Abstract Animal personality traits are defined as consistent individual differences in behavior over time and across contexts. Occasionally this inflexibility results in maladaptive behavioral responses to external stimuli. However, in social groups inflexible behavioral phenotypes might be favored as this could lead to more predictable social interactions. Two hypotheses seek to describe the optimal distribution of personality types within groups. The social niche specialization hypothesis states that individuals within groups should partition social roles, like personality types, to avoid conflict; whereas the conformity hypothesis states that individuals should assort with conspecifics of similar personality. However, no research so far has compared these hypotheses using data from wild animal systems. We tested boldness in the wild on 2 species with different social systems, the Mexican Jay and California Scrub-Jay. We found support for the conformity hypothesis over the social niche specialization hypothesis because individuals within groups of the social species had more similar personalities, and consequently there was a statistically significant group effect. The most likely mechanism for this conformity is social learning of behaviors through development, but more explicit research on this is needed. INTRODUCTION The evolution of animal personalities is not yet well understood. An animal personality trait is defined as a behavior that differs among individuals, but is consistent within an individual over time and across contexts (Gosling 2001). Much research has quantified personality traits in different populations and contexts, particularly along the bold/shy spectrum (Carter et al. 2013). At the population level, variation in a personality trait within a species can lead to population persistence under changing environmental conditions (e.g. Nicolaus et al. 2016), increased niche packing through microhabitat choice (e.g. Pruitt and Goodnight 2014), and alternative reproductive strategies (Smith and Blumstein 2008), or foraging decisions (Sih et al. 2004). Individual personality results from genetic and experiential components (Brown et al. 2007; Edenbrow et al. 2017) leading to consistent, constrained behavioral responses, and this can cause non-adaptive individual behavior in certain situations. For example, high aggression could lead to successfully obtaining food or defending a territory, but could also result in failure to maintain a mate relationship (Johnson and Sih 2005). Additionally, individuals with a strong propensity to take risks (boldness) might be more likely to exploit a new foraging patch or resource, but might also be more likely to be in an exposed position and attacked by a predator (Bell 2005; Smith and Blumstein 2008). To explain how this occasionally maladaptive inflexibility evolved, researchers believe that constrained individual behaviors might be particularly beneficial to species that live in social groups (Bergmüller and Taborsky 2010; Wolf et al. 2011). If the behavior of a group mate is consistent over time and across different contexts then the result of future interactions can be predicted based on past experience with that individual (Dall et al. 2004). The distribution of personality types in groups can affect fitness. For example, guppies put into mixed bold–shy groups in the lab experienced higher rates of feeding than fish in only bold, or only shy shoals (Dyer et al. 2009). Conflicting personality types within groups can have large negative effects on the fitness of the whole group in addition to the individuals involved in the conflict (Oliveira et al. 2001; Sapolsky and Share 2004; Wascher et al. 2008). Consequently, social feedback should exert strong pressure on the distribution of personality types within a group, causing individuals to assort non-randomly based on personality. There are 2 opposing hypotheses that describe the optimal assortment of personality types in group-living species. The “social niche specialization hypothesis” (hereafter SNS) states that systems in which individuals maintain relatively partitioned and stable social roles might show reduced conflict over resources such as food and reproductive opportunities (Bergmüller and Taborsky 2010; Montiglio et al. 2013). Alternatively, the “conformity hypothesis” (hereafter CON) states that groups will be composed of individuals with similar personality types (positive assortment). This could occur when individuals assort with conspecifics of similar personality type (Both et al. 2005; Aplin et al. 2013), or when individuals conform their behaviors to that of group mates (King et al. 2014). A key prediction of both hypotheses is that the repeated interactions with group mates will lead to maintenance of assortment via selective immigration and emigration, or social learning of behaviors during development. As such, we would expect to find the best support for either of these hypotheses in animal groups that exhibit stable compositions, long-term interactions, and low levels of immigration or emigration. Furthermore, comparing two species that differ in the number of consistent social interactions experienced during development can elucidate the importance of this factor as a mechanism (Bergmüller and Taborsky 2010). Several studies have tested for SNS, but were conducted in lab environments with artificially manipulated, temporary social groups (Modlmeier et al. 2014; Bierbach et al. 2017; von Merten et al. 2017). SNS was inferred from greater individual repeatability of behavior (larger between- than within-individual variance) in control groups of familiar individuals than in the manipulated unfamiliar groups. However, this experimental setup assumes short-term group dynamics approximate the natural development of groups assorted by personality. It is possible that unfamiliar groups of adults that cannot disperse would lead to individuals behaving unnaturally or erratically. In addition, individuals do not always react to a stimulus in the same way in social and asocial contexts (Koski and Burkart 2015). Therefore, an important step towards a better understanding of the evolution of personality is to conduct research that quantifies individual personalities, and distribution of personalities in natural populations. We tested for non-random assortment in wild animals by conducting 2 assays of boldness on Mexican Jays (Aphelocoma wollweberi; hereafter MEJA) and California Scrub-Jays (Aphelocoma californica; hereafter CASJ). These 2 species inhabit similar scrub-oak and pine habitat, but differ in social behavior. MEJA live in stable groups year-round on an all-purpose territory. Kin and non-kin help the breeding pairs feed nestlings (Brown 1972) and defend the territory against predators and territory intruders (Brown 1994). Low levels of immigration seem to be sufficient for inbreeding avoidance such that the average relatedness within a group is low (Brown and Brown 1981). There are between 5–25 jays in overlapping generations in each group, and the group territory is maintained indefinitely with a slow turnover in group members. This system could allow for increased stability and persistence of personality assortment because social interactions with group mates over several years of development may strongly shape personality traits of young jays. If so, there could be exaggeration in the degree of the trait within groups over time (Wolf et al. 2011). In contrast, CASJ are solitary monogamous breeders. Young CASJ disperse from the natal territory shortly after independence, so development of personality is likely to occur in the absence of consistent social interactions. Additionally, CASJ mate pairs are unlikely to be genetically closely related, so any pattern in personality assortment more likely results from active mate choice (Gabriel and Black 2012). Once a territory and mate are acquired, pairs remain together for life (given successful nesting attempts), so it is also possible that limited plasticity of adult personality traits might result in weakly assortative or disassortative pairings by personality (Westneat et al. 2015; Edenbrow et al. 2017). As such, if the number and timing of consistent social interactions are an important influencing factor above and beyond genetic factors, then the two different social systems would lead to distinct effects on individual, and group or pair personality. The SNS and CON hypothesis yield contrasting predictions about the direction of the effect of social interactions on personality development (Figure 1). By definition, an individual personality trait results in decreased within-individual variance in behavior for increased response consistency (Dingemanse and Dochtermann 2013; Dochtermann and Dingemanse 2013). In addition, if the SNS hypothesis is supported there will be high between-individual variance, and no difference in group average personality score across groups. In contrast, the CON hypothesis would predict low between-individual variance in personality scores among group mates, and a significant difference in group average scores across groups. In our system, to find support for either hypothesis we specifically tested: 1) if there are significant differences in average boldness among groups (MEJA) or pairs (CASJ), 2) whether there is low between-individual variance in boldness scores within groups or pairs of jays, and 3) whether the number of early social interactions plays an important mechanistic role, as evidenced by MEJA showing smaller within-individual variance than CASJ, which have few consistent social interactions. Figure 1 View largeDownload slide Predicted individual boldness estimates (points) and residual variance (whiskers) under each hypothesis. Under the conformity hypothesis there would be a significant group effect and low between-individual variance. Under social niche specialization, there would be no between-group differences, and high between-individual variance. For either hypothesis, species would differ in that California Scrub-Jays will have greater residual variance than Mexican Jays if social interactions are important for development of consistent personality traits. Figure 1 View largeDownload slide Predicted individual boldness estimates (points) and residual variance (whiskers) under each hypothesis. Under the conformity hypothesis there would be a significant group effect and low between-individual variance. Under social niche specialization, there would be no between-group differences, and high between-individual variance. For either hypothesis, species would differ in that California Scrub-Jays will have greater residual variance than Mexican Jays if social interactions are important for development of consistent personality traits. METHODS Subjects We conducted boldness assessments on 55 adult MEJA around the Southwestern Research Station in Portal, Arizona from May to November 2015. These individuals were naturally grouped into 7 distinct flocks ranging from 6 to 15 jays (mean 7.86). We also assessed boldness of 35 adult CASJ on 20 stable territories in Willamette Mission State Park in Keizer, Oregon from April 2014 to November 2016. All jay subjects of both species were trained to come to a whistle for peanuts, and received the same trapping and handling procedure to apply unique color band combinations. All methods were approved by the University of Washington Institutional Animal Care and Use Committee (protocol #4064-03), and the appropriate permitting agencies (Bird Banding Lab #22802, Oregon Department of Fish and Wildlife #015-16, Oregon State Parks #007-14, US Fish and Wildlife Service #MB51894B-0, Arizona Game and Fish Department #SP697293). We experimentally measured jay boldness with novel object approach (NOA) and flight initiation distance (FID) assessments. We chose these 2 types of assessments because they are commonly used measures of boldness, but they measure boldness in 2 different contexts (e.g. Carter et al. 2010; Jolles et al. 2013). The NOA assessment measures boldness towards a potential threat in a foraging context, whereas the FID assessment measures boldness in a non-foraging threat context. Novel object approach NOA assessments for each species included both control and experimental trials that varied only in the novelty of the object. During experimental trials, a yellow plastic duck, approximately 4 inches in length and height, was placed on the ground within three concentric layers of peanuts on the territory of each subject (Figure 2a). During control trials, the duck was replaced with a rock of similar size and shape, so that the only novel feature was the non-random arrangement of peanuts. The first layer of peanuts was placed touching the central object such that one peanut was touching each side (front, back, left, right). The next layer of peanuts was placed 8 inches (20 cm) further away, but in line with the first layer. The third and last layer of peanuts was placed an additional 8 inches away from the middle layer, 16 inches (40cm) from the central object, and in line with the other 2 peanuts. We used peanuts that appeared to be the same size and shape, and placement of peanuts was random so there would be no correlation between nutritional value and distance from the central object. Figure 2 View largeDownload slide Visual depiction of the methods for measuring boldness: (a) Novel object approach, bolder jays take peanuts from closer to the novel object (i.e. the rubber duck), and (b) Flight initiation distance, jays that allow a human to walk closer before flushing are more bold. Figure 2 View largeDownload slide Visual depiction of the methods for measuring boldness: (a) Novel object approach, bolder jays take peanuts from closer to the novel object (i.e. the rubber duck), and (b) Flight initiation distance, jays that allow a human to walk closer before flushing are more bold. To maintain the novelty of the assessment, we conducted only one trial a day per territory and no more than two total experimental and control trials on each territory. Trials lasted up to 15 min, or until all peanuts were taken, and we randomized the order of control and experimental trials on each territory. Before each trial, we called jays in to the center of the territory and checked for motivation to engage in the task by tossing 2 peanuts on the ground. If jays quickly came down to get these peanuts then we set up the assessment and began the trial. Jays were allowed to take multiple peanuts per trial as a group. Wild MEJA typically move around the territory as a unit and will predominantly encounter novelty as a group (Brown 1963; Koski and Burkart 2015), so our setup is evolutionarily and ecologically relevant. However, it is possible that the order in which individuals within a group approach to take peanuts is not based on boldness, but rather dictated by a dominance hierarchy and resource monopolization behaviors. Previous research on group-foraging decisions in MEJA found that jays will preferentially wait to join group mates of lower dominance rank at a food source in order to scrounge (McCormack et al. 2007). Therefore, the dominance hierarchy can produce consistency in the order which jays approach novel food sources. To account for this possible confound, we quantified to what extent order of first approach to within 2 m of the setup was repeatable across the 2 experimental trials. Additionally, for our measure of boldness we quantified the closest approach distance in centimeters to the central object, including jays that never took a peanut. Jays that never approached to 2 m away were excluded from the analyses (n = 3) because at greater distances we could not exclude confounding possibilities for the decision not to approach, such as lack of food motivation or distraction. Similarly, CASJ pairs were allowed to simultaneously take peanuts during the NOA assessment. We repeated the NOA assessment on 18 CASJ, 9 months after the first trial. Due to a more time-limited field season, we repeated NOA trials on 16 MEJA from 3 flocks 4–11 weeks after the first experimental trial. Flight initiation distance We also conducted up to 7 FID assessments on each banded jay throughout the year. When a solitary, color-banded jay was encountered sitting un-obscured at a low height (between 0 and 3 meters high), and not engaged with food, the experimenter directed his or her gaze towards the jay’s face (Eason et al. 2006) and walked towards the jay at a normal pace. When the jay flushed, the experimenter stopped and measured the distance between his or her location and the jay’s original location (Figure 2b). Smaller FID values indicate a bolder jay that is comfortable letting a potential predator (human) approach closer. Only jays with more than one FID measure (n = 47 MEJA, n = 28 CASJ) were included in repeatability analyses. Statistical analyses FID values, after log-transformation, and residuals were normally distributed (CASJ: Shapiro–Wilk W = 0.98, P = 0.08; MEJA: W = 0.99, P = 0.87), so we used linear regression for all models with FID as the dependent variable. Closest approach (in cm) to the novel object or rock is a count variable, so we used Poisson regression for all NOA analyses. All models included jay ID as a random effect, and all analyses were conducted in RStudio Version 1.0.136 (RStudio Team 2015). To ensure our NOA assessment was eliciting a neophobic response, we used a generalized linear mixed-effect model in the “lme4” package (Bates et al. 2015) to measure the effect of treatment (duck vs. rock) on closest approach. To verify that our methods were capturing an aspect of personality, we quantified repeatability of individual responses in both assessments across time. We only used data from birds with more than one measure and modeled repeatability using the rpt function in the package “rptR” (Nakagawa and Schielzeth 2010). This function uses mixed-effect modeling approaches to partition variance, and calculates repeatability as the ratio of variance among individuals divided by total variance (among individual plus residual variance). The rpt function also conducts parametric bootstrapping for confidence intervals, and likelihood ratio tests for P values for each estimate. Repeatability analyses for each boldness method in each species resulted in 4 models (2 species and 2 boldness metrics) where the dependent variables were the repeated measures of individual boldness scores. Within each trial on each MEJA flock or CASJ pair, the closest approach distance was significantly related to the order that peanuts were taken (MEJA: ß = −0.29, z = −4.6, P < 0.01; CASJ: ß = −1.94, z = −11.17, P < 0.01). Additionally, we found evidence that the dominance hierarchy affected the order that MEJA approached the NOA assessment because order of first approach was significantly repeatable for individual MEJA across the 2 trials (n = 16, R = 0.48, P = 0.02), but not for CASJ (n = 18, R = 0, P = 1). It is possible that jays in larger groups are more bold in their responses (Stamps and Groothuis 2010). However, group size as a group-level fixed effect did not improve the fit of NOA or FID models already including flock ID (NOA: χ2 = 0.65, P = 0.42; FID: χ2 = 1.97, P = 0.16). Therefore, we included a fixed effect of individual order to take a peanut in all NOA repeatability analyses. We did not find a significant effect of sex on CASJ boldness (FID: t = −1.24, P = 0.23; NOA: t = −1.70, P = 0.10). Unfortunately, it was logistically impossible to determine sex in all MEJA individuals during the time of our field season because only a few jays per group build nests each breeding season, and there are no other behavioral signals of sex (Brown 1994). As such, we did not include fixed effects of sex in our models. Previous research has not shown a consistent effect of sex on animal personality (Titulaer et al. 2012; Koski and Burkart 2015; van Horik et al. 2017), and there is low sexual dimorphism in these jay species so it is unlikely we would see consistent sex differences in boldness. Additionally, all data from hatch year birds were excluded from analyses because of the possibility that personality traits are relatively more plastic at this age (Stamps and Groothuis 2010). To test whether our methods measured the same aspect of boldness across contexts, we compared the 2 types of boldness scores to each other using Pearson’s correlation coefficient. In this analysis, we treated each individual as a single observation, and calculated the average of the repeated boldness measures recorded on a single bird. We determined significance using the cor.test function in the “stats” base package. To assess how boldness scores are distributed within groups (MEJA flocks and CASJ pairs), we again used the rpt function to build on our repeatability models by including group ID as an additional random effect. In this way we were able to partition variance in boldness into among group, among individuals within a group, and residual variance in order to test for support of the SNS or CON hypotheses, and to determine whether number of consistent social interactions relates to within-individual variance. Here, we only used data from birds with more than one data point and we also only used CASJ pairs with data on both birds (n = 18 jays). We used the P values and confidence intervals output by the rpt function to determine the repeatability of each random effect in these models. We also determined significance of the random group effect from chi-square values of likelihood ratio tests that compared the change in log-likelihood of a model with the random group effect to one without it. RESULTS Novel object approach Our NOA assessment elicited a neophobic response. Individuals of both species had significantly smaller closest approach distances (MEJA ß = 1.63, z = 25.13, P < 0.01; CASJ ß = 2.86, z = 18.48, P < 0.01), indicating more bold responses, during the control trial when the rock was in the middle (MEJA n = 38, mean ± SE = 8.08 ± 1.99 cm; CASJ n = 26, 1.73 ± 0.73 cm) than in the experimental trial with the duck (MEJA n = 35, 52.26 ± 11.27 cm; CASJ n = 33, 37.94 ± 11.1 cm). This, combined with consistently jumpy and nervous behavior in the presence of the duck but not the rock (personal observation), confirms that the duck stimulus was aversive and jays classified it as a threat (Greggor et al. 2015). After including order of approach as a fixed effect to account for that potential confound, we found that NOA distance was repeatable for CASJ (n = 18, R = 0.43, P = 0.04) and for MEJA (n = 16, R = 0.42, P = 0.04). Flight initiation distance The number of FIDs collected per jay did not differ by species. We collected between 2–7 FIDs for MEJA (n = 45, mean ± SE = 3.39 ± 0.18) and 2–6 for CASJ (n = 28, mean = 3.11 ± 0.2; z = 0.5, P = 0.62). Surprisingly, when comparing average individual scores from the 2 boldness methods, our results suggest that the methods were not measuring the same aspect of personality. There was no significant correlation between FID and NOA for either species (MEJA n = 34, r = 0.05, P = 0.79; CASJ n = 28, r = −0.11, P = 0.57). FID score did not change in relation to breeding season. We classified the breeding season as March–July, and non-breeding season as August–February. Looking only at jays with FIDs from both seasons, we found no difference in FID scores by breeding season (t-test: MEJA n = 18, t = 1.23, P = 0.22; CASJ n = 17, t = 0.77, P = 0.44), so FID data from both seasons were combined for each jay. Repeatability of CASJ FID was extremely low (mean = 21.47 ± 0.74 m; R = 0, P = 1; Figure 3), indicating this may be an unsuitable paradigm for this particular species. In contrast, MEJA FID scores were highly repeatable (mean = 11.12 ± 0.63 m; R = 0.44, P < 0.001). Figure 3 View largeDownload slide The top row of plots shows individual repeatability and 95% confidence intervals of each boldness method (Individual) and the change in repeatability after the group effect is included (Adjusted). The bottom row of plots shows the partitioning of variance components with 95% confidence intervals for the repeatability models that include the group effect. Mexican Jay repeatability drops after group is included because between-individual variance is now accounted for by group membership (support for Conformity hypothesis). Figure 3 View largeDownload slide The top row of plots shows individual repeatability and 95% confidence intervals of each boldness method (Individual) and the change in repeatability after the group effect is included (Adjusted). The bottom row of plots shows the partitioning of variance components with 95% confidence intervals for the repeatability models that include the group effect. Mexican Jay repeatability drops after group is included because between-individual variance is now accounted for by group membership (support for Conformity hypothesis). Boldness distribution We found support for the CON hypothesis over the SNS hypothesis. There was a significant group effect leading to group repeatability and clustering of scores within groups for both MEJA FID (R = 0.38; χ2 = 16.5, P < 0.001) and MEJA NOA (R = 0.54; χ2 = 9.62, P = 0.002) boldness scores. There was no significant group effect for either method in CASJ (FID: R = 0; χ2 = 0, P = 0.5; NOA: R = 0; χ2 = 0, P = 1). This indicates that MEJA overlap in individual boldness scores within a group, and groups differ in average boldness (Figure 4). If frequent social interactions with consistent partners have an impact on personality development, then social species will have smaller within-individual variance components (more repeatable traits) than asocial species (Bergmüller and Taborsky 2010; Webster and Ward 2011). However, we did not find a significant difference in species NOA within-individual variance because all confidence intervals overlap (Figure 3), but species did significantly differ in the FID within-individual component. Due to the unrepeatability of CASJ FID resulting from the extremely large within-individual variance component, this result should be interpreted with caution. Figure 4 View largeDownload slide Scatterplot of jay boldness scores by group. This figure illustrates the clustering of boldness around a group-level mean in Mexican Jays, but not California Scrub-Jays. Dots represent the average score of an individual, and darker areas indicate multiple individuals overlapping in scores. Vertical lines through dots represent standard error of the mean for jays with multiple measures. Jays with only one measure are present in the novel object plots as circles with no vertical lines. Figure 4 View largeDownload slide Scatterplot of jay boldness scores by group. This figure illustrates the clustering of boldness around a group-level mean in Mexican Jays, but not California Scrub-Jays. Dots represent the average score of an individual, and darker areas indicate multiple individuals overlapping in scores. Vertical lines through dots represent standard error of the mean for jays with multiple measures. Jays with only one measure are present in the novel object plots as circles with no vertical lines. DISCUSSION In this experiment, we are among the first to quantify the distribution of personality traits among and within groups of wild animals. These results indicate that the higher frequency of repeated social interactions in MEJA did not lead to partitioning and specialization of degree of boldness, but instead MEJA show evidence for conformity. In contrast, the distribution of CASJ mate-pair boldness scores shows no support for either hypothesis. Our conclusions are based on data from only 2 species, but this information adds to our understanding of how inflexible behavioral phenotypes could be favored through evolutionary time for group-living species. Lastly, our study extends previous research describing animal personality because our subjects were wild, free-flying individuals so it is likely the results from the natural behaviors we quantified are more ecologically relevant than lab-based studies of boldness (Webster and Ward 2011). We found that both methods to measure boldness produced consistent, repeatable results in MEJA, but only the NOA assessment produced repeatable results in CASJ. FID was not repeatable for CASJ, potentially because the open and flat park environment introduced more variance in starting distance. Experimenters could sometimes see CASJ from further away than what was usual in MEJA and were therefore walking towards jays for longer. One way to control for this in future research might be to quantify experimenter starting distance and number of steps taken before the animal moves, as well as the distance to the animal’s original location after it moves. Additionally, species differences in FID repeatability might arise if individual behavioral consistency is more important in MEJA; because they evolved in larger groups with more coordinated behaviors, the need to predict behavioral responses of others may be higher (Wolf et al. 2011). NOA and FID were not correlated, suggesting that these are not equivalent measures of boldness. NOA could potentially be a measure of exploration (Réale et al. 2007), or FID could be measuring activity level (Carter et al. 2012). These results indicate that future studies should take great care in choosing methods for assessing personality traits of interest, and emphasizes the need for researchers to use multiple different methods as well as ecological correlates of personality (territory size, daily activity patterns) to validate their measures (Carter et al. 2013). There was a significant effect of MEJA group membership on boldness scores, and this indicates some level of conformity where group mates’ boldness scores are similarly clustered around a specific flock mean (Figure 4). In group living, cooperatively breeding species like MEJA, conformity, and predictability of behavior could be adaptive for increasing and maintaining altruistic cooperation. The evolution of cooperation among unrelated individuals has been hypothesized to occur in species that exhibit long-term interactions, consistent individual variation in behavior, and the ability to detect and reject cheaters (McNamara et al. 2008). In species that exhibit conformity, there would be little need to develop costly mechanisms for detecting cheaters (Riolo et al. 2001). As such, personality conformity should facilitate cooperation in non-kin group-living species, an effect we also see in cooperatively breeding common marmosets (Koski and Burkart 2015), as well as cooperative dyadic interactions of guppies (Croft et al. 2009), and chimpanzees (Massen and Koski 2014). Due to the evolutionary distance between fish, mammalian and avian taxa, this could indicate a general trend towards personality conformity in altruistic cooperative social groups or pairs. We found no evidence that CASJ are specializing or conforming in their boldness with their mate (see Supplementary Material). Due to the unrepeatability of CASJ FID, we cannot assume we accurately measured personality with this metric, and so do not interpret these results with respect to our hypotheses. However, results from the CASJ NOA assessment can be interpreted, and the distribution of scores appears random; there is no significant group effect as expected under conformity, and the within-individual variance component is not significantly different from the between-individual variance, contrary to predictions under SNS. Unlike in other systems that exhibit personality conformity related to dyad-level cooperation (Croft et al. 2009; Massen and Koski 2014), the cooperation evident in CASJ is not altruistic. Behavioral coordination to raise young and defend the territory yields selfish, as well as cooperative benefits. Therefore, selection for consistent and similar boldness traits may be less strong (McNamara et al. 2008). Nevertheless, mate choice in other avian systems does show evidence for non-random positive assortment (Both et al. 2005; Gabriel and Black 2012) or negative assortment (Houtman and Falls 1994) by personality. Consequently, it is likely that choice of mate in our CASJ population occurs more opportunistically in response to a vacancy in a breeding position (Curry et al. 2002), or could be related to factors such as territory quality or availability, and physical appearance (Overeem et al. 2014). Multiple mechanisms could explain the assortment of personality scores in MEJA. First, previous research has found that personality traits can be heritable (van Oers et al. 2005). Since young MEJA delay dispersal, it is possible that groups are comprised of highly related individuals that share the genetic predisposition for degree of boldness. However, early research on this population found that relatedness within groups was low due to the presence of unrelated immigrant breeders, and ranged from 0.02 to 0.22 (Brown and Brown 1981). Similarly, the level of emigration and immigration seems to be too low to create a high level of conformity if individuals are directing dispersal towards groups with similar phenotypes. Variability in presence of certain ecological components across territories could also shape group-level personality (Bell 2005). At the time of the study, the territories of all of the 7 flocks were contained within an approximately 3.5 km square area. As such, it is unlikely that the flocks experienced ecological differences between their territories large enough to shape distinct flock personalities. Therefore, the most parsimonious explanation is that the within-group conformity likely occurs through social learning and/or social facilitation of behavior (Edenbrow et al. 2017) during juvenile development. Boldness, or propensity to take risks, could be a trait that is particularly susceptible to a mechanism of learning so that individuals are optimally adapted to the particular risks in their local environment. For example, previous research across taxa has shown that observation of conspecific mobbing behavior leads to social learning of novel threats (Curio et al. 1978; Cook et al. 1985; Mathis et al. 1996). A coordinated mobbing response to a visible predator is important to successfully repel it, so it would be adaptive for young jays to learn similar behavioral responses towards novel and potentially threating objects from group members (Croft et al. 2009; Edenbrow et al. 2017). Our study improves on previous research on the evolution of personality because we measured boldness in wild, free-flying animals, and compared results across closely related species with differing degrees of consistent social interactions. Future research should include additional assays to investigate the relationship between food-related and predator-related response to risk. Furthermore, experimental manipulation and longitudinal tracking of individuals would elucidate the relationship between development of individual and group-level variation in personality traits, as well as whether certain group average levels of boldness relate to higher fitness. SUPPLEMENTARY MATERIAL Supplementary data are available at Behavioral Ecology online. FUNDING This work was supported by the University of Washington Psychology Department, the National Science Foundation Graduate Research Fellowship Program, the National Science Foundation Graduate Research Internship Program, and the grant “Narodowe Centrum Nauki (N304 138440)”. The authors thank Corina Logan and Jonathon Valente for manuscript comments and discussion. We thank the Oregon State Parks Department and the staff at Willamette Mission State Park, in particular. We also thank the University of Washington Psychology Department, the director and staff of the American Natural History Museum’s Southwestern Research Station, the director of the Museum and Institute of Zoology PAS, and Prof Wieslaw Bogdanowicz for logistical support. We thank our many student assistants in the field, especially Wonyoung Lee, Choongwon Jeong, Carly Batist, and Brice Lawley for help on data collection beyond color banding. Data accessibility: Analyses reported in this article can be reproduced using the data provided by McCune et al. (2018). REFERENCES Aplin LM , Farine DR , Morand-Ferron J , Cole EF , Cockburn A , Sheldon BC . 2013 . Individual personalities predict social behaviour in wild networks of great tits (Parus major) . Ecol Lett . 16 : 1365 – 1372 . Google Scholar CrossRef Search ADS PubMed Bates D , Maechler M , Bolker B , Walker S . 2015 . Fitting linear mixed-effects models using lme4 . J of Stat Soft . 67 : 1 – 48 . 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Behavioral Ecology – Oxford University Press
Published: Apr 18, 2018
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