The neutrophil to lymphocyte ratio indexes individual variation in the behavioural stress response of wild roe deer across fluctuating environmental conditions Abstract Understanding how wild animals adapt to perturbations and their consequences for life history traits and population dynamics is a current focus of attention in ecology and conservation biology. Here, we analysed variation in the neutrophil to lymphocyte ratio (N:L ratio), a proxy of stress level, in wild roe deer Capreolus capreolus from three populations experiencing markedly different environmental conditions. We first assessed whether among-individual differences in the N:L ratio were consistent over time and across environmental contexts. We then investigated how both individual characteristics (behaviour at capture, age, sex, body mass), and environmental context (habitat and year quality) were linked to this indicator of stress level. We found moderate, but consistent, repeatability of the N:L ratio in all three populations, indicating stable among-individual differences in the way individuals cope physiologically with varying environmental conditions. In addition, we found a weak, but consistent, association between the N:L ratio and behaviour at capture in two of the three populations. Finally, the N:L ratio also varied in relation to temporal changes in environmental conditions. In particular, individuals had, on average, higher levels of stress in poor-quality years in two of the three populations. We discuss our results in relation to the coping styles framework. Significance statement Due to global change, natural populations are increasingly faced with unpredictable fluctuations of their environment. The stress response, via the release of glucocorticoids, is a key mechanism that enables individuals to cope with these variations. However, all individuals do not necessarily cope with life threatening and/or stressful situations in the same way, but as yet, the major drivers underlying variation in stress level remain unclear. We showed that the N:L ratio, reflecting baseline stress level, was repeatable and influenced by both individual and environmental factors. In particular, variation in the N:L ratio was linked to the quality of the year in terms of resource availability and, to a lesser extent, to the individual’s behaviour at capture. Our study demonstrates that both environmental context and individual characteristics drive variation in the N:L ratio in a wild vertebrate population.

Elevation-related differences in the age structure of breeding birds suggest stronger selection at harsher elevations Abstract How environmental variation affects the strength of selection is important for understanding phenotypic variation within a population. An indirect method to evaluate differences in selection pressures is to compare the age composition of a breeding population. A larger adult/first-year breeding ratio, due to higher juvenile mortality, may indicate stronger selection. We investigated the adult/first-year breeding ratios of food-caching mountain chickadees inhabiting different elevations. Chickadees at high elevations experience harsher winter conditions and cache more food items and have better spatial cognition to recover caches compared with their low-elevation counterparts. Cache recovery and spatial cognition are critical for overwinter survival and individuals with better spatial cognitive abilities are more likely to survive their first winter; therefore, these differences are likely a result of stronger selection pressures on spatial cognition and caching behavior at higher elevations. In support of this hypothesis, we found a significantly larger adult/first-year breeding ratio at high elevations compared with low elevations. This may suggest higher overwinter juvenile mortality and hence stronger selection pressures on survival-related traits at high elevations, consistent with elevation-related phenotypic differences. Significance statement The strength of selection on survival-related traits varies across heterogenous environments. One way to assess the strength of selection is to measure the traits of individuals that are able to survive a particular selection event. Here we assessed the adult/first-year breeding ratios of resident, food-caching birds experiencing differentially harsh winter climates. These birds rely on spatial cognition to retrieve food caches and survive the winter; the largest annual selection event. At the individual level, those with better cognitive ability have demonstrated higher overwinter survival. However, at the population level, it remains unclear whether or not the strength of selection differs between harsh and mild winter climates. By assessing the age of breeders, we found a significantly larger adult/first-year breeding ratio in birds inhabiting harsher, high elevations compared with milder, low elevations. This suggests higher overwinter juvenile mortality and hence stronger selection pressures on survival-related traits at high elevations.

Coping with style: individual differences in responses to environmental variation Abstract Between-individual differences in coping with stress encompass neurophysiological, cognitive and behavioural reactions. The coping style model proposes two alternative response patterns to challenges that integrate these types of reactions. The “proactive strategy” combines a general fight-or-flight response and inflexibility in learning with a relatively low HPA (hypothalamic–pituitary–adrenal) response. The “reactive strategy” includes risk aversion, flexibility in learning and an enhanced HPA response. Although numerous studies have investigated the possible covariance of cognitive, behavioural and physiological responses, findings are still mixed. In the present study, we tested the predictions of the coping style model in an unselected population of bank voles (Myodes glareolus) (N = 70). We measured the voles’ boldness, activity, speed and flexibility in learning and faecal corticosterone metabolite levels under three conditions (holding in indoor cages, in outdoor enclosures and during open field test). Individuals were moderately consistent in their HPA response across situations. Proactive voles had significantly lower corticosterone levels than reactive conspecifics in indoor and outdoor conditions. However, we could not find any co-variation between cognitive and behavioural traits and corticosterone levels in the open field test. Our results partially support the original coping style model but suggest a more complex relationship between cognitive, behavioural and endocrine responses than was initially proposed. Significance statement Understanding the proximate mechanisms regulating the individual variation in responses to environmental challenges and changes is fundamental in ecological and evolutionary research. Theory predicts correlations between behavioural, cognitive and physiological traits to form alternative strategies named coping styles but recent studies report contrasting and mixed findings. We examined the relationship between a measure of endocrine state (concentrations of faecal glucocorticoid metabolites), two behavioural traits (boldness and activity) and two cognitive traits (speed and flexibility of learning) in 70 unselected bank voles (Myodes glareolus) under three different conditions. The findings partially support the original coping style model’s hypothesis and predictions. We found individual consistency of all traits. However, correlations between behavioural and cognitive aspects and endocrine state were found only in two of the three tested conditions, highlighting the need for further investigations and testing of theory.

Water turbidity constrains male mating success in a marine fish Abstract Human-induced eutrophication, resulting in increased algal growth and water turbidity, is an alarming problem in aquatic systems. Many studies have focused on the effects of algal turbidity on mate choice and sexual selection in fish, but little emphasis has been given to the ways it can constrain mating success. Here we experimentally investigated the effects of algal turbidity on maximum male mating success and parental care in the sand goby, Pomatoschistus minutus, a fish with a resource-defence mating system and male parental care. For this purpose, we introduced to 1 nest-holding male 5 random-sized ripe females in either clear or in turbid water. After spawning, we observed how many mates and eggs the male received and followed his parental behaviour and egg survival for 6 days under turbid or clear water conditions. When spawning took place in clear water, the number of eggs the male received into his nest increased with the total weight of five females in his tank. However, when spawning took place in turbid water, there was no relationship between female size and the number of eggs laid, although the number of females that spawned was the same as in clear water. The results indicate that females adjust the number of eggs they lay according to water turbidity. This could explain previous findings that mating success is more evenly distributed among males in turbid than clear water conditions. Significance statement The first responses of animals to human-induced changes in the environment are behavioural. Subtle changes in the behaviour of individuals can have profound consequences for populations and communities. Human-induced eutrophication, leading to algal blooms and water turbidity, is a major environmental problem in aquatic systems worldwide. Our results on the sand goby suggest a new mechanism by which water turbidity may affect fish mating systems and weaken sexual selection. When spawning takes place in clear water, the number of eggs accumulated in a males’ nest is an increasing function of the fecundity of the females. However, when spawning in turbid water, this positive relationship between female size and egg numbers disappears. We believe this is because females do not perceive the competition from other females in turbid water and therefore invest less in present reproduction.

Male aggression varies with consortship rate and habitat in a dolphin social network Abstract Coalitions and alliances exemplify the core elements of conflict and cooperation in animal societies. Ecological influences on alliance formation are more readily attributed to within-species variation where phylogenetic signals are muted. Remarkably, male Indo-Pacific bottlenose dolphins in Shark Bay, Western Australia, exhibit systematic spatial variation in alliance behavior, not simply within a species or population, but within a single social network. Moving SE-NW along Peron Peninsula in Shark Bay, males ally more often in trios than pairs, consort females more often, and exhibit greater seasonal movements. Ecological models predict more male-male conflict in the north, but sufficient observations of aggression are lacking. However, dolphins often incur marks, in the form of tooth rakes, during conflicts. Here we report that the incidence of new tooth rake marks varies systematically in the predicted pattern, with greater marking in the north, where males form more trios and consort females at a higher rate. While our previous work demonstrated that alliance complexity has an ecological component, we can now infer that ecological variation impacts the level of alliance-related conflict in Shark Bay. Significance statement To understand ecological influences on animal societies, researchers have focused on differences within species, where confounds due to evolutionary history are minimized. Such differences are usually found among geographically separated populations, but in Shark Bay, Western Australia, male dolphin alliance size and access to females increase along a spatial axis within a single social network. Here we report that aggression levels, evidenced by tooth rake marks, increase along the same axis. Alliances are of particular interest as they represent a complex kind of relationship, often implicated in the evolution of social intelligence. Our discovery of spatial variation in alliance behavior and aggression within a social network provides a unique opportunity to investigate the intersection of cognition, social structure, and ecology.

The role of social network behavior, swimming performance, and fish size in the determination of angling vulnerability in bluegill Abstract An individual’s behavioral and physiological characteristics can have important impacts on fitness, including during interactions with humans. For example, certain traits (metabolic rate, boldness, etc.) have been shown to impact angling vulnerability in fish targeted by recreational anglers. While prior work has focused on boldness behavior and several metrics of metabolic performance, the role of two critical traits, social behavior and swimming performance, have rarely been directly examined. To address this gap, we conducted a study utilizing bluegill Lepomis macrochirus, a highly popular sportfish species found throughout much of North America, to determine the relationship between social behavior, swimming performance, fish size, and angling vulnerability. One hundred and seven bluegill were assessed for social behavior in a laboratory setting, using scoring methods derived from social network analysis. Bluegill were then assessed for swimming performance (critical swimming speed, Ucrit) before being angled in a naturalistic pond setting over nine daily angling sessions. Following angling, a subset of fish were left uncaptured (N = 28), were captured only once (N = 68), or were captured twice (N = 11). Both fish total length and swimming performance were positively linked with vulnerability to initial capture, with fish length also being linked to vulnerability to recapture. In addition to length, social behavior (higher sociability and lower aggression) was positively linked to vulnerability to recapture. Collectively, these results indicate that the drivers of angling vulnerability shift as angled fish populations gain more experience with lures, and that for bluegill, the most vulnerable individuals are likely to be larger and highly social. Significance statement Individuals within a species show several differences in their behavior and physiology. These differences may have major consequences for fitness, especially in environments impacted by humans. One example of this is freshwater fish targeted by recreational anglers, where individuals with certain behavioral or physiological traits might be more likely to be caught. In this study, we assessed the social behavior, aggression, and swimming performance of bluegill Lepomis macrochirus before angling them in a naturalistic pond setting. We found that larger size and higher swimming performance were linked to vulnerability to initial capture, while length and higher sociability caused fish to be more vulnerable to being caught a second time. Collectively, this means these traits may evolve as a result of selective harvest and also that the drivers of vulnerability may change after fish gain experience with anglers.

Evidence for non-random co-occurrences in a white shark aggregation Abstract Groups or aggregations of animals can result from individuals being attracted to a common resource or because of synchronised patterns of daily or seasonal activity. Although mostly solitary throughout its distribution, white sharks (Carcharodon carcharias) seasonally aggregate at a number of sites worldwide to feed on calorie-rich pinnipeds. At the Neptune Islands, South Australia, large numbers of white sharks can be sighted throughout the year, including during periods of low seal abundance. We use a combination of photo-identification and network analysis based on co-occurrence of individuals visiting the site on the same day to elucidate the population structure and aggregatory behaviour of Australia’s largest aggregation of sub-adult and adult white sharks. We photo-identified 282 sharks (183 males, 97 females, 2 unknown) over a 4.5-year period (June 2010–November 2014) and found that white sharks did not randomly co-occur with their conspecifics, but formed four distinct communities. Tendency to co-occur varied across months with males co-occurring with more individuals than females. Sex-dependent patterns of visitation at the Neptune Islands and resulting intraspecific competition likely drive the observed community structure and temporal variability in co-occurrences. This study provides new insights into the aggregatory behaviour of white sharks at a seal colony and shows for the first time that white shark co-occurrence can be non-random. Significance statement White sharks are top marine predators that are typically solitary but can also form aggregations around seal colonies to feed. Using a combination of photo-identification and network analysis, we investigated the co-occurrence patterns of white sharks. We showed, for the first time, that white sharks form non-random associations with conspecifics. We hypothesise that the observed sex-dependant variations in co-occurrence are linked to intraspecific competition for resources, providing new insights into the aggregatory behaviour of white sharks at a seal colony. Our study adds to the growing literature of animals showing non-random interaction or co-occurrence patterns, providing important comparative knowledge for other researchers as it broadens our knowledge to marine top predators.

Intersexual conflict influences female reproductive success in a female-dispersing primate Abstract In group-living mammals, individual efforts to maximize reproductive success result in conflicts and compromises between the sexes. Females utilize counterstrategies to minimize the costs of sexual coercion by males, but few studies have examined the effect of such behaviors on female reproductive success. Secondary dispersal by females is rare among group-living mammals, but in western gorillas, it is believed to be a mate choice strategy to minimize infanticide risk and infant mortality. Previous research suggested that females choose males that are good protectors. However, how much female reproductive success varies depending on male competitive ability and whether female secondary dispersal leads to reproductive costs or benefits has not been examined. We used data on 100 females and 229 infants in 36 breeding groups from a 20-year long-term study of wild western lowland gorillas to investigate whether male tenure duration and female transfer rate had an effect on interbirth interval, female birth rates, and offspring mortality. We found that offspring mortality was higher near the end of males’ tenures, even after excluding potential infanticide when those males died, suggesting that females suffer a reproductive cost by being with males nearing the end of their tenures. Females experience a delay in breeding when they dispersed, having a notable effect on birth rates of surviving offspring per female if females transfer multiple times in their lives. This study exemplifies that female counterstrategies to mitigate the effects of male-male competition and sexual coercion may not be sufficient to overcome the negative consequences of male behavior. Significance Individual reproductive success is maximized through conflicts and compromises between the sexes in social mammals. In species with high dimorphism, females can experience sexual coercion by males and develop counterstrategies to reduce these costs. We studied this link in western lowland gorillas, which exhibit female secondary dispersal, a strategy that is believed to reflect female choice for the protective abilities of males. We found that females are faced with the dilemma of staying with a silverback at the end of his tenure and risk higher infant mortality versus dispersing and suffering reproductive delays and lower birth rates. This study shows that female reproductive strategies, namely dispersal, used to counter the effects of sexual coercion by males are not sufficient to overcome the negative consequences of male behavior.

Overmarking behaviour of zebra males: no scent masking, but a group cohesion function across three species Abstract Among many hypotheses in the literature that explain overmarking in mammals, most studies favour a sexual selection hypothesis. However, results in the literature are conflicting. In this study, we tested two hypotheses that could explain overmarking by males: (i) as a part of sexual selection, more specifically to mask scent of receptive females and (ii) as a form of communication serving to aid group cohesion. We observed each of the three zebra species in eight different herds at four zoos. In total, we recorded 1395 eliminations (760 defecations, 635 urinations) performed by 78 individuals including 8 stallions. Stallions investigated 248 eliminations and overmarked 124. The rate of overmarking by stallions was higher than those of all other sex and age categories. Stallions of all species overmarked all age and sex categories, except Grévy’s zebra stallions did not overmark foal eliminations. In contrast to our first hypothesis, when attracted to the elimination, stallions overmarked non-oestrus females more often than oestrus ones. Thus, our results did not support the hypothesis that overmarking by males has a sexual selection function, but it could be explained by the group cohesion hypothesis. Based on our results, it seems that overmarking by equid males plays a greater role in intra-specific communication than in intra-sexual competition. In addition, this behaviour might play different roles in different species based on their social organisation. Significance statement Overmarking is ubiquitous among terrestrial mammals; however, the function of this behaviour has not been fully explained. In addition, previous studies came to differing conclusions. In this study, we tested two hypotheses possibly explaining overmarking by adult males in three equid species. We found that adult males overmarked at a much higher rate than individuals of all other sex and age categories. In contrast to many other studies, our results did not support the hypothesis that overmarking by males has a sexual selection function, but it could be explained by a group cohesion hypothesis.

Galápagos lava lizards ( Microlophus bivittatus ) respond dynamically to displays from interactive conspecific robots Abstract In many species, outcomes of male duels determine access to females and, ultimately, male reproductive success. Ritualization of behavior in male contests can reduce the probability of injury, which benefits both contestants. Components of ritualized combat often include postures and displays that showcase a male’s quality in a sequential assessment of fighting ability. Among the most common contest acts in iguanine lizards are bobbing displays. Investigations of bobbing display dynamics often include experimental “playbacks,” in which video or robotic representations of conspecifics are presented to subjects. In most “playback” research, pre-programmed stimuli exhibit behavior that is independent of subjects’ responses, despite the fact that actual animal contests are highly interactive. In the present study, we utilized a robotic Galápagos lava lizard (Microlophus bivittatus) to investigate the importance of interaction in simulated contests under field conditions. Using a matched pairs design where each subject experienced two behavioral variants of the robotic stimulus, we tested the effect of a robot that displayed immediately following a subject’s display versus when the same robot display was postponed 30 s. Results showed that immediate response from the robot stimulated subjects to display significantly more often than when the stimulus was delayed. We speculate that subjects perceived a rapid response from their robotic contestant as being more aggressive than a delayed response. We discuss our results in light of relevant previous work, and we suggest possibilities for future research using interactive lizard robots. Significance statement Some of the most impressive examples of ritualized animal behavior can be observed in male contests for access to reproductive females. The use of stereotyped displays in such duels allows males to assess one another’s quality while avoiding dangerous fighting that can lead to injury. For example, males in many lizard species perform bobbing displays where contestants respond to each other in reciprocal fashion. In this study, we used a realistic lizard robot as a stand-in for a contestant in simulated contests. We found that an immediate display response by the robot to a subject’s bobbing display stimulated subjects to display significantly more often than when the robot’s response was delayed by 30 s. To our knowledge, this is the first interactive robot “playback” experiment with lizards. Future research will further explore the “rules” underlying display behavior in lizard contests.