; Amundsen and Forsgren 2001), one would expect female dence is not unequivocal and the functions and dynamics of carote- visual signaling to be particularly important late in the season. noids in animals are clearly more complex than initially suggested Svensson et al. (2009a) found belly coloration (as expressed by a* in (Olson and Owens 1998; Svensson and Wong 2011; Royle et al. the CIE L*a*b* color space) to be dynamically regulated by gonad 2015). pigmentation (fixed) and 2 dynamic aspects of skin coloration: the Analyzing for relationships between natural belly coloration, ca- redness of the belly, and the degree of skin transparency (causing rotenoid content, and measures of reproductive success in G. flaves- variation in the degree to which the colorful gonads are visible cens, we found that colorful females produced significantly larger through the skin) (Figure 4 in Svensson et al. 2009a). Comparing clutches than drab ones, but did not find any significant effect of coloration of mature females between early and late season, we Downloaded from https://academic.oup.com/cz/article-abstract/64/3/363/4986908 by Ed 'DeepDyve' Gillespie user on 21 June 2018 376 Current Zoology, 2018, Vol. 64, No. 3 found that females sampled late in the season had indeed more mature females (Massironi et al. 2005). The colorful belly is actively colorful bellies. This was partly due to more colorful gonads with a displayed to males during courtship, appears to be a signal of female higher carotenoid content (Figure 8e), but also to more red- quality (Massironi et al. 2005), and males prefer more colorful pigmented, and at the same time more transparent, bellies late in the females, like in G. flavescens (Pizzolon et al. 2008). However, con- season (Figure 3 in Svensson et al. 2009a). Analyzing belly color- trary to G. flavescens, the yellow belly of K. panizzae is solely ation across stages of egg maturation (“roundness” of females), we caused by skin pigmentation, with gonads basically unpigmented confirmed that belly coloration increased as the females matured, (Massironi et al. 2005). The apparent similarity of the 2 systems raises the intriguing question of what evolved first in G. flavescens: due to a combination of more colorful gonads and higher skin trans- the male preference for colorful females, or the belly transparency parency. At the same time, female belly coloration was highly vari- that makes gonad coloration externally visible to males. The “sand able among fully mature females (Figure 2 in Svensson et al. 2009b), goby group,” including K. panizzae and G. flavescens, share com- showing that female coloration in G. flavescens is not merely a sig- mon ancestry about 5 million years ago (Huyse et al. 2004). nal of readiness to spawn. To our knowledge, the work on G. flavescens is the only detailed When females court males, as occurs frequently late in the season (Forsgren et al. 2004), and often involve multiple females (Myhre exploration of a female ornament that is, at least partly, a display of et al. 2012), they typically become more transparent and enter a pigmented gonads. However, gonads are visible through the skin in state which we informally denote “the glow” (Figure 8b, and Figure a range of fishes including several gobies and wrasses (Fam. 1d in Sko ¨ ld et al. 2008). This effect is due to aggregation of dermal Labridae, Baird 1988), potentially with similar functions and dy- pigment cells (Svensson et al. 2005). We explored the hormonal namics as revealed in G. flavescens. regulation of pigment cells by exposing skin biopsies to hormonal Taken together, our studies of female ornamentation in G. fla- treatments. When all pigment cells in skin biopsies were aggregated vescens have documented a strong male preference for more colorful by noradrenaline treatment, the skin got more transparent and at females, a complex causation and regulation of the female ornament the same time less colorful (Figure 2 in Svensson et al. 2005). which clearly reveals a signaling function, and potential, yet so far tentative, benefits from coloration. However, more work is clearly However, during the glow, it appears that the basal-body dark-pig- needed to fully understand this complex type of female sexual mented melanophore cells are “turned off” (pigment aggregated) signaling. whereas the red-pigmented erythrophore cells in the belly are at the same time “turned on” (pigment dilated). By exposing skin biopsies to a range of hormones (and hormone blends) present in fish, we Mate Sampling, Mate Competition, and Mate found no single hormone to cause simultaneous aggregation of mel- Choice anophores and dilation of erythrophores. However, a combination of melatonin and melanocyte-stimulating hormone (MSH) caused While mate preferences and mate choice by females have been exten- the skin to get more transparent (melanophore aggregation) while at sively explored in animals, we know surprisingly little about how fe- the same time becoming more red (erythrophore dilation) (Figures 2 male animals behave during mate search, and how many potential and 3 in Sko ¨ ld et al. 2008). This is the effect observed during “the mates they visit before mating (Amundsen 2003). Much of extant glow,” suggesting that female belly coloration during display is work, theoretically and empirically, have focused on sampling tac- modulated by a blend of hormones. Thus, pigment cell modulation tics and decision rules. An initial focus was whether females would affects the degree to which the colorful gonads are visible through employ a best-of-n or threshold criterion tactic (Janetos 1980; Real the skin, but may also add “extra redness” to the effect of gonads on 1990; Gibson and Langen 1996). Empirical work on birds have belly coloration. Notably, we found no effect of sex steroids largely supported best-of-n models, as females have often been (T, 11kT, E2) on pigment regulation (Table 1 in Sko ¨ ld et al. 2008). found to revisit and mate with previously visited males. By contrast, The complex and dynamic interaction between skin transparency fishes may appear to more often employ a threshold tactic, mating and egg pigmentation in producing a colorful belly appears unique with the last male visited (Amundsen 2003). Later, more sophisti- to G. flavescens among its Nordic goby relatives (Svensson et al. cated, models that take information theory into account have 2009a). Comparing belly coloration, egg carotenoids, and skin painted a more complex yet probably more realistic picture (Luttbeg transparency among 6 goby species (including G. flavescens) that 2002; Wiegmann et al. 2010a, 2010b; Castellano and Cermelli occur in the same area (Figure 1 in Svensson et al. 2009a), we found 2011). Most of what we know empirically about mate sampling G. flavescens to be the only to have a strongly colored belly, despite stems from a relatively small number of studies of birds (e.g., Dale that 2 other species (P. microps and G. niger) had significant (yet et al. 1990; Fiske and Ka ˚la ˚ s 1995; Dakin and Montgomerie 2014) more variable) concentrations of gonad carotenoids. These 2 spe- and fishes (e.g., Gronell 1989; Forsgren 1997b; Fagundes et al. cies, however, had much less transparent skin (Figure 2 in Svensson 2007). A general insight from these studies is that females typically et al. 2009a). What made G. flavescens stand out as conspicuously sample quite few males (median numbers often  5), but with exten- colorful was the combination of highly transparent skin and consist- sive individual variation. ently high gonad carotenoid concentrations (Figure 3 in Svensson The strength of sexual selection by mate choice can be affected et al. 2009a). By contrast, females of other goby species inhabiting by the number of potential partners that are sampled before a mat- the same waters were largely camouflaged, either by gray–brown ing decision is made (Jennions and Petrie 1997; Benton and Evans patterns that blend in with the substrate (benthic species) or by 1998). From the mate-searching individual’s perspective, the likeli- transparency in the case of the pelagic Aphia minuta (Svensson et al. hood of encountering a high-quality mate increases as more poten- 2009a). tial mates are sampled, yet in a deceleration function (Real 1990). The closely related goby K. panizzae displays a female ornament However, rejecting a potential partner to continue searching may that may shed light on the evolution of the belly color signaling sys- entail costs, in terms of time and energy, but also in lost mating tem in G. flavescens. As gonads mature, K. panizzae females develop opportunities (Real 1990). The latter cost is because the highest an increasingly colorful yellow belly that is also variable among quality male in a sampled set is increasingly likely to get mated with Downloaded from https://academic.oup.com/cz/article-abstract/64/3/363/4986908 by Ed 'DeepDyve' Gillespie user on 21 June 2018 Amundsen  Gobies as models in sexual selection 377 time spent searching. If potential mates are in short supply, a search- females (Lindstro ¨ m and Lehtonen 2013). It may be noteworthy that ing individual may remain unmated if it rejects one or more mating the median number of males sampled by G. flavescens (Myhre et al. options. Mate search and mating decisions become increasingly 2012) was higher than in most, if not all, other studies of mate complex and dynamic when both sexes may execute mate choice search in fishes or other taxa. (Bergstrom and Real 2000), as in G. flavescens (Myhre et al. 2012). Competition and choice during mate sampling Number of males sampled During a G. flavescens female visit to a male, some sort of courtship In species with male territoriality, like G. flavescens, mate search is interaction would usually occur, even if most visits do not lead to primarily conducted by females, who may visit a number of males mating. These interactions could be initiated by either the female or before mating. In G. flavescens, the opportunity cost of extensive the male, and likewise be terminated by either the male or the female sampling is likely small early in the season, due to the male-biased if mating does not commence. The potential for males, and not only OSR, with an abundance of mating-ready males. By contrast, the females, to assess and reject potential partners during mate sampling opportunity cost may be significantly late in the season, when a had, to our knowledge, not previously been empirically investigated. strongly female-biased OSR implies that mating-ready males are Instead, extant work on mate sampling has generally made the im- scarce. A female G. flavescens that rejects a mating opportunity in plicit assumption that visits without mating would be due to rejec- late season could easily find herself without a nest in which she tion on the part of the female. This is likely to be mostly true in could spawn her eggs and have them cared for. Thus, we predicted many species, but is less likely in mutual choice systems. females to sample fewer males late than early in the season. This The sex experiencing the strongest mating competition would be was exactly the pattern found: females visited on average about 3 expected to initiate courtship more often. In G. flavescens, this times as many males early as they did late in the season (Figure 9a; would be males early in the season and females late in the season Myhre et al. 2012). A negative effect of female competition on the (Forsgren et al. 2004). As predicted, we found males to initiate the extent of mate sampling has also been found in the pied flycatcher majority of courtship interactions early in the season, whereas al- Ficedula hypoleuca (Dale et al. 1992). Moreover, a recent experi- most all courtship interactions were initiated by females late in the ment on sand gobies P. minutus, simulating female mate sampling in season (Figure 9b; Myhre et al. 2012). the laboratory, found that a low male density and the presence of Termination of courtship, on the other hand, would represent re- potential female competitors reduced the time until mating for jection of a potential mate. The more-choosy sex should therefore Figure 9. Effects of time of season on female mate search behavior in G. flavescens. “Early” refers to late April and May (male-biased OSR), “Late” to mid-June to mid-July (female-biased OSR). Females from one locality were captured, individually marked, released at another locality, and followed during mate search [see Myhre et al. (2012) for details]. (A) Number of males visited (sampled) during 30 min observations of mate-searching females, (B) female propensity to initi- ate courtship, (C) female propensity to terminate courtship, (D) frequency of multi-female courtship, and (E) likelihood of male courtship in relation to the number of simultaneously courting females. About 30% of the females mated during 30 min of observation. (A–D) Open boxes represent females that did not mate during observation, shaded boxes those that mated during observation. Reproduced (A) from Figure 2, (B–C) from Figure 1 and (D–E) from Figure 2 in Myhre et al. (2012), American Naturalist 179:741–755, with permission of University of Chicago Press. Downloaded from https://academic.oup.com/cz/article-abstract/64/3/363/4986908 by Ed 'DeepDyve' Gillespie user on 21 June 2018 378 Current Zoology, 2018, Vol. 64, No. 3 terminate courtship interactions more often. As for courtship initi- for larger mates. Using a standard 2-stimulus mate choice set-up, ation, we found a strong effect of time of season on which sex termi- with a response (choosing) female seeing a small male at one end of nated courtship most often. Late in the season, females almost never the aquarium and a large male at the other end, we found females to terminated courtship, whereas females were more likely than males express a clear preference for larger males early in the breeding sea- to terminate courtship early in the season (Figure 9c; Myhre et al. son. Late in the season, however, females appeared indiscriminate 2012). These patterns indicate that females become indiscriminate with respect to male size (Figure 10; Borg et al. 2006). This finding with whom to mate when female competition is strong late in the is as predicted from OSR theory, as mature females in the wild have season. At this time, about 75% of courtship events included 2 or an abundance of males to choose from early in the season, but a more competing females (Figure 9d). A similar seasonal pattern has shortage of potential mates late in the season (Forsgren et al. 2004). been found in the stickleback G. aculeatus, where “by the end of the Moreover, mating with a large male may confer greater benefits to breeding season, any male with a nest was seldom found without females when competition is strong early in the season, because a several females courting him” (Kynard 1978). Female–female com- large male is less likely to have his nest overtaken and the brood petition during courtship may negatively impact the likelihood of lost. Like in many other species, larger-bodied G. flavescens males mating to a point where the male refrains from courting the females, are more likely to keep a nest under competition (Figure 2 in as invariably happened when >5 females simultaneously courted a Wacker et al. 2012). It is noteworthy that test females experienced male (Figure 9e; Myhre et al. 2012). Taken together, female mate no sex ratio treatment in the laboratory. Thus, their change in pref- search behavior changed over the season as predicted from sex role erence with season must either reflect an ontogenetic change in pref- theory, with much fewer males sampled, a strong increase in eager- erences or “memorized” recent experience of mating competition ness to court, and very infrequent mate rejection, late in the season. from the wild (Borg et al. 2006). In contrast to females, we found no or at most a weak preference for female size among males, using the same type of experimental Mate Choice and Body Size in Males and Females set-up (Pe ´ labon et al. 2003). These results were obtained studying Body size is closely related to fitness across the animal kingdom apparently healthy males (Pe ´ labon et al. 2003); an absence of prefer- (e.g., Blanckenhorn 2000). A large body size may reflect both genet- ence for larger females was also found among males infected by ic and phenotypic quality in both sexes. In the sex experiencing the microsporidian parasites (Pe ´ labon et al. 2005, the nature of infec- strongest mating competition, a large body size (either skeletally or tion shown in Figure 14). Both these studies were conducted during in terms of condition) would usually imply a high resource-holding the latter half of the breeding season (late June to mid-July), at a potential (RHP, Parker 1974), giving large-bodied individuals an ad- time when gravid females would be present in excess and males thus vantage in gaining and maintaining resources including territories, have an opportunity to be choosy (Forsgren et al. 2004). While the lack of a clear male preference for female size may appear in con- nests, and mates (e.g., Magnhagen and Kvarnemo 1989). Such an trast to theory, a further investigation suggests that fitness benefits advantage would have significant fitness consequences in the sex to a male from choosing larger females (i.e., rejecting smaller ones) experiencing the strongest mating competition. As outlined above, would be small, for a number of reasons. First, variation in female this is usually but not always males. In females, a large body size is size is relatively modest, reflecting the fact that G. flavescens is an often indicative of high fecundity (e.g., Honek 1993; Koops et al. annual species for which individual age would vary by only a few 2004; Harding et al. 2007). Variation in RHP and fecundity is par- months (all having hatched during May–July the previous year). ticularly extensive in taxa with indeterminate growth. In such organ- Thus, in G. flavescens, a male that rejects a relatively small female isms, including fishes (e.g., Fleming 1996; Marteinsdottir and Begg would be unlikely to soon be visited by a much larger one (Pe ´ labon 2002), the largest adult individuals may be several times larger than et al. 2003). Second, variation in fecundity was less strongly related the smallest ones, with the greatest variation displayed by long-lived species. This creates large contrasts in size among intra-sexual com- petitors as well as among potential mates, for both sexes. In males of many species, intra-sexual competition and female mate choice reinforce each other in causing positive selection on body size (Hunt et al. 2009). Effects of partner size As predicted from the extensive size variation in fishes, larger- bodied males are often preferred by females (e.g., Bisazza and Marconato 1988) and males (e.g., Co ˆte ´ and Hunte 1989; Wong and Jennions 2003). In monogamous fishes, mutual mate choice for size can lead to assortative mating (Rueger et al. 2016). However, actual mate choice may be affected by factors (e.g., parental quality, Forsgren 1997a) that could interact with or override size (e.g., Qvarnstro ¨ m and Forsgren 1998; Wong and Candolin 2005). It is far from given that males or females would always express a preference for large-bodied partners, either because other factors are more im- portant or because of a cost to choosiness (e.g., Wong and Jennions Figure 10. Female preference for large males in relation to time of season, in 2003), for instance when mating competition is strong. G. flavescens. The figure shows the proportions of females that responded The dynamics of the G. flavescens system makes it possible to more strongly to large and small males, respectively, in a 2-choice aquarium analyze how mate choice relates to mating competition regime. set-up. Reprinted, with permission from Elsevier, from Figure 1 in Borg et al. Interestingly, we found females but not males to display a preference (2006), Animal Behaviour 72:763–771. Downloaded from https://academic.oup.com/cz/article-abstract/64/3/363/4986908 by Ed 'DeepDyve' Gillespie user on 21 June 2018 Amundsen  Gobies as models in sexual selection 379 to female size than in many other fishes (female body length explain- from Bateman’s (1948) classical analysis of variance in reproductive ing only 37% of variation in fecundity, Figure 1 in Pe ´ labon et al. success, showing that variation in number of mates had a stronger 2003). This would further reduce the likelihood that rejecting a effect on reproductive success in males than in females. This insight, small female to later mate with a larger one would provide a signifi- which is broadly applicable (Trivers 1972), was the basis for the cant fecundity benefit. Finally, while late-season males have the op- “Bateman gradient”(b )(Andersson and Iwasa 1996) and the ss portunity to exert choice of partners, it is not obvious that the “opportunity for selection (I),” and “opportunity for sexual selec- benefit from doing so would always outweigh its costs. With an tion (I )” indices (Wade and Arnold 1980). The Bateman gradient abundance of gravid females willing to mate late in the season, a (b ) quantifies the degree to which increased mating success is asso- ss male first accepting to mate with a less-than-average fecund females ciated with reproductive success. The opportunity for selection (I)is could likely mate again with another female shortly afterward, and simply an index reflecting variation in reproductive success; the op- so on until his nest was full, which might take less than a day. Thus, portunity for sexual selection (I ) is the equivalent index reflecting the benefit of choosing larger females for mating (i.e., rejecting small variation in mating success. The opportunity for selection indices ex- ones) would likely have been negligible even if there had been press the maximum potential strength of selection given a certain greater variation in female size and a more consistent relationship variance in mating and reproductive success. These indices say noth- between female size and fecundity than found in our study (Pe ´ labon ing about which traits may be sexually selected but quantifies the et al. 2003). This situation may be different from that of choice for potential for phenotypic selection. colorful females, where the male could gain an egg quality benefit By contrast, realized sexual selection expresses the degree to from choice, and not just more eggs. which specific traits (e.g., body size or ornaments) are subject to sex- ual selection in a given system. Such selection is typically analyzed by standardized selection differentials (s ) and selection gradients Effects of own size (b )(Lande and Arnold 1983), describing the strength of selection The size of the choosing individual may affect its choosiness. This on the phenotypic trait in question. This approach relies on the iden- could for instance happen when there is mutual mate choice (e.g., Jones and Hunter 1993) for size. In such a case, smaller males may tification and measurement of phenotypic traits potentially subject face fewer mating options and have less scope for choice (e.g., Foote to sexual selection. 1988). When competition is strong, low-quality individuals may There has been extensive debate on how best to measure sexual benefit from either relaxing their choosiness or reversing their pref- selection during the last decade, in parallel with our work on the erence in favor of less-sought-after mates (Fawcett and Johnstone G. flavescens system. The debate has centered on a number of 2003). topics, including: (i) the value of indices of potential selection vs. Having established that male G. flavescens overall prefer more realized selection for understanding sexual selection (Jones 2009; ornamented females late in the season (Amundsen and Forsgren Krakauer et al. 2011; Jennions et al. 2012; Henshaw et al. 2016; Janicke and Morrow 2018), (ii) the effects of mate monopolization 2001), we explored whether variation in preference was related to and random mating on the relationship between OSR and sexual se- the choosing male’s own size. That turned out to be the case, with lection indices (Sutherland 1985; Klug et al. 2010a; Jennions et al. the smallest males seemingly indiscriminate with respect to female 2012), (iii) which index of selection best predicts total sexual selec- coloration despite showing a clear eagerness to mate (Figure 3 in tion in nature (Mills et al. 2007; Jones 2009; Fitze and Le Galliard Amundsen and Forsgren 2003). We interpreted this to suggest that 2011; Henshaw et al. 2016), and (iv) which individuals to include in small males could not afford to be choosy, as they would be less at- real-world measures of sexual selection (Klug et al. 2010b). tractive to females and/or loose out in contest competition over nests Elaborating on this complex yet important debate is beyond the (Amundsen and Forsgren 2003). This argument likely applies in the first part of the breeding season, as large males are generally com- scope of the present paper. petitively superior in G. flavescens (Figure 2 in Wacker et al. 2012). However, in our work on sexual selection in G. flavescens,we It is less likely that small males would be discriminated against have adopted a complementary approach, analyzing both the poten- (Borg et al. 2006) or face strong nest competition (Wacker et al. tial for selection and realized phenotypic selection on specific traits. 2014) late in the season, when the study was conducted. Taken at In one of the studies, we also analyzed how I and I related to 2 face value, the result may suggest that small males are selected to be other indices of sexual selection: the Bateman gradient (b ) ss indiscriminate in response to strong male–male competition early in (Andersson and Iwasa 1996) and the maximum standardized sexual the season, and that this preference is not plastic and hence main- selection differential (s )(Jones 2009). An advantage of the po- max tained also when it entails no benefit late in the season. However, tential for selection approach (e.g., I and I ) is that it neither requires the study included relatively few small (indiscriminate) males measurement nor knowledge of the particular traits targeted by sex- (Amundsen and Forsgren 2003), which call for caution in ual selection, which makes it relatively easy to measure in many sys- interpretations. tems (including ones where realized selection on particular traits is hard to measure). However, variance-based approaches only pro- vide an upper limit to the strength of sexual selection (potential se- OSR, Mating Competition, and Sexual Selection lection), but do not answer how much of the potential selection is realized in selection on phenotypic traits. Ultimately, understanding Mating competition and mate choice are the major processes caus- ing sexual selection (Andersson 1994). However, the 2 processes how animal phenotypes (morphology and behavior) are shaped by only lead to sexual selection if success is related to heritable pheno- sexual selection requires knowledge of which traits are targeted by typic traits. Sexual selection on a trait requires the trait to explain a selection, and the nature of selection on these traits. Such knowledge significant component of fitness variation among individuals, with either requires successful a priori assumptions on which traits are certain trait values associated with higher fitness. Phenotypic sexual under selection, or systematic exploration of candidate traits. When selection can be analyzed at 2 levels: as potential selection or as real- animals have multiple traits that could be under sexual selection, as ized selection. The “potential for selection” approach originated is usually the case (e.g., in G. flavescens), such exploration is Downloaded from https://academic.oup.com/cz/article-abstract/64/3/363/4986908 by Ed 'DeepDyve' Gillespie user on 21 June 2018 380 Current Zoology, 2018, Vol. 64, No. 3 required. Thus, it may be harder to detect significant realized sexual up over the 6-day course of the experiment, the male-bias was some- what reduced but far from eliminated. selection on a specific trait than to document a significant potential Notably, variation in mating success inevitably increases with a for sexual selection. This practical limitation to analyses of realized change from a female-biased to a male-biased sex ratio even in the selection is exacerbated by the fact that measures of sexually absence of any selection (i.e., with random mating), for purely nu- selected traits often have significant measurement error, and more merical reasons (Jennions et al. 2012). This has been overlooked in so than measures of mating or reproductive success. Thus, the 2 much work on OSR in relation to selection. When nest size puts a approaches to measuring sexual selection have different strengths limit to mating success, as in G. flavescens, this limitation will affect and limitations and cannot replace each other (Wacker 2013). We variation in mating success both under random mating and strong therefore recommend a combination of the 2 approaches whenever selection. Thus, we simulated the lower bound of I (which we feasible. termed I ) under random mating, taking nest size into account, min and assuming that a maximum of 4 females could spawn in an aver- Measuring sexual selection by experiment age nest (Forsgren et al. 2004). However, sexual selection may also In the mesocosm sex ratio experiment outlined above (“Support for have an upper bound, either because of nest-space limitations, or be- cause care is mildly depreciable (Clutton-Brock 1991) at large brood a causal effect of OSR on sex roles,” Wacker et al. 2013), we investi- sizes and leads to an optimal brood size beyond which males should gated both the potential for selection and realized (actual) selection refrain from attracting additional mates. Care of excessively large on a range of male traits. We analyzed the potential for sexual selec- broods can be depreciable, if increases in brood size above some tion by estimating the opportunity for selection (I) based on repro- level constrains efficient ventilation (by fanning), cleaning, and de- ductive success measured as number of eggs in the nest (Wacker fence against predators. In G. flavescens, nest size may considerably et al. 2013). The breeding success (and consequent fitness) of a male affect the upper bound to potential sexual selection, as no male is the product of number of mates, fecundity of these mates, and sur- could have more mates than required to fill up his nest. The upper vival of eggs and later larvae. While larval survival is generally in- bound to potential selection (I ) was simulated based on maximal max tractable in G. flavescens, egg survival is somewhat affected by filial mate monopolization (i.e., when no female would spawn in an cannibalism (Bjelvenmark and Forsgren 2003) but hatching success empty nest until all nests with eggs were full). We found the oppor- of eggs present at hatching time is very high (>90%; Table 2 in tunity for selection (I) in our experiment to be significantly greater Svensson et al. 2006). The number of eggs in a nest correlated than it would be under random mating (I ), whereas it was close min strongly with number of mates (Figure 2 in Mobley et al. 2009)ina to and not significantly different from the upper theoretical bound field study, implying that the number of eggs in a male’s nest largely to selection (I ) in the study system (Figure 11b; Wacker et al. max reflects his success in attracting many and fecund partners under the 2013). These results reveal a strong potential for sexual selection. constraint of limited nest size (Table 2 in Mobley et al. 2009). Thus, The most parsimonious interpretation of the results would be that we interpreted the opportunity for selection I, as measured from there is phenotypic selection for male traits that promote high mating numbers of eggs in the nests, to reflect sexual selection. We found I success (e.g., body size, ornamentation, and courtship). However, in to be significantly greater with an even than with a female-biased substrate brooders like G. flavescens, mate monopolization (and con- sex ratio (Wacker et al. 2013). Notably, an even sex ratio in this ex- sequent high I) can theoretically occur even in the absence of pheno- periment implied a male-biased OSR, as each male could cater for typic selection (Wacker et al. 2013). This could happen because eggs from several females in their nest. As nests were gradually filled females of several substrate-brooding species (e.g., Unger and Sargent Figure 11. Sexual selection in male G. flavescens.(A) Male morphological (upper) and ornamental (lower) traits potentially subject to sexual selection. (B) Measured opportunity (I, open circles and line) for selection in relation to theoretical minimum (filled triangles) and maximum (filled circles) opportunity for selec- tion under different sex ratios. Measured opportunity is closer to the upper than the lower theoretical bound. Note that a change in sex ratio by itself increases the opportunity for selection. The shaded areas are outside the theoretical bound for sex ratio effects on opportunity for selection in the model system [see Wacker et al. (2013) for further explanation]. Reproduced, by permission of John Wiley and Sons, from (A) Figure 1 and (B) Figure 3 in Wacker et al. (2013), Evolution 67:1937–1949. Downloaded from https://academic.oup.com/cz/article-abstract/64/3/363/4986908 by Ed 'DeepDyve' Gillespie user on 21 June 2018 Amundsen  Gobies as models in sexual selection 381 1988), including the goby P. minutus (Forsgren et al. 1996a), seeming- breeding season (Forsgren et al. 2004; Myhre et al. 2012; Wacker ly prefer to spawn in nests where there are eggs from before. This et al. 2014). Thus, they validate our claim that the seasonal dynam- could be a form of mate choice copying (“parasitizing” mate assess- ics of mating competition are causally related to the change in OSR, ment by previous mates), or in order to reduce the risk of egg canni- and not caused by some unknown factor that happens to vary in balism by the male. An experiment on P. minutus provided support concert with OSR over the the breeding season. for the cannibalism hypothesis but not for mate choice copying Methodologically, the study provided a novel and strong way to (Forsgren et al. 1996a). It is unknown whether female G. flavescens test for true OSR effects on the potential for sexual selection, by comparing the slope of the opportunity for selection (I) in response have a preference for males with eggs in their nest. However, it is not to OSR variation, with the slope simulated from random mating. It unlikely that such a preference exists and may explain part of the vari- also represents a new way of exploring the opportunity for selection, ation in mating success. by simulating its upper and lower bounds and testing how experi- More importantly, the study showed that the opportunity for se- mental results relate to these. The study is likely unique in testing lection changed more steeply in response to OSR than it would have effects of OSR variation at 3 different levels in the same experiment: done with random mating (significantly different slopes, Figure 11b) (i) mating competition behavior, (ii) the potential for selection, and (Wacker et al. 2013). This conclusively documents a true effect of OSR on the opportunity for sexual selection. Our approach to (iii) realized selection on phenotypic traits. This comprehensive ap- analyzing how OSR affects sexual selection is novel, and overcomes proach is obviously labor-intensive, but provides a coherent and the “problem” that estimates of I change with OSR even in the ab- integrated picture of how OSR affects sexual selection that could sence of selection (Jennions et al. 2012). We encourage future stud- not otherwise be achieved. ies of OSR and sexual selection to similarly test whether the slope of In the wild, our findings would suggest that male ornamentation measured variation in success (I) is greater than the slope simulated and body build is subject to selection early but not late in the season, from random mating. Doing so would provide stronger tests of OSR in line with a male-biased OSR and a stronger potential for sexual effects on the potential for sexual selection than in most extant stud- selection early in the season. Such temporal variation in selection regimes would weaken overall sexual selection on males, and could ies, and could significantly improve our understanding. Notably, even contribute in maintaining variation in male traits, for instance OSR effects on the slope of I cannot be explained by a preference as a result of opposing selection on male size between early and late for males with eggs. season. In G. flavescens, obvious candidates for sexual selection in males are the iridescent-blue lateral spots, and also the colorful dorsal fins (Figures 1 and 11a). These extravagant traits are conspicuously dis- Sexual selection in the wild played both in courtship and in male–male interactions (Amundsen Sexual selection is harder to detect in the field than in experimental and Forsgren 2001; Forsgren et al. 2004). Another obvious candi- laboratory set-ups, due to the many factors that affect mating suc- date is body size and related bodily traits (Figure 11a). Male size is cess in natural environments. While some obvious confounding vari- more variable than that of females and the largest males weigh 4–5 ables (e.g., nest size) can be taken into account, there may be times that of the smallest ones of the same population (T. Amundsen important ones that are not even known. Nonetheless, ongoing sex- et al., unpublished data). Body size has been shown to affect nest oc- ual selection in the wild can be investigated by comparing individu- cupation both in G. flavescens (Figure 2 in Wacker et al. 2012) and als that breed and those that do not breed (by selection differentials) other gobies. In line with this, we found significant positive selection and variation in individual success (analyzed by selection gradients). for torso area and size of lateral blue spots (Table 2 in Wacker et al. The G. flavescens model system has both strengths and weaknesses 2013), but only in the even sex ratio treatment where males had to for such analyses. The strengths include the fact that nests can be compete for females. Torso area and blue spot area were corrected fairly easily found and reasonable numbers of nests and males easily for standard length in analyses, meaning that the selected individuals collected. A limitation is that it is very hard to distinguish breeders were more heavily built and more colorful than the average fish of from non-breeders except when breeders defend a nest. Our analyses their size. In this study, there was no selection for fish length per se. of sexual selection in the wild are therefore based on males found to Torso area (Figure 11a; Wacker et al. 2013) is a “non-standard” hold a nest (mostly in a mussel, Figure 2)(Mobley et al. 2009; measure in fishes, possibly because it requires standardized photo- Wacker et al. 2014). Given the strongly female-biased OSR late in graphs of individuals and not only the usual weight and length meas- the season (Forsgren et al. 2004), we predicted males to have higher ures. It is a compound measure of skeletal size, body musculature, mating and reproductive success late than early in the season. This fat deposits, and stomach fullness. The positive selection for torso turned out to be true, with on average about twice as many eggs (ca. area (controlled for fish length) suggests that “body build” matters 2700 vs. 1400) in the nests late than early in the season. The differ- in mating competition. ence was to a large extent due to fewer unmated nest-holders late in We could not reliably measure the size and coloration of the dor- the season, but also to broods being larger among those that were sal fins because we were unable to sufficiently standardize fin exten- mated (Table 1 in Wacker et al. 2014). This resulted in a significant- sion while photographing live fish. Likewise, we did not measure ly higher opportunity for selection (I, based on number of eggs) and color qualities (e.g., spectral reflectance) of the iridescent blue spots. sexual selection (I , based on number of mates) early than late in the It is possible but yet to be demonstrated that these aspects of male season (Wacker et al. 2014), as would be expected from OSR ornamentation were also subject to sexual selection. theory. The most important finding from the experiment was the conclu- As argued above, not only I , but also I, reflects sexual selection sive evidence for a causal effect of OSR on mating competition in this system: I reflects the number of females that has spawned in behaviors, on the potential for sexual selection, and on realized sex- a nest whereas I is a composite measure of number of mates and the ual selection on phenotypic traits (ornamental coloration and fecundity of these mates. We could measure both in this study be- morphology). These experimentally documented effects mirror the cause, unlike in the mesocosm experiment of the previous section, covariation of mating competition with OSR over the course of the all broods were genotyped for parentage analyses (Wacker et al. Downloaded from https://academic.oup.com/cz/article-abstract/64/3/363/4986908 by Ed 'DeepDyve' Gillespie user on 21 June 2018 382 Current Zoology, 2018, Vol. 64, No. 3 2014). We similarly found the maximum standardized sexual selec- tion differential (s )(Jones 2009) to be greater early than late in max the season, whereas the Bateman gradient (b )(Andersson and ss Iwasa 1996) was similar early and late. This was because, in our study, the reduced potential for sexual selection was due to reduced variation in mating success and reproductive success, rather than a change in the degree to which mating success translated into repro- ductive success (Wacker et al. 2014). The results revealed that the Bateman gradient failed to detect important changes in sexual selec- tion in our study, despite that it has proven successful in predicting sexual selection in other model systems (e.g., Jones et al. 2000; Fritzsche and Arnqvist 2013). This calls for caution in drawing inferences about sexual selection from one index alone (Wacker et al. 2014), as the nature of selection may vary between systems and may be more or less well captured by each index. The patterns emerging when analyzing for selection on specific male traits (Table 2 in Wacker et al. 2014) were less clear and partly contradictory to results found in the laboratory (Table 2 in Wacker et al. 2013). However, small sample sizes for these analyses along with the males having been collected at various stages of breeding render these results less conclusive (Wacker et al. 2014). With a male-biased OSR early in the breeding season, one would Figure 12. Gobiusculus flavescens males that breed late in the season are expect competitively superior males to exclude small ones from smaller than those breeding early in the season. Reproduced, by permission breeding, especially if high-quality nest sites are limited. This seems of John Wiley and Sons, from Figure 2 in Wacker et al. (2014), Molecular to be the case, as artificial PVC nest tubes introduced to the breeding Ecology 23:3587–3599. habitat are usually quickly occupied (K. de Jong, unpublished data). Artificial nests have also had high occupancy rates in other popula- such breeding resources. A typical example is migratory birds, where tions (W Norway, Monroe et al. 2016, Mid-Norway: T. Amundsen males often compete for territories even before the females have and S. Wacker, unpublished data). Thus, small males may be forced arrived to the breeding grounds. Similarly, a goby male needs a nest to or strategically postpone breeding until competition over nests (mussel or other) to breed. Accordingly, male G. flavescens (Wacker and females is relaxed later in the season, when male density is lower et al. 2012), as well as other gobies (e.g., Lindstro ¨ m 1988; and gravid females abundant. By contrast, one would expect large Magnhagen and Kvarnemo 1989), may compete for ownership of males to be the ones successful in getting a nest when male density is nests. When nests are in short supply, only competitively superior high and competition strong, as it is early in the breeding season. goby males may be able to obtain a nest (e.g., Forsgren et al. One would therefore expect nest-holding males to be larger than the 1996b). Assuming that competition for nests precedes competition population average early in the season, and also larger than those for mates, Ahnesjo ¨ et al. (2001) argued that only males that have breeding later (Wacker et al. 2014). Late in the season, male density succeeded in nest competition are “qualified to mate” and thus is much lower (Figure 1 in Forsgren et al. 2004) and male competi- involved in mating competition (see also Kvarnemo and Ahnesjo ¨ tion almost absent (Figure 6b, c). At this time, one might expect any 2002). In G. flavescens, however, anecdotal observations in the wild male still alive to be able to gain a nest, and thus no difference in suggest that males sometimes engage in courtship without having size between nest-holding males and the population average. As pre- previously defended a single, well-defined, nest. With the species dicted, we found early season breeders to be clearly larger than the being highly opportunistic in choice of breeding substrate, and with population mean, resulting in significant positive selection for male more-or-less good nesting substrates available in excess in most ter- size. However, late season breeders were clearly smaller than the ritories, a male may be able to find a suitable substrate to which he population mean (Figure 1 in Wacker et al. 2014) and thus also can lead an interested female for spawning, even if he has not smaller than early-season breeders (Figure 12), with consequent resided there on beforehand. Males are also much more prone to negative selection on male body size (Wacker et al. 2014). This was compete for and spend time in nests when there is a female present a result that we did not predict, and which we therefore can only in- (personal observation). Such observations suggest that resource terpret post hoc. We suggest that large males, having started to competition and mating competition may not be as clearly separated breed early in the season and having cared for several consecutive broods, have paid a greater cost of care than smaller ones that have as often assumed. This may render the distinction between males either employed a sneaker tactic or bred fewer times, if at all. This qualified and unqualified to mate less clear. may render small males more fit for costly care than large ones late In order to test for potential interactions between mate competi- in the season, and thus more attractive to females at that time. tion and resource competition, we experimentally varied OSR and Whether or not this hypothesis is true, the results indicate opposing nest abundance. Males were either faced with a shortage of nests selection on male body size between early and late season (Wacker but not of females, a shortage of females but not of nests, or no et al. 2014). shortage of either (Wacker and Amundsen 2014). Notably, nest shortage (fewer nests than males) did not increase competition behavior (aggression and courtship) above the baseline level when Resource competition vs. mating competition In animals that require a territory or a particular nesting structure to neither mates nor nests were in short supply. Thus, nest shortage per breed, competition for mates is often preceded by competition for se did not seem to significantly affect competition. By contrast, a Downloaded from https://academic.oup.com/cz/article-abstract/64/3/363/4986908 by Ed 'DeepDyve' Gillespie user on 21 June 2018 Amundsen  Gobies as models in sexual selection 383 shortage of females induced a strong increase in competition behav- suitable nest sites if visited by a female, even if he has not inspected ior relative to the “no-shortage baseline” (Figures 1 and 2 in Wacker these sites before. Under such scenarios, Q< ASR (Ahnesjo¨et al. and Amundsen 2014). Fewer nests were occupied by males when 2001) would be a reality (roaming males are not “in”) but would rep- there was a shortage of females than when there was no female resent the number of territorial (stationary) males rather than the shortage (Figure 3 in Wacker and Amundsen 2014). With a shortage number of males occupying a nest. This is essentially the approach we of nests but not of females, males behaved less aggressively and have taken when calculating OSR in G. flavescens in the field: we courted less than when females were in short supply, and in fact no have included stationary males whether or not we have seen any nest, but excluded roaming ones (Forsgren et al. 2004). In any species or more than in a treatment with ample access to both nests and population where potential nest sites are present in excess within terri- females. Thus, nest shortage seems not to elicit increased male com- tories, a scenario may apply in which males “control” potential nest petition, unless there is a shortage of females with whom to mate. sites by territoriality without spending much (or any) time in the nests These results call into question the traditional assumption that before engaging in courtship. Hence, the relevance of “strict sense Q” resource (nest) competition comes before mating competition and, (males occupying a nest) for estimating mating competition, and the more fundamentally, that these are truly distinguishable processes. While this may often be the case, as in many hole-nesting birds, our degree to which resource and mating competition are separated, may results suggest that it need not always be so. Instead, the findings vary not only between species, but also within species. suggest that, in G. flavescens, the 2 processes are inter-related, and Notably, we found significant selection on male size in the mate- maybe impossible to fully separate, both empirically and theoretical- limited, but not in the nest-limited, treatment (Figure 4 in Wacker ly (Wacker and Amundsen 2014). If this is true and to a greater or and Amundsen 2014): nest-holding males were larger than non-nest- smaller extent also true for other species, it calls into question holders with female shortage, but not with nest shortage. The find- whether one can distinguish between males that are “qualified to ing that males engage in nest competition mainly when there is a mate” (nest-holders) and those that are not (non-nestholders) shortage of females makes sense from a cost–benefit perspective. (Ahnesjo ¨ et al. 2001), in G. flavescens and any species where male Given that nest defence is costly, the cost may only be balanced by nest-related behaviors are affected by presence or absence of sufficient benefits when the likelihood of mating is high. The strength of sexual selection under mate limitation decreased over the females. The general argument by Ahnesjo ¨ et al. (2001) is that only breeding season (Wacker and Amundsen 2014). those males that hold a nest take part in mating competition (see also Szekely et al. 2014b). Hence, Ahnesjo ¨ et al. (2001) suggest that this subset (which they term Q, for Qualified), instead of ASR, is Environmental variation, mating behavior, and what should be combined with potential reproductive rate to deter- sexual selection mine the OSR and thus predict mating competition. We fully en- dorse this argument on general grounds: provided nest (resource) In the wild, most animals live in complex environments, with habi- competition precedes mating competition, Q should be both quanti- tat type and structure varying among and within populations. fiable and more relevant than ASR. If, however, males do not always Similarly, critical breeding resources (for instance suitable nest sites) establish in a nest before engaging in mate attraction (i.e., resource may be abundant or scarce (e.g., Forsgren et al. 1996b; Borg et al. and mating competition does not occur in sequence), it becomes less 2002), and they may be anything from uniformly distributed to clear which males are qualified to mate. When this is the case, quan- highly clumped. Environmental heterogeneity may affect male re- tifying Q from numbers of males residing in nests may underesti- productive behavior (e.g., aggression) directly, or via effects on mate the true number of males in the mating pool, and thus bias inter-nest distances (Bakker 1994). The nature of the habitat, and OSR estimates “in the female direction” (i.e., overestimating any fe- also the availability of suitable nest sites, is today commonly male bias or underestimating any male bias). We encourage more affected by anthropogenic disturbances. For example, the wide- studies to test, in similar systems, whether resource competition and spread eutrophication of freshwater and marine environments has mating competition are truly sequential and independent, as is gen- affected sexual selection in a range of fish species including gobies, erally assumed (e.g., Ahnesjo ¨ et al. 2001), or whether they are in- via effects on turbidity or habitat structure (e.g., Seehausen et al. stead simultaneous and inter-related, as suggested by our study. It 1997; Ja ¨ rvenpa ¨a ¨ and Lindstro ¨ m 2004; Wong et al. 2007; Candolin should be noted that these 2 alternatives are the extremes of a con- and Wong 2012; Sundin et al. 2016). tinuum: the most commonplace situation may be one where re- The natural habitat structure of G. flavescens is highly variable source (e.g., nest) shortage by itself elicits competition, but where and often complex (Figure 2), and is also temporally dynamic, be- resource competition significantly increases when there is a shortage tween and especially within years. For instance, kelp forests (often of, and thus competition for, mates. dominated by Saccharina spp., Laminaria spp., or a combination) In systems like that of G. flavescens, it is conceivable that the con- typically have high structural complexity, whereas seaweed beds cept of individuals qualified to mate (Q) is relevant, but not necessar- (e.g., Fucus serratus-dominated) have less “3D-complexity.” The ily reflected in nest occupation. As outlined at the start of this article, type and degree of structural complexity can vary considerably males are either relatively stationary (i.e., mostly hovering over a among nearby islands, between windward and leeward sides of the small area, often <1m ) or roaming. They will often but not always same island, and between sheltered bays and exposed rock-faces. It behave aggressively to other males getting near. We have interpreted may also vary temporally, as perennial macro-algae become increas- such stationary males to be territorial, and to be available for mating, ingly overgrown by annual filamentous algae as the season pro- even when never observed in a nest. A territory in the kelp forest is gresses (Figure 2h). Such filamentous algae can both reduce and likely to include a number of potential nest sites to which a male can increase structural complexity, depending on the initial species com- lead an interested female. He may be aware of and have inspected sev- position and habitat structure. When it comes to breeding substrate, eral such potential nest sites, without spending much time there, as we G. flavescens appears to favor breeding in empty mussels, but such have occasionally observed in the wild (T. Amundsen, personal breeding sites are in short supply in most of the habitat and their dis- observation). Alternatively, he may be able to instantaneously locate tribution can be highly variable. In some cases, available breeding Downloaded from https://academic.oup.com/cz/article-abstract/64/3/363/4986908 by Ed 'DeepDyve' Gillespie user on 21 June 2018 384 Current Zoology, 2018, Vol. 64, No. 3 sites can be highly clumped, as in a cluster of live and dead blue Effects of structural complexity mussels attached to a rock-face. In G. flavescens, 2 studies have addressed the role of environmental While there is reason to believe that G. flavescens is faced with characteristics for mating competition and consequent reproduction. an unusually complex and variable habitat, both when it comes to Creating spatial complexity by means of opaque acrylic barriers in structure and nest substrates (Figure 2), structural and other within- 2  2 m mesocosm tanks, and comparing this with a non-structured population habitat variation is almost ubiquitous among animals. environment in the same sort of tanks (Figure 13a, b), Myhre et al. Thus, analyses of how environmental complexity and variation (2013) analyzed how environmental structure affected courtship, affects processes related to mating and breeding should be of great competition, and breeding. They found strong effects of structure on relevance for understanding natural systems. For instance, a less the behavior of both sexes. For females, a structured environment structured environment may facilitate mate detection and compari- caused less movement and less frequent encounters with males but, son of potential mates but may at the same time increase the risk of despite the latter, a shorter time until mating (Figures 2 and 3 in mating interference, because behavioral interactions including mat- Myhre et al. 2013). This may have been because fewer courtship ing are more often visible to nearby conspecifics and to heterospe- interactions were interrupted by other males in the structured envir- cific predators. Similarly, a highly clumped distribution of nests may onment, as was also found, likely for the simple reason that ongoing facilitate mate assessment by females, but also lead to more male– courtship interactions were not detected by other males. The same male aggression and potentially preclusion of less competitive males reason may explain less frequent multi-male courtship in the struc- from breeding. Despite this, most experimental work on mating tured environment (Figure 4 in Myhre et al. 2013). Taken together, competition in the laboratory (including that on G. flavescens) has the results indicate that habitat structure negatively impacts both fe- been conducted in structurally simple environments, motivated by male mate choice (fewer encounters) and male–male competition logistic tractability and, for behavioral work, also observability. (less direct mating competition). This is consistent with the finding Figure 13. Experimental designs for testing effects of habitat structure (A, B) and nest spacing (C, D)in G. flavescens. Tubes (8 in each treatment) indicate artificial PVC nests, the branched structures artificial algae placed in the aquaria. The upper 2 panels illustrate a study comparing mating behavior and sexual selection be- tween an open (A) and a physically structured (B) environment (Myhre et al. 2013). In (B), spatial structuring is achieved by opaque Plexiglas dividers formed to allow fish to move across but significantly preventing visual contact between the compartments containing nests. The lower 2 panels illustrate a study comparing mating behavior and sexual selection between environments with a dispersed (C) or clumped (D) nest distribution (Mu¨ ck et al. 2013). Reprinted, by permission of Oxford University Press, from Figure 1 in Myhre et al. (2013), Behavioral Ecology 24:553–563 (A, B), and, by permission from Springer, from Figure 1 in Mu¨ck et al. (2013), Behavioral Ecology and Sociobiology 67:609–617 (C, D). Downloaded from https://academic.oup.com/cz/article-abstract/64/3/363/4986908 by Ed 'DeepDyve' Gillespie user on 21 June 2018 Amundsen  Gobies as models in sexual selection 385 Figure 14. Microsporidian Kabatana sp. infection of male G. flavescens.(A) Heavily infected male. Each spot is a colony of microsporidians. (B) Longitudinal sec- tion through infected flank muscle, with Masson’s trichrome staining indicating the spore mass (red) and surrounding, intact myofibrils (blue). (C) VOF-stained V C longitudinal section of non-infected (blue) and enlarged infected muscle fibers (brown) in the mandible. (A) Photo: Anders Salesjo¨ , (B, C) modified from Figure 2 in Barber et al. (2009), Diseases of Aquatic Organisms 83:145–152. When the fish gets infected, microsporidian spores migrate to the striated muscles of the fish (B, C), where they multiply to replace the muscle fibers, essentially leaving infected musculature gelatinous and non-functional. Muscles in any part of the body can be infected. Skeletal musculature is often infected and, when extensive like in (A), the swimming ability of the fish is impaired. of positive selection on male size in the open but not in the struc- G. flavescens and many other animals, including substrate brooding tured environment. The results indicate that a more complex habitat fishes, the same population can include both clumped nest substrates may relax sexual selection (Myhre et al. 2013). Given the often high- (e.g., mussel clusters) and individual nest substrates (e.g., solitary ly variable environments of many species, including G. flavescens, mussels) that are widely dispersed. In such cases, sexual selection this insight may be of importance for understanding variation in sex- may vary on a micro-scale, with strong selection on male competi- ual selection in the wild. The study suggests that the structurally tive ability in nest-structure aggregations and less competition but simple environments often employed in laboratory experiments, more scope for female choice where there is a greater distance be- including several of those on G. flavescens, runs a risk of overesti- tween favored substrates (and thus males). This situation may pro- mating sexual selection compared with a natural situation in the mote a dynamic in which less competitive males aim to avoid nest wild. This emphasizes the value of complementing laboratory aggregations and rather try to take up nests further away from com- experiments with systematic field studies, in order to evaluate the petitors. Taken together, variation in structural complexity and dis- “real-life” relevance of laboratory findings to natural ecosystems. tribution of favored nesting resources may have significant impacts on the type and strength of sexual selection processes. If traits selected for in competition are not the same as those favored in mate Effects of nest distribution choice, such environmental variation may contribute in maintaining Mu ¨ ck et al. (2013) manipulated nest distribution (artificial PVC variation in male traits. tubes) to be either highly clumped (aggregated nests) or maximally dispersed, in tanks of the same size (Figure 13c, d). When nests were aggregated, a larger proportion of the males behaved aggressively Effects of parasite infection (Figure 2 in Mu ¨ ck et al. 2013) but fewer of them succeeded in occu- Parasites often have a severe impact on body condition and perform- pying a nest and becoming mated. Moreover, those males that ance of animals (Poulin 2007), and significantly impact animal mated had smaller broods in their nests (Figure 3 in Mu ¨ ck et al. behavior (Moore 2002). Parasites are central to sexual selection, 2013). These effects resulted in a higher variance in reproductive both because of their commonplace negative effect on performance success and, hence, a higher opportunity for selection (I), in the and sexual signaling, and because sexual ornaments may evolve as aggregated treatment (Figure 5 in Mu ¨ ck et al. 2013). In environ- signals of parasite resistance and immuno-competence (Hamilton ments where nest sites are usually clumped, we may therefore expect and Zuk 1982; Folstad and Karter 1992). Studies of G. flavescens in Germany have revealed that the spe- strong sexual selection by male–male competition. Clumped nests may also facilitate female comparison of potential partners, but may cies can be host to a wide range of parasites (Zander 2003, 2004, at the same time increase the risk of mating interference, limit the 2005). In the W Sweden G. flavescens study population, a propor- range of males available for mating (as many males are unable to de- tion of both males and females is infected by a unicellular Kabatana fend a nest in aggregations), and increase the risk of sneaking by sp. microsporidian parasite (Barber et al. 2009). The microspori- competitively inferior males. Thus, clumping of breeding resources dians multiply in colonies in the musculature, destroying the muscle may severely constrain female choice and could lead to a system fibers of infected tissue and being visible as ulcerous whitish spots where male–male competition overrides female choice. As females on the exterior of the fish (Figure 14; Barber et al. 2009). Heavily do not always prefer to mate with the more dominant males (e.g., infected individuals appear to have difficulties swimming properly Forsgren 1997a; Qvarnstro ¨ m and Forsgren 1998; Candolin 2004; (personal observation) but most individuals are less heavily infected. Wong and Candolin 2005), nest site distribution may therefore have While there has been much research on effects of parasites on mor- a significant impact on realized sexual selection. Notably, in phological traits including ornaments, less knowledge exists on how Downloaded from https://academic.oup.com/cz/article-abstract/64/3/363/4986908 by Ed 'DeepDyve' Gillespie user on 21 June 2018 386 Current Zoology, 2018, Vol. 64, No. 3 parasites affect mating behaviors of males and females. The goby- depending on competitive situation and own resource-holding po- microsporidian infection system is well suited to explore such tential (RHP) (Parker 1974), display an ontogenetic change from sneaker to territorial as they grow (Magnhagen 1992), or a combin- effects, both because the parasite directly affects the musculature and because infections are visible externally. The external visibility ation (Oliveira et al. 2008). allows quantification of infection in live animals, significantly facili- tating study designs for behavioral work. Equally importantly, it Low level of sneaking also means that the fishes can visually assess the parasite status of Sneaking is widespread among substrate-brooding fishes (Coleman conspecific males and females, which is not the case for internal par- and Jones 2011), and has been reported to frequently occur both in asites. Comparing courtship behavior between parasitized and non- the gobies P. minutus (e.g., Jones et al. 2001a, 2001b)and P. microps parasitized males, we found a 30% reduction in courtship among (e.g., Magnhagen 1994). However, the frequency of sneaking and parasitized males (Figure 2 in Pe ´ labon et al. 2005). As high- proportion of clutches that are parasitized vary greatly among studies courtship males are often preferred by females in gobies (e.g., and species (Coleman and Jones 2011). In 2 populations of P. minu- Forsgren 1997b) and other animals (e.g., Grant and Green 1996), tus, the proportion of broods with parasitic eggs was similar at about such a reduction may significantly decrease mating success. 35–50% (Jones et al. 2001a, 2001b). In the West Swedish study popu- Notably, the negative effect on courtship was present despite no de- lation of G. flavescens, however, we found that the proportion of eggs tectable negative effect of parasite infection on body condition. fertilized by parasitic (non-nest-holding) males was generally very Hence, the study suggests that courtship may be a sensitive index of small (<1%, Mobley et al. 2009; Wacker et al. 2014). In particular, sub-lethal effects of parasites and other stressors. sneaking was almost absent late in the season (Table 2 in Mobley et al. 2009), when male competition is minimal or non-existent (e.g., Forsgren et al. 2004). At this time, there would be little if any compe- Alternative Reproductive Tactics tition for nest sites, so that even competitively inferior (small) males In many fishes, individual males employ alternative reproductive could establish as territorials. As described above, we indeed found tactics, with some being territorial (bourgeois), aiming to monopol- nest-holding (bourgeois) males to be mostly small at this time (Wacker et al. 2014). Evidence from a W Norwegian population has ize breeding resources and females, others displaying a “sneaker” documented that the same male can act as a sneaker early in the sea- tactic of parasitic spawning, and others again mimicking females to son and as a nest-holder later on (Monroe et al. 2016). gain access to spawning male–female pairs (Taborsky 1997; Oliveira et al. 2008). Territorial and sneaker tactics are co-occurring in populations of very many species, whereas female mimicry tactics Geographic variation in tactics seem less widespread (e.g., Taborsky 1998). Sometimes, sneaking Notably, the dynamics of alternative reproductive tactics in and female mimicry tactics are hard to distinguish, because small G. flavescens appear to be very different in the population studied in males often resemble females and because both tactics are parasitic. W Norway (Utne-Palm et al. 2015; Monroe et al. 2016). In this Alternative reproductive tactics can be obligate or facultative. If fac- population, a major proportion of the males are very small, and typ- ultative, males can either variably act as sneakers or territorials ically smaller than females (Figure 15a; Utne-Palm et al. 2015), Figure 15. Alternative reproductive tactics are prevalent in a West Norwegian population of G. flavescens. In this population, males are generally smaller than females and there is an abundance of very small males (A), the smallest males have much larger testes (gonado-somatic index GSI) for their size (B, C), and sneaking occurs both early and late in the season (D). Reproduced from (A) Figure 1, (B) Figure 2 and (C) Figure 6 in Utne-Palm et al. (2015), PLoS One 10:e0143487, and from (D) Figure 2d in Monroe et al. (2016), Journal of Evolutionary Biology 29:2362–2372, by permission of John Wiley and Sons. Downloaded from https://academic.oup.com/cz/article-abstract/64/3/363/4986908 by Ed 'DeepDyve' Gillespie user on 21 June 2018 Amundsen  Gobies as models in sexual selection 387 unlike the situation in W Sweden where very small males are rare particular how spatial and temporal variation interact in shaping (T. Amundsen et al., unpublished data). The small males in the W sexual selection. Norway population tend to have very large testes for their size (Figure 15b, c), and to store sperm in the seminal vesicles (Utne- Acknowledgments Palm et al. 2015) that would produce mucus for lining the nest be- fore spawning among territorial males. Thereby, sneaker males, I thank all my collaborators in research on two-spotted gobies over nearly 2 decades. In particular, I would like to thank Elisabet Forsgren who introduced who do not need to produce any mucus, likely maximizes their fer- me to gobies and co-managed the two-spotted goby project with me during tilizing success. Consistent with the prevalence of an apparent many years. I also thank Elisabet Forsgren, Sebastian Wacker, Karen de Jong, sneaker morph, nearly a third of male nests both early and late in Jarle Tryti Nordeide, Ingo Schlupp, and 3 anonymous reviewers for com- the breeding season contained eggs sired by one or more parasitic ments on the manuscript. I thank Ingo Schlupp for the invitation to Behaviour males (Figure 15d; Monroe et al. 2016). The differences in mating 2017 that spurred this paper. Any insights on sex roles and sexual selection tactics between populations in W Sweden and W Norway raise the emanating from work on the G. flavescens model system are due to joint intriguing question whether sex roles and sexual selection in G. fla- efforts, empirically and intellectually, by all the authors on the various papers. vescens are dynamic not only on a temporal, but also on a spatial, I am deeply grateful to collaborators Iain Barber, Jon Blount, Bjørn Bjerkeng, scale. the late Angela Davies, Sam Dupont, Elizabet Espy, Joe Ironside, Adam Jones, Fredrik Jutfelt, Carin Magnhagen, Ian Mayer, Melanie Monroe, Hanno Sandvik, Helen Sko ¨ ld, Peter Surai, and Anne Christine Utne Palm; to Conclusions and Prospects former post-docs Kenyon B. Mobley and Christophe Pe ´ labon; to former Ph.D. students Jens Bjelvenmark, Asa A. Borg, Karen de Jong, Lise Cats I hope to have shown that the G. flavescens model system has great Myhre, P. Andreas Svensson, and Sebastian Wacker; to former M.Sc. students potential in analyzing and understanding temporal dynamics of sex Uwe Berger, Camilla Brevik, Jorunn M. Eriksen, Aleksandra I. Johansen, roles and sexual selection. The work presented here is only a start Isabel Mu ¨ ck, and Gry Sagebakken, and not the least to the countless field for future studies to build upon. A main quality of the system is its assistants (none named, none forgotten) who made the intensive field and la- extremely dynamical nature, to my knowledge so far unparalleled in boratory work possible. I have been greatly inspired by collaborations and any other vertebrate model. The system is also exceptional in the discussions with colleagues and friends in the Nordic Goby Network, includ- ing Katja Heubel, Lotta Kvarnemo, Kai Lindstro ¨ m, Carin Magnhagen, Ola ease by which large-scale field and laboratory studies can be com- Svensson, and Anne Christine Utne Palm. The work would not have been pos- bined. In the field, the species is easy to observe in its shallow habi- sible without generous scientific advice from many scientific colleagues, in tat with typically clear oceanic water, is highly abundant yet mostly particular those at the Sven Love ´ n Centre for Marine Sciences at Kristineberg, relatively stationary, and is basically unaffected by close-range pres- and similarly generous help from technical staff at Kristineberg and at the ence of an observer. Gobiusculus flavescens is also easy to collect in Espeland Marine Biological Station of the University of Bergen. I would final- large numbers for laboratory experiments and population analyses, ly like to thank those who have recently worked with me to establish a Mid- and behaves naturally and breeds readily in captivity. None of these Norway study system for G. flavescens, first of all Sebastian Wacker to whom characteristics are unique to the G. flavescens system, but only few I am greatly indebted, but also NTNU technical staff and the many volunteers systems have all these qualities present at the same time, and to the who have made great efforts in the field. This work has greatly inspired my same degree as G. flavescens. current thinking about sexual dynamics and the potential of the G. flavescens model system in sexual selection research. Taken together, the G. flavescens studies indicate a central role of the OSR in regulating the strength and “direction” of mating competition. Insights gained from this model system should be of Funding relevance to any animal that experiences natural fluctuations in The work on which this review article is based has been funded by grants from adult and operational sex ratios. Such fluctuations appear to be the Research Council of Norway [Grant Nos. 133553, 146744, 166596, and common but have not been extensively studied. This is particularly 178444], the Norwegian University of Science and Technology, the Norwegian the case for within-breeding season changes in competition regime. I Academy of Science and Letters, the Royal Swedish Academy of Sciences, the encourage scientists to explore model systems with similarly exten- Nordic Marine Academy, the EU Transnational Access to Research sive variation in OSR (including both male-biased and female-biased Infrastructures Scheme, the Nordic Council program NORDFORSK, and the situations), in order to establish to what extent insights from studies National Science Foundation [USA, Grant No. OISE/0701086]. of G. flavescens are broadly applicable. The most suitable model systems would be ones where behaviors and breeding can be easily recorded both in the field and in the laboratory, like in References G. flavescens. I encourage a search for further species with extensive Adams S, Mapstone BD, Russ GR, Davies CR, 2000. 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Sex roles and sexual selection: lessons from a dynamic model system

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