Competition for food in 2 populations of a wild-caught fish

Competition for food in 2 populations of a wild-caught fish Aggressive behavior when competing for resources is expected to increase as the ratio of competitors-to-resource ratio (CRR) units increases. Females are expected to be more aggressive than males when competing for food when body size is more strongly related to reproductive suc- cess in females than in males, whereas aggression is predicted to decrease under high ambient predation risk by natural selection. Under the risk allocation model, however, individuals under high ambient predation risk are expected to be more aggressive, and forage more in the absence of imminent risk than their low risk counterparts. An interaction between adult sex ratio (i.e., adult males/females), ambient predation risk (high vs. low), and sex on intrasexual competition for mates in Trinidadian guppies Poecilia reticulata has been shown. The interaction suggested an increase in aggression rates as CRR increased, except for males from the high predation population. To compare the patterns of competition for food versus mates, we replicated this study by using food patches. We allowed 4 male or 4 female guppies from high and low predation populations to com- pete for 5, 3, or 1 food patches. The foraging rate was higher in a high rather than low ambient pre- dation risk population. Surprisingly, CRR, sex, and population of origin had no effect on aggression rates. Despite other environmental differences between the 2 populations, the effect of ambient predation risk may be a likely explanation for differences in foraging rates. These results highlight the importance for individuals to secure food despite the cost of competition and predation. Key words: aggression, competitor-to-resource ratio, foraging, Poecilia reticulata, population differences, predation risk, sex Interference competition, when individuals use aggression or other foraging context (e.g., Nummelin 1988; Uccheddu et al. 2015), means to prevent others from consuming a resource (Keddy 2001), while males tend to compete for females. The effects of resource for food is common when resources are clumped and predictable in availability, predation risk, and sex on intraspecific competitive pat- space and time, and at intermediate levels of abundance (e.g., Grand terns have been studied intensively, but in most cases in isolation and Grant 1994; Schmidt et al. 1998; Weir and Grant 2004; Hodge from one another, thus ignoring any potential interactions. et al. 2009; Tanner et al. 2011; Morandini and Ferrer 2015). The term competitor-to-resource ratio (CRR; Grant et al. 2000) Among prey populations, competitive aggression (sensu Archer was introduced as a measure to allow the comparison of patterns of 1988) tends to be balanced against antipredator behavior to increase competition for access to different resources (i.e., food, mates, and survival (Huntingford 1982). In addition to resource availability territories) based on the predictions of operational sex ratio (OSR) and predation risk, individuals within prey populations are expected theory regarding mating competition (Emlen and Oring 1977). Just to show different competitive patterns based on their sex. Females as operational sex ratio predicts the rates of aggression (Weir et al. have a greater pre-natal investment in reproduction because they 2011), CRR, the ratio of individual competitors over the number of produce larger gametes than males (Trivers 1972; Kokko and resource units available (e.g., patches of food, mates; Grant et al. Jennions 2008), and tend to be the more competitive sex in a 2000), predicts the rate of aggression (Noel et al. 2005). The rate of V C The Author(s) (2018). Published by Oxford University Press. 1 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/advance-article-abstract/doi/10.1093/cz/zox078/4797078 by Ed 'DeepDyve' Gillespie user on 12 July 2018 2 Current Zoology, 2018, Vol. 0, No. 0 competitive aggression peaks at intermediate values of CRR, reproductive success quite variable in males (Magurran and Garcia approximately 2 (e.g., Kvarnemo et al. 1995; Grant et al. 2000). 2000), potentially leading to more male–male aggression. Conversely, female–female competition may be more prevalent in a Game theory models also predict that hawk will be an ESS at a CRR of 2, if the gain from the resource is greater than the cost related to foraging context (e.g., Nummelin 1988; Uccheddu et al. 2015) aggression (Parker 1984). CRR also predicts a decrease in aggression because body size is usually more strongly related to reproductive rates as resource units (i.e., amount of resource) become relatively success in females than in males (Charnov 1993). abundant or scarce (Grant et al. 2000; Noel et al. 2005). When the Recent findings suggest an interaction between CRR, ambient predation risk, and sex on mating competition in Trinidadian gup- resource is abundant, aggression is not necessary as all individuals can forage to satiation. Conversely, if the resource is too scarce, the pies (Chuard et al. 2016). Both males and females typically cost of aggression exceeds the potential gain in foraging opportuni- increased their aggression rates toward same-sex individuals as the ties (Brown 1964), resulting in a decrease in aggression rates (Grant relative number of mates decreased, except for males from the high ambient predation population: hence, the significant interaction. et al. 2002; Toobaie and Grant 2013). However, these patterns Chuard et al. (2016) argue that this exception might be due to the might be altered by predation risk as both the availability of resour- ces and the risk of predation are known to affect aggression rates. use of less risky alternative mating tactics by males instead of aggres- The non-consumptive effects of predation strongly affect the sion to secure mates under high-ambient predation risk. We are not behavior of potential prey organisms (Preisser et al. 2005). The risky aware of any study on the simultaneous effects of CRR and ambient competition hypothesis (Chuard et al. 2016) predicts a decrease in predation risk on foraging competition that directly compares males to females. Here, we explored whether similar patterns of competi- intraspecific aggression rates under high ambient predation risk (i.e., in populations adapted to high predation regime), in the absence of an tion were observed in a foraging context, and determine the effects imminent predation threat (e.g., Qvarnstrom et al. 2012); there is pre- of any potential interaction. sumably a trade-off between conspicuously competing for limited We compared intrasexual aggression and foraging rates of wild- caught male and female Trinidadian guppies, from a high versus low resources and predator detection and avoidance (Huntingford 1982). ambient predation risk population (i.e., the same 2 populations used In the absence of an imminent threat, individuals may perceive the by Chuard et al. 2016), and under different food CRRs, to test the fol- risk of a predation event as constant or variable. If the former, then, an elevated ambient predation risk should lead to a decrease in the lowing predictions (Table 1). (1) Individuals will increase their aggres- sion rates as CRR initially increases up to a CRR of 2, above which rates of foraging (e.g., Romero et al. 2011) and intraspecific aggres- aggression rates should decrease due to the cost of competition (Grant sion (e.g., Magurran and Seghers 1991; Herczeg and Valimaki 2011; et al. 2000; Noel et al. 2005). Female Trinidadian guppies show inde- Heinen et al. 2013), in favor of antipredator behavior, even in the terminate growth and forage for longer periods than males, whereas absence of an imminent risk of predation. Under high ambient preda- tion risk, individuals need to trade-off acquiring resources (e.g., com- male guppies stop growing after sexual maturity (Magurran 2005) and quickly switch from foraging to courting after ingesting some peting for resources, foraging) with survival. In the latter, the risk- food (Abrahams 1993). For these reasons, (2) females will be more allocation model (Lima and Bednekoff 1999; Ferrari et al. 2009) sug- aggressive than males when competing for food. Based on the risky- gests higher rates of resource acquisition (e.g., aggression to secure competition hypothesis, in the absence of an imminent risk of preda- resources, foraging, mating) in populations experiencing high versus tion, individuals from the high versus low ambient predation risk pop- low ambient predation risk in the absence of an imminent predation ulation will be (3) less aggressive, and (4) forage less. Alternatively, risk. Based on this model, individuals perceive predation risk as varia- following the risk-allocation model (Lima and Bednekoff 1999), we ble and take advantage of opportunities when predation risk is per- expect the opposite of predictions 3 and 4, if the absence of an immi- ceived as low (i.e., no imminent risk of predation). For instance, in the absence of an imminent risk, female sand tilefish Malacanthus plu- nent predation risk indicates a “safe period.” mieri from high-predation risk sites have higher foraging rates than their low-predation risk counterparts (Baird and Baird 2006;see also Materials and Methods Magurran and Seghers 1994). Another determinant of competitive patterns is the sex of indi- Collection and holding of individuals viduals. When competing for mates, males are typically more aggres- To test the effect of ambient predation risk, we used wild-caught sive than females (Clutton-Brock and Parker 1992) likely due to the adult individuals from 2 populations: high versus low levels of back- indirect effect of higher reproductive rates of males compared with ground predation risk. The Upper Aripo River, a low-risk popula- females. This difference in rates of reproduction leads to stronger tion, experiences predation from 2 species which prey upon sexual selection on males by females, which in turn makes newborns, juveniles, and small male guppies: Hart’s rivulus Table 1. Predictions and results based on the effects of CRR , sex, and ambient predation risk population differences on foraging competition Explanatory variables Predictions Results As CRR increases (1) Intrasexual aggression rate increases initially to then decrease above a CRR of 2 No effect Sex (2) Intrasexual aggression rate is greater in females than in males No effect High versus low ambient (3) Intrasexual aggression rate is lower or higher No effect Predation risk population (4) Foraging rate is lower or higher Significant effect – higher CRR is defined here as the ratio of individual competitors over the number of food patches available. Activities expected to decrease if the cost of ambient predation risk is high (e.g., foraging is conspicuous to predators) OR increase in the absence of a perceived imminent predation risk as it would indicate a “safe” period, as predicted by the risk allocation model (Lima and Bednekoff 1999). Downloaded from https://academic.oup.com/cz/advance-article-abstract/doi/10.1093/cz/zox078/4797078 by Ed 'DeepDyve' Gillespie user on 12 July 2018 Chuard et al.  Foraging competition in guppies 3 Anablepsoides hartii (Magurran 2005), and a freshwater prawn We placed 4 individuals from the same holding tank in a test Macrobrachium crenulatum (personal observations). Further down- tank (45  30  30 cm) and allowed them 1 h to acclimate. We chose stream, the Lower Aripo River population has a high-background individuals who were noticeably different in size so that individuals predation risk (Croft et al. 2006) with species preying upon both could be readily recognized. The percentage standard length (6SD) adult and juvenile guppies. These predators include, but are not lim- of the 2nd, 3rd, and 4th female ranked by size compared with the ited to: pike cichlids Crenicichla sp.; blue acara cichlids 1st were, respectively, 83% (611), 72% (611), and 64% (611); Andinoacara pulcher; and brown coscorub cichlids Cichlasoma and the percentage standard length (6SD) of the 2nd, 3rd, and 4th bimaculatum (Croft et al. 2006; Botham et al. 2008). While high male ranked by size compared with the 1st were, respectively, 96% ambient predation risk sites tend to correlate with low guppy den- (63), 92% (64), and 88% (65). The slides were introduced 10 min sities, high stream productivity (Grether et al. 2001), and higher- before the beginning of observations so individuals could acclimate quality diets for guppies (Zandona ` et al. 2011), we will refer to the to and begin feeding from the food patches, which avoided hunger- Lower Aripo and Upper Aripo populations as “high predation” and biased behavior (i.e., increased foraging attempts in males, Griffiths “low predation” sites for now (see Discussion, “Population 1996). We removed loose flakes by blowing on slides before intro- differences”). ducing them into test tanks. In the one-patch treatment, the single We collected guppies using seine nets between 29 April and 7 slide was placed on the substrate, in the center of the tank. For the June 2013 throughout the duration of the experimental trials. We 3- and 5-patches treatments, slides were placed evenly across the transported fish to the laboratory, a 45-min drive, in 30-L buckets tank, but at least 25 mm from the side of the tank, to make it diffi- filled with 30–40 guppies and approximately 10 L of water from the cult for a single fish to defend more than one patch (i.e., >30 mm individuals’ original river. Once in the laboratory, individuals were from one another, Magurran and Seghers 1991). All 4 individuals held in mixed-sex groups by population of origin. The standard could potentially forage on the same patch without direct physical lengths (6SD) of individuals by sex and population were interaction. The observer recorded behavior from the front of the 18.26 1.2 mm for males and 19.16 4.8 mm for females in the low tank; we covered the outside of the remaining sides with white plas- predation site and 14.66 1.1 mm for males and 15.36 3.1 mm for tic sheets to prevent disturbance. A single observer (P.J.C. Chuard) females in the high predation site. As expected, stronger predation recorded behavior for 10 min, divided into two 5-min periods. pressures on high predation individuals seem to have selected Guppies were individually identified by a combination of color pat- against larger size and later age of sexual maturation compared with terns, size, and shape. Within each period of 5 min, we observed the the low predation population (Magurran 2005). We ensured high 4 fish in a randomized sequence for 75 s each, without observing a water quality in the holding tanks by continuously aerating the fish twice consecutively (i.e., the last focal individual of the first water using air stones, and by continuously filtering the water with period was not used as the first focal individual of the second filters filled with floss and activated charcoal. We removed the period). We summed all focal observations from the 2 periods. excess food and wastes twice a day to avoid bacterial outbreaks. We recorded the frequency of agonistic behavior, performed and Regarding testing tanks, we changed the water after each trial to received separately, including chasing, biting (Gorlick 1976), push- maintain high levels of oxygen and water quality. All fish were fed ing (Magurran and Seghers 1991), and tail beating (Liley 1966). We TM commercial flakes (TetraMin provided by Tetra, Blacksburg, VA, did not record encounter rate to measure aggression propensity (i.e., USA) and brine shrimp twice daily, except the day before a trial for aggression rate corrected by the number of encounters; sensu de individuals to be tested the next day (see below). We released gup- Jong et al. 2012) as individuals could see one another (i.e., no visual pies back to their original rivers using hand nets after a maximum of barrier). In addition, the frequency of foraging was quantified, 41 days (min: 1 day; mean: 20.5 days) in the laboratory. defined as when an individual pecked directly on a food patch, or pecked within one body length of a patch as food might be found here quickly after the beginning of a trial (i.e., flakes detached from Experimental procedure the patch due to foraging). As food rarely detached and fell more To enhance foraging competition, we did not feed individuals in the than one body-length away from a patch, the difference in body size 24 h preceding observations. The day before testing, we made between individuals is not likely to have biased the foraging rate defendable patches of food by dipping standard microscope slides TM recorded per focal individual (i.e., more foraging for longer individ- (75 25 mm) into unflavored gelatine (Indulge , General Foods uals as the area where foraging is recorded depends on the body Corporation, White Plains, NY, USA) using about 20 g gelatine/ length). 100 mL water. Once the slides were covered with a thin layer of TM gelatine, we applied flake food (Tetramin ), fragmented into Statistical analysis smaller pieces, to a square area (25  25 mm) at the center of one side of the slide and allowed the gelatine to set. All slides had We performed all analyses using generalized linear mixed models approximately the same amount of food as only one thin layer of (GLMM) in the R software (3.1.2; R Development Core Team flakes would stick to the gelatine. Enough food was applied for the 2015) with the glmmadmb() function of the glmmADMB package. patches to last for the entire length of a trial (10 min of acclimation Due to a right-skewed distribution of our count data (i.e., many and 10 min of observation). We observed males and females sepa- zeros), we first attempted to fit our data to a Poisson distribution. rately from each population to avoid any confounding effects of As the models were significantly over-dispersed, we ran each model mating competition (Nordell 1998). To manipulate CRR, we used 4 fitted to the negative binomial distribution. As expected, when vali- fish exposed to 5, 3, or 1 food patches (i.e., CRRs¼ 0.8, 1.3, or 4). dated, the negative binomial distribution effectively dealt with the Thus, we used a 3-way factorial design (i.e., 2 populations  2 over-dispersion issue (P> 0.99; Linde ´ n and Ma ¨ ntyniemi 2011). We sexes3 CRRs) with 30 replicates of each. Each individual was used used population, sex, and CRR (quadratic contrasts to test for a only once, for a total of 1,440 individuals. We tested unused individ- dome-shaped relationship, and linear contrasts to detect a linear uals after a median of 3 days (range¼ 1–7 days) in captivity. We ran- increase in aggression; see Chuard et al. 2016) as fixed factors in all domly assigned groups of 4 individuals to the different treatments. analyses. We also added the coefficient of variation of individual Downloaded from https://academic.oup.com/cz/advance-article-abstract/doi/10.1093/cz/zox078/4797078 by Ed 'DeepDyve' Gillespie user on 12 July 2018 4 Current Zoology, 2018, Vol. 0, No. 0 Table 2. Results of the GLMM testing for CRR (quadratic and/or linear contrasts), population of origin (lower: high risk versus upper Aripo: low risk), sex, and CV of individual size (i.e., only for aggression rates) on intrasexual aggression and foraging rates in Trinidadian guppies Variable Main effect Regression coefficient 95% confidence interval zP Intrasexual aggression rates CRR (quadratic contrasts) 0.10 0.39, 0.18 0.71 0.48 CRR (linear contrasts) 0.12 0.17, 0.41 0.82 0.41 Population 0.070 0.31, 0.16 0.61 0.54 Sex 0.27 0.024, 0.56 1.80 0.072 CV of individual size 0.94 1.54, 0.34 3.06 0.0022 Foraging rates CRR (linear contrasts) 0.072 0.48, 0.33 0.35 0.73 Population 0.40 0.74, 0.069 2.36 0.018 Sex 0.29 0.038, 0.62 1.74 0.083 CRR is defined here as the ratio of individual competitors over the number of food patches available. Figure 2. Mean (6SE, N ¼ 30) foraging rate per trial in relation to 3 CRRs (4 individuals competing for 5, 3, and 1 food patches, respectively 0.8, 1.33, 4) Figure 1. Mean (6SE, N ¼ 30) aggression rate, sum of given and received, per and 2 populations of origin: high predation (HP; open diamonds) and low pre- trial in relation to 3 CRR (4 individuals competing for 5, 3, and 1 food patches, dation (LP; shaded squares)in (A) males and (B) females. respectively 0.8, 1.33, 4) and 2 populations of origin: high predation (HP; open diamonds) and low predation (LP; shaded squares; low predation) in (A) males and (B) females. possible competing pairs. However, our main goal was to compare the relative aggression across treatments rather than estimating the size within a trial (CV; standard deviation/mean for each trial) as a total aggression within a given trial. Second, we analyzed total for- covariate to take into account size differences within groups. We aging rates fitted to a negative binomial distribution. Since we based used the principal component of standard length and weight of indi- our tests on a priori predictions, we did not apply any statistical cor- viduals as a proxy for size. As expected, standard length and weight rection to our tests. were highly correlated (98%). We used trial number as a random factor in all GLMM analyses. First, using GLMM, we tested total aggression rate per trial Results (given and received aggression summed up per trial) fitted to a nega- Contrary to our 3 first predictions, overall aggression rates (Table 2 tive binomial distribution. Using focal individual observations allowed us to estimate the total per capita rates of aggression, which and Figure 1) were not significantly affected by CRR, population of were summed for the 4 fish to estimate total aggression in the trial. origin, sex, nor their interactions (Appendix 1). However, aggres- This total aggression for 5 min will underestimate the total aggres- sion rates increased as CV of individual size decreased within a trial sion by 50% on average compared with simultaneously watching all (Table 2). Consistent with our fourth prediction, following the risk- 4 fish for 5 min. Out of the 6 possible pairs of competing individu- allocation model (Lima and Bednekoff 1999), foraging rates were als, we only recorded aggression given and received for the focal higher in the high than in the low predation population (Table 2 and individual during a 5-min period, which accounts for 3 of the 6 Figure 2). However, CRR, sex, and the interactions of the 3 Downloaded from https://academic.oup.com/cz/advance-article-abstract/doi/10.1093/cz/zox078/4797078 by Ed 'DeepDyve' Gillespie user on 12 July 2018 Chuard et al.  Foraging competition in guppies 5 above-mentioned factors (Appendix 2) had no significant effects on coefficient of variation of net aggression) between CRRs (see foraging rate (Table 2 and Figure 2). Chuard 2017). Because an increase in density leads to an increase in aggression rates (Magurran and Seghers 1991) but a decrease in the number of food patches does not (our results), seasonal changes in Discussion guppy density might be more ecologically relevant to guppies than changes in food abundance. Indeed, there is no strong evidence that Overall, our results support 1 of our 4 original predictions (Table 1). food availability varies across seasons in Trinidadian streams Surprisingly, CRR, sex, and population of origin did not influence (Magurran 2005; but see Reznick 1989). aggression rates among guppies competing for access to foraging patches. Rather, foraging rates followed the risk-allocation model (Lima and Bednekoff 1999) with higher foraging rates in the high Sex versus the low predation population. These results suggest that We found no difference in aggression and foraging rates between decreasing food availability at a constant competitor density does not males and females. While male guppies forage just enough to satisfy affect aggression rates in guppies. However, the effect of elevated their immediate hunger (Griffiths 1996), female guppies devote a ambient predation risk seems to favor individuals able to forage greater portion of their time budget to foraging (Magurran and more when an imminent risk is absent. As expected (Parker 1974), Seghers 1994), presumably to produce eggs, and to match the ener- aggression rates increased as size differences between competing indi- getic requirements associated with indeterminate growth (Magurran viduals decreased. These conclusions should be tested in future stud- 2005). Given that individuals fasted for 24 h before testing, it is pos- ies using more populations as our experimental design only included sible that a 10-min observation period was not sufficient for males 2 of them. The composition of the predatory community of these 2 to start reducing their foraging rates, and associated aggression, rivers, among other environmental factors, might be specific to them. compared with females. For example, after at least a 3-h fast, male For instance, the freshwater prawn, M. crenulatum, was not found in guppies switched from primarily feeding to courting after about the Upper Aripo River in previous studies (Magurran 2005) but is 10 min [see Figure 3 in Abrahams (1993)]. now found. This recent invasion might have had an effect on how this specific population responds to ambient predation. Population differences These results contrast with those of Chuard et al. (2016) where We found that the low and high predation populations showed simi- both CRR and ambient predation risk had an effect on aggression lar levels of aggression. These results do not support the risky com- rates in a mating competition context (see Figure 1 in Chuard et al. petition hypothesis (Chuard et al. 2016; see also Magurran and 2016). Indeed, aggression rates increased as CRR increased, and Seghers 1991). However, those 2 same populations did differ in low-predation guppies were more aggressive than their high- aggression rates related to mating competition (Chuard et al. 2016). predation counterparts as expected under the risky-competition As courtship displays have the potential to attract predators (Zuk hypothesis (Chuard et al. 2016). However, similar to our findings in and Kolluru 1998), aggression might be more likely traded-off for a foraging context, males and females did not differ significantly in antipredator behavior in a mating rather than a foraging competi- their aggression rates (Chuard et al. 2016). The most notable differ- tion context. The absence of a difference in aggression rates between ence between the 2 experiments was the observed rates of aggres- the 2 populations might also be due to the predator assemblage of sion, which were more than 3 times greater in the food- rather than each population. Indeed, guppy populations have been shown to be the mating-competition experiment once we corrected for methodo- adapted to their local environments along a continuous environmen- logical differences (i.e., scanning vs. focal observations, trial length). tal gradient, where the predatory community plays an important Perhaps fixed food patches are easier to monopolize and defend role (Torres-Dowdall et al. 2012). The 2 populations we used are than mobile mates, resulting in a greater pay-off for individuals who not located at the extreme ends of the predation risk gradient invest energy in aggressive behavior when competing for food. encountered in nature (Torres-Dowdall et al. 2012), and thus preda- tion risk might not be different enough to cause differences in Competitor-to-resource ratio aggression rates related to foraging. Unlike Magurran and Seghers (1991), we found no effect of CRR on Foraging rates between populations were consistent with the pre- aggression rate. However, Magurran and Seghers (1991) manipu- dictions of the risk allocation theory in the absence of an imminent lated CRR in one of their 4 experiments (i.e. “the effect of tank size risk of predation (Lima and Bednekoff 1999); high-predation indi- and group size”) by increasing the number of competitors foraging viduals foraged more than low-predation conspecifics. In the on a single food patch, whereas we decreased the number of resour- absence of an imminent risk of predation, individuals from a high ces (i.e., food patches) while holding the number of competitors con- versus low ambient predation risk site seem to compensate for lost stant. A possible explanation for this discrepancy is that aggression foraging opportunities during previous periods of high imminent rates increased significantly above a CRR of 4 in Magurran and predation risk. Future studies should replicate this experiment with Seghers’ (1991) experiment, while 4 was the highest CRR in our the addition of imminent predation risk treatments to validate this study. Future studies should also try to disentangle the effects of hypothesis. However, due to the cost of predation, high ambient CRR, fish density, and food abundance, by using higher CRRs while predation risk guppies seem to spend less time foraging than their keeping the number of individuals constant (i.e., more than 4 indi- low ambient predation risk counterparts (Magurran and Seghers viduals per trial), as well as test aggression rates at equal CRRs but 1994), suggesting selection for higher foraging rates under high varying fish density (e.g., Clark and Grant 2010). In addition, the ambient predation risk, and more time spent on antipredator behav- food patches might have been close enough together in our experi- ior. Given the smaller size of individuals from high versus low pre- ment such that the dominant fish could defend them against 3 other dation risk sites (Magurran 2005), foraging more to sustain higher competitors. This explanation is supported by the high rates of growth rates in low predation risk females is not likely. As males aggression in our experiment compared with Chuard et al. (2016), stop growing after sexual maturity (Magurran 2005), this explana- and the absence of differences in dominance structure (i.e., tion is even less likely for them. Downloaded from https://academic.oup.com/cz/advance-article-abstract/doi/10.1093/cz/zox078/4797078 by Ed 'DeepDyve' Gillespie user on 12 July 2018 6 Current Zoology, 2018, Vol. 0, No. 0 Our conclusions cannot be limited to the effect of ambient preda- References tion risk alone, as low ambient predation risk streams tend to have Abrahams MV, 1993. 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The effects of adult sex ratio on compared with populations living in high-productivity streams (high mating competition in male and female guppies Poecilia reticulata in two predation; Magurran and Seghers 1991; but see Kolluru et al. 2007). wild populations. Behav Process 129:1–10. Clark L, Grant JWA, 2010. Intrasexual competition and courtship in female While the 2 populations did not differ in rates of aggression (see and male Japanese medaka Oryzias latipes: effects of operational sex ratio above), guppies from the low-predation/low-productivity popula- and density. Anim Behav 80:707–712. tion had lower foraging rates than their high-predation/high produc- Clutton-Brock TH, Parker GA, 1992. Potential reproductive rates and the tivity counterparts. These results suggest that differences in density, operation of sexual selection. 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Science 197:215–223. as they can select habitats that lower this risk (e.g., Main et al. Ferrari MCO, Sih A, Chivers DP, 2009. The paradox of risk allocation: a 1996). Conversely, individuals facing low ambient predation risk review and prospectus. Anim Behav 78:579–585. can still face high imminent predation risk, such as just prior to a Gorlick DL, 1976. Dominance hierarchies and factors affecting dominance in predator attack. Consequently, it would be of great value to explore the guppy Poecilia reticulata (Peters). Anim Behav 24:336–346. aggression rates in relation to food abundance under varying inten- Grand TC, Grant JWA, 1994. Spatial predictability of food influences its sities of imminent predation risk to fully investigate the risk alloca- monopolization and defence by juvenile convict cichlids. Anim Behav 47: 91–100. tion hypothesis, in populations that also vary in ambient predation Grant JWA, Gaboury CL, Levitt HL, 2000. Competitor-to-resource ratio, a risk. general formulation of operational sex ratio, as a predictor of competitive aggression in Japanese medaka (Pisces: Oryziidae). Behav Ecol 11:670–675. Grant JWA, Girard IL, Breau C, Weir LK, 2002. Influence of food abundance Acknowledgments on competitive aggression in juvenile convict cichlids. Anim Behav 63: We are grateful to Anne-Christine Auge, Glaeson Ramnarine, Pierre-Jean 323–330. Recondo, and Drs Heather Auld, Chris Elvidge, Jean-Guy Godin, and Indar Grether GF, Millie DF, Bryant MJ, Reznick DN, Mayea W, 2001. Rain forest W. Ramnarine for their help in the field. We also thank Dr Guillaume canopy cover, resource availability, and life history evolution in guppies. Larocque for his expertise in statistics, as well as the Director of Fisheries in Ecology 82:1546–1559. the Trinidadian Ministry of Agriculture, Land, and Marine Resources for per- Griffiths SW, 1996. Sex differences in the trade-off between feeding and mat- mission to sample individuals from the Aripo River and use them in our study. ing in the guppy. J Fish Biol 48:891–898. All work reported herein was conducted in accordance with guidelines of the Heinen JL, Coco MW, Marcuard MS, White DN, Peterson MN et al., 2013. Canadian Council on Animal Care and the laws of Canada, and was Environmental drivers of demographics, habitat use, and behavior during a approved by the Concordia University Animal Research Ethics Committee. post-Pleistocene radiation of Bahamas mosquitofish Gambusia hubbsi. Evol Ecol 27:971–991. Herczeg G, Valimaki K, 2011. Intraspecific variation in behaviour: effects of Funding evolutionary history, ontogenetic experience and sex. J Evol Biol 24: This work was financially supported by Concordia University (Faculty of Arts 2434–2444. and Science Graduate Fellowship [to P.J.C.C.]) and the Natural Sciences and Hodge SJ, Thornton A, Flower TP, Clutton-Brock TH, 2009. Food limitation Engineering Research Council of Canada (to G.E.B. and J.W.A.G.). increases aggression in juvenile meerkats. Behav Ecol 20:930–935. Downloaded from https://academic.oup.com/cz/advance-article-abstract/doi/10.1093/cz/zox078/4797078 by Ed 'DeepDyve' Gillespie user on 12 July 2018 Chuard et al.  Foraging competition in guppies 7 Huntingford FA, 1982. Do inter- and intra-specific aggression vary in relation Qvarnstrom A, Vallin N, Rudh A, 2012. The role of male contest competition over mates in speciation. Curr Zool 58:493–509. to predation pressure in sticklebacks? Anim Behav 30:909–916. Keddy PA, 2001. Competition. Hoboken: John Wiley & sons. R Development Core Team, 2015. A Language and Environment for Kokko H, Jennions MD, 2008. Parental investment, sexual selection and sex Statistical Computing [cited 2017 December 30]. Austria: The R ratios. J Evol Biol 21:919–948. Foundation. Available from: http://www.R-project.org/. Kolluru GR, Grether GF, Contreras H, 2007. Environmental and genetic influ- Reznick DN, 1989. Life-history evolution in guppies: 2. repeatability of field ences on mating strategies along a replicated food availability gradient in observations and the effects of season on life histories. Evolution 43: guppies Poecilia reticulata. Behav Ecol Sociobiol 61:689–701. 1285–1297. Kvarnemo C, Forsgren E, Magnhagen C, 1995. Effects of sex ratio on intra- Romero GQ, Antiqueira PAP, Koricheva J, 2011. A meta-analysis of predation and inter-sexual behaviour in sand gobies. Anim Behav 50:1455–1461. risk effects on pollinator behaviour. PLoS ONE 6:e20689. Liley NR, 1966. Ethological isolates mechanisms in four sympatric species of Schmidt KT, Seivwright LJ, Hoi H, Staines BW, 1998. The effect of depletion Poeciliids fishes. Behaviour 13:1–197. and predictability of distinct food patches on the timing of aggression in red Lima SL, Bednekoff PA, 1999. Temporal variation in danger drives antipredator deer stags. Ecography 21:415–422. behavior: the predation risk allocation hypothesis. Am Nat 153:649–659. Tanner CJ, Salalt GD, Jackson AL, 2011. Feeding and non-feeding aggression Linde ´ n A, Ma ¨ ntyniemi S, 2011. Using the negative binomial distribution to can be induced in invasive shore crabs by altering food distribution. Behav model overdispersion in ecological count data. Ecology 92:1414–1421. Ecol Sociobiol 65:249–256. Magurran AE, 2005. Evolutionary Ecology: The Trinidadian Guppy. Oxford Toobaie A, Grant JWA, 2013. Effect of food abundance on aggressiveness and Series in Ecology and Evolution. Oxford: Oxford University Press. territory size of juvenile rainbow trout Oncorhynchus mykiss. Anim Behav Magurran AE, Garcia M, 2000. Sex differences in behaviour as an indirect 85:241–246. consequence of mating system. J Fish Biol 57:839–857. Torres-Dowdall J, Handelsman CA, Ruell EW, Auer SK, Reznick DN et al., Magurran AE, Seghers BH, 1991. Variation in schooling and aggression amongst 2012. 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Downloaded from https://academic.oup.com/cz/advance-article-abstract/doi/10.1093/cz/zox078/4797078 by Ed 'DeepDyve' Gillespie user on 12 July 2018 8 Current Zoology, 2018, Vol. 0, No. 0 Appendix Table A1. Results of the interactions of the GLMM testing for the effects of CRR (quadratic and linear contrasts), population of origin (lower: high risk versus upper Aripo: low risk), and sex on intrasexual aggression rates in Trinidadian guppies Interaction Regression coefficient 95% confidence interval zP CRR (quadratic contrasts)population 0.014 0.39, 0.42 0.070 0.95 CRR (linear contrasts)population 0.038 0.45, 0.37 0.18 0.86 CRR (quadratic contrasts)sex 0.31 0.099, 0.72 1.49 0.14 CRR (linear contrasts)sex 0.069 0.34, 0.48 0.33 0.74 Populationsex 0.072 0.26, 0.40 0.43 0.67 CRR (quadratic contrasts)populationsex 0.047 0.53, 0.62 0.16 0.87 CRR (linear contrasts)populationsex 0.15 0.73, 0.42 0.52 0.60 CRR is defined here as the ratio of individual competitors over the number of food patches available. Table A2. Results of the interactions of the GLMM testing for the effects of CRR (linear contrasts), population of origin (lower: high risk ver- sus upper Aripo: low risk), and sex on foraging rates in Trinidadian guppies Interaction Regression coefficient 95% confidence interval zP CRR (linear contrasts)population 0.13 0.71, 0.45 0.45 0.65 CRR (linear contrasts)sex 0.23 0.80, 0.34 0.80 0.42 Populationsex 0.29 0.76, 0.18 1.21 0.23 CRR (linear contrasts)populationsex 0.021 0.80, 0.84 0.050 0.96 CRR is defined here as the ratio of individual competitors over the number of food patches available. Downloaded from https://academic.oup.com/cz/advance-article-abstract/doi/10.1093/cz/zox078/4797078 by Ed 'DeepDyve' Gillespie user on 12 July 2018 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Current Zoology Oxford University Press

Competition for food in 2 populations of a wild-caught fish

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Abstract

Aggressive behavior when competing for resources is expected to increase as the ratio of competitors-to-resource ratio (CRR) units increases. Females are expected to be more aggressive than males when competing for food when body size is more strongly related to reproductive suc- cess in females than in males, whereas aggression is predicted to decrease under high ambient predation risk by natural selection. Under the risk allocation model, however, individuals under high ambient predation risk are expected to be more aggressive, and forage more in the absence of imminent risk than their low risk counterparts. An interaction between adult sex ratio (i.e., adult males/females), ambient predation risk (high vs. low), and sex on intrasexual competition for mates in Trinidadian guppies Poecilia reticulata has been shown. The interaction suggested an increase in aggression rates as CRR increased, except for males from the high predation population. To compare the patterns of competition for food versus mates, we replicated this study by using food patches. We allowed 4 male or 4 female guppies from high and low predation populations to com- pete for 5, 3, or 1 food patches. The foraging rate was higher in a high rather than low ambient pre- dation risk population. Surprisingly, CRR, sex, and population of origin had no effect on aggression rates. Despite other environmental differences between the 2 populations, the effect of ambient predation risk may be a likely explanation for differences in foraging rates. These results highlight the importance for individuals to secure food despite the cost of competition and predation. Key words: aggression, competitor-to-resource ratio, foraging, Poecilia reticulata, population differences, predation risk, sex Interference competition, when individuals use aggression or other foraging context (e.g., Nummelin 1988; Uccheddu et al. 2015), means to prevent others from consuming a resource (Keddy 2001), while males tend to compete for females. The effects of resource for food is common when resources are clumped and predictable in availability, predation risk, and sex on intraspecific competitive pat- space and time, and at intermediate levels of abundance (e.g., Grand terns have been studied intensively, but in most cases in isolation and Grant 1994; Schmidt et al. 1998; Weir and Grant 2004; Hodge from one another, thus ignoring any potential interactions. et al. 2009; Tanner et al. 2011; Morandini and Ferrer 2015). The term competitor-to-resource ratio (CRR; Grant et al. 2000) Among prey populations, competitive aggression (sensu Archer was introduced as a measure to allow the comparison of patterns of 1988) tends to be balanced against antipredator behavior to increase competition for access to different resources (i.e., food, mates, and survival (Huntingford 1982). In addition to resource availability territories) based on the predictions of operational sex ratio (OSR) and predation risk, individuals within prey populations are expected theory regarding mating competition (Emlen and Oring 1977). Just to show different competitive patterns based on their sex. Females as operational sex ratio predicts the rates of aggression (Weir et al. have a greater pre-natal investment in reproduction because they 2011), CRR, the ratio of individual competitors over the number of produce larger gametes than males (Trivers 1972; Kokko and resource units available (e.g., patches of food, mates; Grant et al. Jennions 2008), and tend to be the more competitive sex in a 2000), predicts the rate of aggression (Noel et al. 2005). The rate of V C The Author(s) (2018). Published by Oxford University Press. 1 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/advance-article-abstract/doi/10.1093/cz/zox078/4797078 by Ed 'DeepDyve' Gillespie user on 12 July 2018 2 Current Zoology, 2018, Vol. 0, No. 0 competitive aggression peaks at intermediate values of CRR, reproductive success quite variable in males (Magurran and Garcia approximately 2 (e.g., Kvarnemo et al. 1995; Grant et al. 2000). 2000), potentially leading to more male–male aggression. Conversely, female–female competition may be more prevalent in a Game theory models also predict that hawk will be an ESS at a CRR of 2, if the gain from the resource is greater than the cost related to foraging context (e.g., Nummelin 1988; Uccheddu et al. 2015) aggression (Parker 1984). CRR also predicts a decrease in aggression because body size is usually more strongly related to reproductive rates as resource units (i.e., amount of resource) become relatively success in females than in males (Charnov 1993). abundant or scarce (Grant et al. 2000; Noel et al. 2005). When the Recent findings suggest an interaction between CRR, ambient predation risk, and sex on mating competition in Trinidadian gup- resource is abundant, aggression is not necessary as all individuals can forage to satiation. Conversely, if the resource is too scarce, the pies (Chuard et al. 2016). Both males and females typically cost of aggression exceeds the potential gain in foraging opportuni- increased their aggression rates toward same-sex individuals as the ties (Brown 1964), resulting in a decrease in aggression rates (Grant relative number of mates decreased, except for males from the high ambient predation population: hence, the significant interaction. et al. 2002; Toobaie and Grant 2013). However, these patterns Chuard et al. (2016) argue that this exception might be due to the might be altered by predation risk as both the availability of resour- ces and the risk of predation are known to affect aggression rates. use of less risky alternative mating tactics by males instead of aggres- The non-consumptive effects of predation strongly affect the sion to secure mates under high-ambient predation risk. We are not behavior of potential prey organisms (Preisser et al. 2005). The risky aware of any study on the simultaneous effects of CRR and ambient competition hypothesis (Chuard et al. 2016) predicts a decrease in predation risk on foraging competition that directly compares males to females. Here, we explored whether similar patterns of competi- intraspecific aggression rates under high ambient predation risk (i.e., in populations adapted to high predation regime), in the absence of an tion were observed in a foraging context, and determine the effects imminent predation threat (e.g., Qvarnstrom et al. 2012); there is pre- of any potential interaction. sumably a trade-off between conspicuously competing for limited We compared intrasexual aggression and foraging rates of wild- caught male and female Trinidadian guppies, from a high versus low resources and predator detection and avoidance (Huntingford 1982). ambient predation risk population (i.e., the same 2 populations used In the absence of an imminent threat, individuals may perceive the by Chuard et al. 2016), and under different food CRRs, to test the fol- risk of a predation event as constant or variable. If the former, then, an elevated ambient predation risk should lead to a decrease in the lowing predictions (Table 1). (1) Individuals will increase their aggres- sion rates as CRR initially increases up to a CRR of 2, above which rates of foraging (e.g., Romero et al. 2011) and intraspecific aggres- aggression rates should decrease due to the cost of competition (Grant sion (e.g., Magurran and Seghers 1991; Herczeg and Valimaki 2011; et al. 2000; Noel et al. 2005). Female Trinidadian guppies show inde- Heinen et al. 2013), in favor of antipredator behavior, even in the terminate growth and forage for longer periods than males, whereas absence of an imminent risk of predation. Under high ambient preda- tion risk, individuals need to trade-off acquiring resources (e.g., com- male guppies stop growing after sexual maturity (Magurran 2005) and quickly switch from foraging to courting after ingesting some peting for resources, foraging) with survival. In the latter, the risk- food (Abrahams 1993). For these reasons, (2) females will be more allocation model (Lima and Bednekoff 1999; Ferrari et al. 2009) sug- aggressive than males when competing for food. Based on the risky- gests higher rates of resource acquisition (e.g., aggression to secure competition hypothesis, in the absence of an imminent risk of preda- resources, foraging, mating) in populations experiencing high versus tion, individuals from the high versus low ambient predation risk pop- low ambient predation risk in the absence of an imminent predation ulation will be (3) less aggressive, and (4) forage less. Alternatively, risk. Based on this model, individuals perceive predation risk as varia- following the risk-allocation model (Lima and Bednekoff 1999), we ble and take advantage of opportunities when predation risk is per- expect the opposite of predictions 3 and 4, if the absence of an immi- ceived as low (i.e., no imminent risk of predation). For instance, in the absence of an imminent risk, female sand tilefish Malacanthus plu- nent predation risk indicates a “safe period.” mieri from high-predation risk sites have higher foraging rates than their low-predation risk counterparts (Baird and Baird 2006;see also Materials and Methods Magurran and Seghers 1994). Another determinant of competitive patterns is the sex of indi- Collection and holding of individuals viduals. When competing for mates, males are typically more aggres- To test the effect of ambient predation risk, we used wild-caught sive than females (Clutton-Brock and Parker 1992) likely due to the adult individuals from 2 populations: high versus low levels of back- indirect effect of higher reproductive rates of males compared with ground predation risk. The Upper Aripo River, a low-risk popula- females. This difference in rates of reproduction leads to stronger tion, experiences predation from 2 species which prey upon sexual selection on males by females, which in turn makes newborns, juveniles, and small male guppies: Hart’s rivulus Table 1. Predictions and results based on the effects of CRR , sex, and ambient predation risk population differences on foraging competition Explanatory variables Predictions Results As CRR increases (1) Intrasexual aggression rate increases initially to then decrease above a CRR of 2 No effect Sex (2) Intrasexual aggression rate is greater in females than in males No effect High versus low ambient (3) Intrasexual aggression rate is lower or higher No effect Predation risk population (4) Foraging rate is lower or higher Significant effect – higher CRR is defined here as the ratio of individual competitors over the number of food patches available. Activities expected to decrease if the cost of ambient predation risk is high (e.g., foraging is conspicuous to predators) OR increase in the absence of a perceived imminent predation risk as it would indicate a “safe” period, as predicted by the risk allocation model (Lima and Bednekoff 1999). Downloaded from https://academic.oup.com/cz/advance-article-abstract/doi/10.1093/cz/zox078/4797078 by Ed 'DeepDyve' Gillespie user on 12 July 2018 Chuard et al.  Foraging competition in guppies 3 Anablepsoides hartii (Magurran 2005), and a freshwater prawn We placed 4 individuals from the same holding tank in a test Macrobrachium crenulatum (personal observations). Further down- tank (45  30  30 cm) and allowed them 1 h to acclimate. We chose stream, the Lower Aripo River population has a high-background individuals who were noticeably different in size so that individuals predation risk (Croft et al. 2006) with species preying upon both could be readily recognized. The percentage standard length (6SD) adult and juvenile guppies. These predators include, but are not lim- of the 2nd, 3rd, and 4th female ranked by size compared with the ited to: pike cichlids Crenicichla sp.; blue acara cichlids 1st were, respectively, 83% (611), 72% (611), and 64% (611); Andinoacara pulcher; and brown coscorub cichlids Cichlasoma and the percentage standard length (6SD) of the 2nd, 3rd, and 4th bimaculatum (Croft et al. 2006; Botham et al. 2008). While high male ranked by size compared with the 1st were, respectively, 96% ambient predation risk sites tend to correlate with low guppy den- (63), 92% (64), and 88% (65). The slides were introduced 10 min sities, high stream productivity (Grether et al. 2001), and higher- before the beginning of observations so individuals could acclimate quality diets for guppies (Zandona ` et al. 2011), we will refer to the to and begin feeding from the food patches, which avoided hunger- Lower Aripo and Upper Aripo populations as “high predation” and biased behavior (i.e., increased foraging attempts in males, Griffiths “low predation” sites for now (see Discussion, “Population 1996). We removed loose flakes by blowing on slides before intro- differences”). ducing them into test tanks. In the one-patch treatment, the single We collected guppies using seine nets between 29 April and 7 slide was placed on the substrate, in the center of the tank. For the June 2013 throughout the duration of the experimental trials. We 3- and 5-patches treatments, slides were placed evenly across the transported fish to the laboratory, a 45-min drive, in 30-L buckets tank, but at least 25 mm from the side of the tank, to make it diffi- filled with 30–40 guppies and approximately 10 L of water from the cult for a single fish to defend more than one patch (i.e., >30 mm individuals’ original river. Once in the laboratory, individuals were from one another, Magurran and Seghers 1991). All 4 individuals held in mixed-sex groups by population of origin. The standard could potentially forage on the same patch without direct physical lengths (6SD) of individuals by sex and population were interaction. The observer recorded behavior from the front of the 18.26 1.2 mm for males and 19.16 4.8 mm for females in the low tank; we covered the outside of the remaining sides with white plas- predation site and 14.66 1.1 mm for males and 15.36 3.1 mm for tic sheets to prevent disturbance. A single observer (P.J.C. Chuard) females in the high predation site. As expected, stronger predation recorded behavior for 10 min, divided into two 5-min periods. pressures on high predation individuals seem to have selected Guppies were individually identified by a combination of color pat- against larger size and later age of sexual maturation compared with terns, size, and shape. Within each period of 5 min, we observed the the low predation population (Magurran 2005). We ensured high 4 fish in a randomized sequence for 75 s each, without observing a water quality in the holding tanks by continuously aerating the fish twice consecutively (i.e., the last focal individual of the first water using air stones, and by continuously filtering the water with period was not used as the first focal individual of the second filters filled with floss and activated charcoal. We removed the period). We summed all focal observations from the 2 periods. excess food and wastes twice a day to avoid bacterial outbreaks. We recorded the frequency of agonistic behavior, performed and Regarding testing tanks, we changed the water after each trial to received separately, including chasing, biting (Gorlick 1976), push- maintain high levels of oxygen and water quality. All fish were fed ing (Magurran and Seghers 1991), and tail beating (Liley 1966). We TM commercial flakes (TetraMin provided by Tetra, Blacksburg, VA, did not record encounter rate to measure aggression propensity (i.e., USA) and brine shrimp twice daily, except the day before a trial for aggression rate corrected by the number of encounters; sensu de individuals to be tested the next day (see below). We released gup- Jong et al. 2012) as individuals could see one another (i.e., no visual pies back to their original rivers using hand nets after a maximum of barrier). In addition, the frequency of foraging was quantified, 41 days (min: 1 day; mean: 20.5 days) in the laboratory. defined as when an individual pecked directly on a food patch, or pecked within one body length of a patch as food might be found here quickly after the beginning of a trial (i.e., flakes detached from Experimental procedure the patch due to foraging). As food rarely detached and fell more To enhance foraging competition, we did not feed individuals in the than one body-length away from a patch, the difference in body size 24 h preceding observations. The day before testing, we made between individuals is not likely to have biased the foraging rate defendable patches of food by dipping standard microscope slides TM recorded per focal individual (i.e., more foraging for longer individ- (75 25 mm) into unflavored gelatine (Indulge , General Foods uals as the area where foraging is recorded depends on the body Corporation, White Plains, NY, USA) using about 20 g gelatine/ length). 100 mL water. Once the slides were covered with a thin layer of TM gelatine, we applied flake food (Tetramin ), fragmented into Statistical analysis smaller pieces, to a square area (25  25 mm) at the center of one side of the slide and allowed the gelatine to set. All slides had We performed all analyses using generalized linear mixed models approximately the same amount of food as only one thin layer of (GLMM) in the R software (3.1.2; R Development Core Team flakes would stick to the gelatine. Enough food was applied for the 2015) with the glmmadmb() function of the glmmADMB package. patches to last for the entire length of a trial (10 min of acclimation Due to a right-skewed distribution of our count data (i.e., many and 10 min of observation). We observed males and females sepa- zeros), we first attempted to fit our data to a Poisson distribution. rately from each population to avoid any confounding effects of As the models were significantly over-dispersed, we ran each model mating competition (Nordell 1998). To manipulate CRR, we used 4 fitted to the negative binomial distribution. As expected, when vali- fish exposed to 5, 3, or 1 food patches (i.e., CRRs¼ 0.8, 1.3, or 4). dated, the negative binomial distribution effectively dealt with the Thus, we used a 3-way factorial design (i.e., 2 populations  2 over-dispersion issue (P> 0.99; Linde ´ n and Ma ¨ ntyniemi 2011). We sexes3 CRRs) with 30 replicates of each. Each individual was used used population, sex, and CRR (quadratic contrasts to test for a only once, for a total of 1,440 individuals. We tested unused individ- dome-shaped relationship, and linear contrasts to detect a linear uals after a median of 3 days (range¼ 1–7 days) in captivity. We ran- increase in aggression; see Chuard et al. 2016) as fixed factors in all domly assigned groups of 4 individuals to the different treatments. analyses. We also added the coefficient of variation of individual Downloaded from https://academic.oup.com/cz/advance-article-abstract/doi/10.1093/cz/zox078/4797078 by Ed 'DeepDyve' Gillespie user on 12 July 2018 4 Current Zoology, 2018, Vol. 0, No. 0 Table 2. Results of the GLMM testing for CRR (quadratic and/or linear contrasts), population of origin (lower: high risk versus upper Aripo: low risk), sex, and CV of individual size (i.e., only for aggression rates) on intrasexual aggression and foraging rates in Trinidadian guppies Variable Main effect Regression coefficient 95% confidence interval zP Intrasexual aggression rates CRR (quadratic contrasts) 0.10 0.39, 0.18 0.71 0.48 CRR (linear contrasts) 0.12 0.17, 0.41 0.82 0.41 Population 0.070 0.31, 0.16 0.61 0.54 Sex 0.27 0.024, 0.56 1.80 0.072 CV of individual size 0.94 1.54, 0.34 3.06 0.0022 Foraging rates CRR (linear contrasts) 0.072 0.48, 0.33 0.35 0.73 Population 0.40 0.74, 0.069 2.36 0.018 Sex 0.29 0.038, 0.62 1.74 0.083 CRR is defined here as the ratio of individual competitors over the number of food patches available. Figure 2. Mean (6SE, N ¼ 30) foraging rate per trial in relation to 3 CRRs (4 individuals competing for 5, 3, and 1 food patches, respectively 0.8, 1.33, 4) Figure 1. Mean (6SE, N ¼ 30) aggression rate, sum of given and received, per and 2 populations of origin: high predation (HP; open diamonds) and low pre- trial in relation to 3 CRR (4 individuals competing for 5, 3, and 1 food patches, dation (LP; shaded squares)in (A) males and (B) females. respectively 0.8, 1.33, 4) and 2 populations of origin: high predation (HP; open diamonds) and low predation (LP; shaded squares; low predation) in (A) males and (B) females. possible competing pairs. However, our main goal was to compare the relative aggression across treatments rather than estimating the size within a trial (CV; standard deviation/mean for each trial) as a total aggression within a given trial. Second, we analyzed total for- covariate to take into account size differences within groups. We aging rates fitted to a negative binomial distribution. Since we based used the principal component of standard length and weight of indi- our tests on a priori predictions, we did not apply any statistical cor- viduals as a proxy for size. As expected, standard length and weight rection to our tests. were highly correlated (98%). We used trial number as a random factor in all GLMM analyses. First, using GLMM, we tested total aggression rate per trial Results (given and received aggression summed up per trial) fitted to a nega- Contrary to our 3 first predictions, overall aggression rates (Table 2 tive binomial distribution. Using focal individual observations allowed us to estimate the total per capita rates of aggression, which and Figure 1) were not significantly affected by CRR, population of were summed for the 4 fish to estimate total aggression in the trial. origin, sex, nor their interactions (Appendix 1). However, aggres- This total aggression for 5 min will underestimate the total aggres- sion rates increased as CV of individual size decreased within a trial sion by 50% on average compared with simultaneously watching all (Table 2). Consistent with our fourth prediction, following the risk- 4 fish for 5 min. Out of the 6 possible pairs of competing individu- allocation model (Lima and Bednekoff 1999), foraging rates were als, we only recorded aggression given and received for the focal higher in the high than in the low predation population (Table 2 and individual during a 5-min period, which accounts for 3 of the 6 Figure 2). However, CRR, sex, and the interactions of the 3 Downloaded from https://academic.oup.com/cz/advance-article-abstract/doi/10.1093/cz/zox078/4797078 by Ed 'DeepDyve' Gillespie user on 12 July 2018 Chuard et al.  Foraging competition in guppies 5 above-mentioned factors (Appendix 2) had no significant effects on coefficient of variation of net aggression) between CRRs (see foraging rate (Table 2 and Figure 2). Chuard 2017). Because an increase in density leads to an increase in aggression rates (Magurran and Seghers 1991) but a decrease in the number of food patches does not (our results), seasonal changes in Discussion guppy density might be more ecologically relevant to guppies than changes in food abundance. Indeed, there is no strong evidence that Overall, our results support 1 of our 4 original predictions (Table 1). food availability varies across seasons in Trinidadian streams Surprisingly, CRR, sex, and population of origin did not influence (Magurran 2005; but see Reznick 1989). aggression rates among guppies competing for access to foraging patches. Rather, foraging rates followed the risk-allocation model (Lima and Bednekoff 1999) with higher foraging rates in the high Sex versus the low predation population. These results suggest that We found no difference in aggression and foraging rates between decreasing food availability at a constant competitor density does not males and females. While male guppies forage just enough to satisfy affect aggression rates in guppies. However, the effect of elevated their immediate hunger (Griffiths 1996), female guppies devote a ambient predation risk seems to favor individuals able to forage greater portion of their time budget to foraging (Magurran and more when an imminent risk is absent. As expected (Parker 1974), Seghers 1994), presumably to produce eggs, and to match the ener- aggression rates increased as size differences between competing indi- getic requirements associated with indeterminate growth (Magurran viduals decreased. These conclusions should be tested in future stud- 2005). Given that individuals fasted for 24 h before testing, it is pos- ies using more populations as our experimental design only included sible that a 10-min observation period was not sufficient for males 2 of them. The composition of the predatory community of these 2 to start reducing their foraging rates, and associated aggression, rivers, among other environmental factors, might be specific to them. compared with females. For example, after at least a 3-h fast, male For instance, the freshwater prawn, M. crenulatum, was not found in guppies switched from primarily feeding to courting after about the Upper Aripo River in previous studies (Magurran 2005) but is 10 min [see Figure 3 in Abrahams (1993)]. now found. This recent invasion might have had an effect on how this specific population responds to ambient predation. Population differences These results contrast with those of Chuard et al. (2016) where We found that the low and high predation populations showed simi- both CRR and ambient predation risk had an effect on aggression lar levels of aggression. These results do not support the risky com- rates in a mating competition context (see Figure 1 in Chuard et al. petition hypothesis (Chuard et al. 2016; see also Magurran and 2016). Indeed, aggression rates increased as CRR increased, and Seghers 1991). However, those 2 same populations did differ in low-predation guppies were more aggressive than their high- aggression rates related to mating competition (Chuard et al. 2016). predation counterparts as expected under the risky-competition As courtship displays have the potential to attract predators (Zuk hypothesis (Chuard et al. 2016). However, similar to our findings in and Kolluru 1998), aggression might be more likely traded-off for a foraging context, males and females did not differ significantly in antipredator behavior in a mating rather than a foraging competi- their aggression rates (Chuard et al. 2016). The most notable differ- tion context. The absence of a difference in aggression rates between ence between the 2 experiments was the observed rates of aggres- the 2 populations might also be due to the predator assemblage of sion, which were more than 3 times greater in the food- rather than each population. Indeed, guppy populations have been shown to be the mating-competition experiment once we corrected for methodo- adapted to their local environments along a continuous environmen- logical differences (i.e., scanning vs. focal observations, trial length). tal gradient, where the predatory community plays an important Perhaps fixed food patches are easier to monopolize and defend role (Torres-Dowdall et al. 2012). The 2 populations we used are than mobile mates, resulting in a greater pay-off for individuals who not located at the extreme ends of the predation risk gradient invest energy in aggressive behavior when competing for food. encountered in nature (Torres-Dowdall et al. 2012), and thus preda- tion risk might not be different enough to cause differences in Competitor-to-resource ratio aggression rates related to foraging. Unlike Magurran and Seghers (1991), we found no effect of CRR on Foraging rates between populations were consistent with the pre- aggression rate. However, Magurran and Seghers (1991) manipu- dictions of the risk allocation theory in the absence of an imminent lated CRR in one of their 4 experiments (i.e. “the effect of tank size risk of predation (Lima and Bednekoff 1999); high-predation indi- and group size”) by increasing the number of competitors foraging viduals foraged more than low-predation conspecifics. In the on a single food patch, whereas we decreased the number of resour- absence of an imminent risk of predation, individuals from a high ces (i.e., food patches) while holding the number of competitors con- versus low ambient predation risk site seem to compensate for lost stant. A possible explanation for this discrepancy is that aggression foraging opportunities during previous periods of high imminent rates increased significantly above a CRR of 4 in Magurran and predation risk. Future studies should replicate this experiment with Seghers’ (1991) experiment, while 4 was the highest CRR in our the addition of imminent predation risk treatments to validate this study. Future studies should also try to disentangle the effects of hypothesis. However, due to the cost of predation, high ambient CRR, fish density, and food abundance, by using higher CRRs while predation risk guppies seem to spend less time foraging than their keeping the number of individuals constant (i.e., more than 4 indi- low ambient predation risk counterparts (Magurran and Seghers viduals per trial), as well as test aggression rates at equal CRRs but 1994), suggesting selection for higher foraging rates under high varying fish density (e.g., Clark and Grant 2010). In addition, the ambient predation risk, and more time spent on antipredator behav- food patches might have been close enough together in our experi- ior. Given the smaller size of individuals from high versus low pre- ment such that the dominant fish could defend them against 3 other dation risk sites (Magurran 2005), foraging more to sustain higher competitors. This explanation is supported by the high rates of growth rates in low predation risk females is not likely. As males aggression in our experiment compared with Chuard et al. (2016), stop growing after sexual maturity (Magurran 2005), this explana- and the absence of differences in dominance structure (i.e., tion is even less likely for them. Downloaded from https://academic.oup.com/cz/advance-article-abstract/doi/10.1093/cz/zox078/4797078 by Ed 'DeepDyve' Gillespie user on 12 July 2018 6 Current Zoology, 2018, Vol. 0, No. 0 Our conclusions cannot be limited to the effect of ambient preda- References tion risk alone, as low ambient predation risk streams tend to have Abrahams MV, 1993. The trade-off between foraging and courting in male higher guppy densities (as a direct effect of predation) and lower guppies. Anim Behav 45:673–681. productivity (Grether et al. 2001), resulting in higher competition Archer J, 1988. The Behavioural Biology of Aggression. Cambridge: for food. Differences in productivity could then act as a factor select- Cambridge University Press. Baird TA, Baird TD, 2006. Phenotypic plasticity in the reproductive behavior ing for high versus low productivity-adapted behaviors (Walsh and of female sand tilefish Malacanthus plumieri. Ethology 112:52–63. Reznick 2010). These differences in the intensity of foraging compe- Bassar RD, Marshall MC, Lo ´ pez-Sepulcre A, Zandona ` E, Auer SK et al., tition between sites lead to differences in diets. As a response to 2010. Local adaptation in Trinidadian guppies alters ecosystem processes. intense competition, guppies from low-predation localities tend to Proc Natl Acad Sci USA 107:3616–3621. feed on proportionally more low-quality food than their high- Botham MS, Hayward RK, Morrell LJ, Croft DP, Ward JR et al., 2008. predation sites counterparts [Bassar et al. 2010; Zandona ` et al. Risk-sensitive antipredator behavior in the Trinidadian guppy Poecilia retic- 2011; but see Zandona ` et al. (2017) for a counterargument during ulata. Ecology 89:3174–3185. the rainy season]. Because they feed on low-quality food, we would Brown GE, Bongiorno T, DiCapua DM, Ivan LI, Roh E, 2006. Effects of group expect these individuals to forage more than conspecifics from high- size on the threat-sensitive response to varying concentrations of chemical alarm cues by juvenile convict cichlids. Can J Zool 84:1–8. predation sites in order to meet their metabolic requirements. These Brown JL, 1964. The evolution of diversity in avian territorial systems. Wilson adaptations could lead to different energy allocation trade-offs Bull 76:160–169. between foraging competition and antipredator behavior, opposite Charnov EL, 1993. Life History Invariants. Oxford: Oxford University Press. to the effect of the risk allocation theory. Indeed, contrary to our Chuard PJC, 2017. Competition in Trinidadian guppies, Poecilia reticulata: results, individuals inhabiting low-productivity streams (low preda- effects of competitor-to-resource ratio, sex, resource type, and tempo of pre- tion) should invest more energy in foraging and competing for forag- dation risk [doctoral thesis]. Canada: Concordia University. ing opportunities, and less energy into antipredator behavior Chuard PJC, Brown GE, Grant JWA, 2016. The effects of adult sex ratio on compared with populations living in high-productivity streams (high mating competition in male and female guppies Poecilia reticulata in two predation; Magurran and Seghers 1991; but see Kolluru et al. 2007). wild populations. Behav Process 129:1–10. Clark L, Grant JWA, 2010. Intrasexual competition and courtship in female While the 2 populations did not differ in rates of aggression (see and male Japanese medaka Oryzias latipes: effects of operational sex ratio above), guppies from the low-predation/low-productivity popula- and density. Anim Behav 80:707–712. tion had lower foraging rates than their high-predation/high produc- Clutton-Brock TH, Parker GA, 1992. Potential reproductive rates and the tivity counterparts. These results suggest that differences in density, operation of sexual selection. Q Rev Biol 67:437–456. productivity, and diet between the 2 populations are not likely Croft DP, Morrell LJ, Wade AS, Piyapong C, Ioannou CC et al., 2006. explanations for the observed competitive foraging patterns. Predation risk as a driving force for sexual aggregation: a cross-population In conclusion, relative food density did not seem to affect intra- comparison. Am Nat 167:867–878. sexual aggression rates in guppies. Ambient predation risk reflects, de Jong K, Forsgren E, Sandvik H, Amundsen T, 2012. Measuring mating competition correctly: available evidence supports operational sex ratio at least in part, the long-term exposure to imminent predation risk theory. Behav Ecol 23:1170–1177. (Brown et al. 2006). Individuals experiencing high levels of ambient Emlen ST, Oring LW, 1977. Ecology, sexual selection, and the evolution of predation risk do not always face high imminent predation threats, mating systems. Science 197:215–223. as they can select habitats that lower this risk (e.g., Main et al. Ferrari MCO, Sih A, Chivers DP, 2009. The paradox of risk allocation: a 1996). Conversely, individuals facing low ambient predation risk review and prospectus. Anim Behav 78:579–585. can still face high imminent predation risk, such as just prior to a Gorlick DL, 1976. Dominance hierarchies and factors affecting dominance in predator attack. Consequently, it would be of great value to explore the guppy Poecilia reticulata (Peters). Anim Behav 24:336–346. aggression rates in relation to food abundance under varying inten- Grand TC, Grant JWA, 1994. Spatial predictability of food influences its sities of imminent predation risk to fully investigate the risk alloca- monopolization and defence by juvenile convict cichlids. Anim Behav 47: 91–100. tion hypothesis, in populations that also vary in ambient predation Grant JWA, Gaboury CL, Levitt HL, 2000. Competitor-to-resource ratio, a risk. general formulation of operational sex ratio, as a predictor of competitive aggression in Japanese medaka (Pisces: Oryziidae). Behav Ecol 11:670–675. Grant JWA, Girard IL, Breau C, Weir LK, 2002. Influence of food abundance Acknowledgments on competitive aggression in juvenile convict cichlids. Anim Behav 63: We are grateful to Anne-Christine Auge, Glaeson Ramnarine, Pierre-Jean 323–330. Recondo, and Drs Heather Auld, Chris Elvidge, Jean-Guy Godin, and Indar Grether GF, Millie DF, Bryant MJ, Reznick DN, Mayea W, 2001. Rain forest W. Ramnarine for their help in the field. We also thank Dr Guillaume canopy cover, resource availability, and life history evolution in guppies. Larocque for his expertise in statistics, as well as the Director of Fisheries in Ecology 82:1546–1559. the Trinidadian Ministry of Agriculture, Land, and Marine Resources for per- Griffiths SW, 1996. Sex differences in the trade-off between feeding and mat- mission to sample individuals from the Aripo River and use them in our study. ing in the guppy. J Fish Biol 48:891–898. All work reported herein was conducted in accordance with guidelines of the Heinen JL, Coco MW, Marcuard MS, White DN, Peterson MN et al., 2013. Canadian Council on Animal Care and the laws of Canada, and was Environmental drivers of demographics, habitat use, and behavior during a approved by the Concordia University Animal Research Ethics Committee. post-Pleistocene radiation of Bahamas mosquitofish Gambusia hubbsi. Evol Ecol 27:971–991. Herczeg G, Valimaki K, 2011. Intraspecific variation in behaviour: effects of Funding evolutionary history, ontogenetic experience and sex. J Evol Biol 24: This work was financially supported by Concordia University (Faculty of Arts 2434–2444. and Science Graduate Fellowship [to P.J.C.C.]) and the Natural Sciences and Hodge SJ, Thornton A, Flower TP, Clutton-Brock TH, 2009. 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Downloaded from https://academic.oup.com/cz/advance-article-abstract/doi/10.1093/cz/zox078/4797078 by Ed 'DeepDyve' Gillespie user on 12 July 2018 8 Current Zoology, 2018, Vol. 0, No. 0 Appendix Table A1. Results of the interactions of the GLMM testing for the effects of CRR (quadratic and linear contrasts), population of origin (lower: high risk versus upper Aripo: low risk), and sex on intrasexual aggression rates in Trinidadian guppies Interaction Regression coefficient 95% confidence interval zP CRR (quadratic contrasts)population 0.014 0.39, 0.42 0.070 0.95 CRR (linear contrasts)population 0.038 0.45, 0.37 0.18 0.86 CRR (quadratic contrasts)sex 0.31 0.099, 0.72 1.49 0.14 CRR (linear contrasts)sex 0.069 0.34, 0.48 0.33 0.74 Populationsex 0.072 0.26, 0.40 0.43 0.67 CRR (quadratic contrasts)populationsex 0.047 0.53, 0.62 0.16 0.87 CRR (linear contrasts)populationsex 0.15 0.73, 0.42 0.52 0.60 CRR is defined here as the ratio of individual competitors over the number of food patches available. Table A2. Results of the interactions of the GLMM testing for the effects of CRR (linear contrasts), population of origin (lower: high risk ver- sus upper Aripo: low risk), and sex on foraging rates in Trinidadian guppies Interaction Regression coefficient 95% confidence interval zP CRR (linear contrasts)population 0.13 0.71, 0.45 0.45 0.65 CRR (linear contrasts)sex 0.23 0.80, 0.34 0.80 0.42 Populationsex 0.29 0.76, 0.18 1.21 0.23 CRR (linear contrasts)populationsex 0.021 0.80, 0.84 0.050 0.96 CRR is defined here as the ratio of individual competitors over the number of food patches available. Downloaded from https://academic.oup.com/cz/advance-article-abstract/doi/10.1093/cz/zox078/4797078 by Ed 'DeepDyve' Gillespie user on 12 July 2018

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