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Understanding the causes and consequences of repeatable among-individual differences in behavior (i.e., animal personality) is a major area of research in behavioral and evolutionary ecology. Recently, attention has turned to understanding the processes behind changes in repeatability through ontogeny because of their implications for populations. We evaluated the relative importance of selective disappearance (i.e., differential mortality), an among-individual mechanism, in generating age-related changes in the repeatability of aggression and activity in juvenile North American red squirrels (Tamiasciurus hudsonicus). We observed age-related decreases in the repeatability of aggression across ages, arising from lower among-individual variance. Although we found evidence for directional selection on aggressiveness, it was insufficient to erode among-individual variance. Thus, ontogenetic decreases in the repeatability of aggression do not appear to be due to selective disappearance. In contrast, the repeatability of activity was higher across ages due to higher among-individual variance in activity, but there was no support for selective disappearance based on activity. Taken together, our results suggest that age-related changes in trait repeatability in red squirrels are not the result of selective disappearance and instead may be the result of within-individual developmental processes, such as individual differences in developmental trajectories.

Organisms can affect one another's phenotypes when they socially interact. Indirect genetic effects occur when an individual's phenotype is affected by genes expressed in another individual. These heritable effects can enhance or reduce adaptive potential, thereby accelerating or reversing evolutionary change. Quantifying these social effects is therefore crucial for our understanding of evolution, yet estimates of indirect genetic effects in wild animals are limited to dyadic interactions. We estimated indirect phenotypic and genetic effects, and their covariance with direct effects, for the date of spring breeding in North American red squirrels (Tamiasciurus hudsonicus) living in an array of territories of varying spatial proximity. Additionally, we estimated indirect effects and the strength of selection at low and high population densities. Social effects of neighbours on the date of spring breeding were different from zero at high population densities but not at low population densities. Indirect phenotypic effects accounted for a larger amount of variation in the date of breeding than differences attributable to the among-individual variance, suggesting social interactions are important for determining breeding dates. The genetic component to these indirect effects was however not statistically significant. We therefore showcase a powerful and flexible method that will allow researchers working in organisms with a range of social systems to estimate indirect phenotypic and genetic effects, and demonstrate the degree to which social interactions can influence phenotypes, even in a solitary species.

Background Our understanding of gut microbiota has been limited primarily to findings from human and laboratory animals, but what shapes the gut microbiota in nature remains largely unknown. To fill this gap, we conducted a comprehensive study of gut microbiota of a well-studied North American red squirrel (Tamiasciurus hudsonicus) population. Red squirrels are territorial, solitary, and live in a highly seasonal environment and therefore represent a very attractive system to study factors that drive the temporal and spatial dynamics of gut microbiota. Result For the first time, this study revealed significant spatial patterns of gut microbiota within a host population, suggesting limited dispersal could play a role in shaping and maintaining the structure of gut microbial communities. We also found a remarkable seasonal rhythm in red squirrel’s gut microbial composition manifested by a tradeoff between relative abundance of two genera Oscillospira and Corpococcus and clearly associated with seasonal variation in diet availability. Our results show that in nature, environmental factors exert a much stronger influence on gut microbiota than host-associated factors including age and sex. Despite strong environmental effects, we found clear evidence of individuality and maternal effects, but host genetics did not seem to be a significant driver of the gut microbial communities in red squirrels. Conclusion Taken together, the results of this study emphasize the importance of external ecological factors rather than host attributes in driving temporal and spatial patterns of gut microbiota in natural environment. Electronic supplementary material The online version of this article (10.1186/s40168-017-0382-3) contains supplementary material, which is available to authorized users.

Nearly all animals interact with members of their own species at some point during their lives. These behavioral interactions range from courtship, mating, and parental care to the complex cooperative behavior among related or unrelated individuals in group-living species. A number of theoretical models have attempted to explain how cooperation can evolve through natural selection. Although tremendously influential in animal behavior research, these traditional models have largely ignored individual variation in cooperative behavior and its underlying developmental and proximate mechanisms. However, a set of emerging models suggest that the evolution of cooperation can be heavily influenced by the degree of individual variation in cooperative behavior, as well as the complexity of the underlying mechanisms. Yet, while theoreticians argue the importance of studying individual variation in cooperation and the mechanisms underlying it, empiricists have not focused upon these aspects. The main objectives of our symposium at the 2017 meeting of the Society for Integrative and Comparative Biology is to establish new research avenues to study variation in cooperative behavior using both proximate and ultimate explanations and to produce a road map to study the developmental and proximate mechanisms in generating individual variation in cooperative behavior. This symposium brought together empiricists and theoreticians investigating cooperative behavior in diverse taxa and across multiple levels of analysis. Here we briefly describe the rationale for this symposium and why we thought it was needed as well as provide a brief overview of the contributions.

The National Environment Research Council (RG53472 to T.H.C-B.), the European Research Council (294494 to T.H.C-B.), the University of Zurich and the Mammal Research Institute at the University of Pretoria.

Quantifying how whole organisms respond to challenges in the external and internal environment (“stressors”) is difficult. To date, physiological ecologists have mostly used measures of glucocorticoids (GCs) to assess the impact of stressors on animals. This is of course too simplistic as Hans Seyle himself characterized the response of organisms to “noxious stimuli” using multiple physiological responses. Possible solutions include increasing the number of biomarkers to more accurately characterize the “stress state” of animal or just measuring different biomarkers to more accurately characterize the degree of acute or chronic stressors an animal is experiencing. We focus on the latter and discuss how heart rate (HR) and heart rate variability (HRV) may be better predictors of the degree of activation of the sympathetic–adrenal–medullary system and complement or even replace measures of GCs as indicators of animal health, welfare, fitness, or their level of exposure to stressors. The miniaturization of biological sensor technology (“bio-sensors” or “bio-loggers”) presents an opportunity to reassess measures of stress state and develop new approaches. We describe some modern approaches to gathering these HR and HRV data in free-living animals with the aim that heart dynamics will be more integrated with measures of GCs as bio-markers of stress state and predictors of fitness in free-living animals.

Elevations in glucocorticoid levels (GCs) in breeding females may induce adaptive shifts in offspring life histories. Offspring produced by mothers with elevated GCs may be better prepared to face harsh environments where a faster pace of life is beneficial. We examined how experimentally elevated GCs in pregnant or lactating North American red squirrels (Tamiasciurus hudsonicus) affected offspring postnatal growth, structural size, oxidative stress levels (two antioxidants and oxidative protein damage) in three different tissues (blood, heart, liver), and liver telomere lengths. We predicted that offspring from mothers treated with GCs would grow faster but would also have higher levels of oxidative stress and shorter telomeres, which may predict reduced longevity. Offspring from mothers treated with GCs during pregnancy were 8.3% lighter around birth but grew (in body mass) 17.0% faster than those from controls, whereas offspring from mothers treated with GCs during lactation grew 34.8% slower than those from controls and did not differ in body mass around birth. Treating mothers with GCs during pregnancy or lactation did not alter the oxidative stress levels or telomere lengths of their offspring. Fast-growing offspring from any of the treatment groups did not have higher oxidative stress levels or shorter telomere lengths, indicating that offspring that grew faster early in life did not exhibit oxidative costs after this period of growth. Our results indicate that elevations in maternal GCs may induce plasticity in offspring growth without long-term oxidative costs to the offspring that might result in a shortened lifespan.Summary StatementWe show that experimental increases in glucocorticoids in breeding female North American red squirrels affects offspring postnatal growth but not levels of oxidative damage and antioxidants or telomere lengths.

Museum genomics has transformed the field of collections-based research, opening up a range of new research directions for paleontological specimens as well as natural history specimens collected over the past few centuries. Recent work demonstrates that it is possible to characterize epigenetic markers such as DNA methylation in well preserved ancient tissues. This approach has not yet been tested in traditionally prepared natural history specimens such as dried bones and skins, the most common specimen types in vertebrate collections. In this study, we developed and tested methods to characterize cytosine methylation in dried skulls up to 76 years old. Using a combination of ddRAD and bisulphite treatment, we characterized patterns of cytosine methylation in two species of deer mouse (Peromyscus spp.) collected in the same region in Michigan in 1940, 2003, and 2013-2016. We successfully estimated methylation in specimens of all age groups, although older specimens yielded less data and showed greater interindividual variation in data yield than newer specimens. Global methylation estimates were reduced in the oldest specimens (76 years old) relative to the newest specimens (1-3 years old), which may reflect post-mortem hydrolytic deamination. Methylation was reduced in promoter regions relative to gene bodies and showed greater bimodality in autosomes relative to female X chromosomes, consistent with expectations for methylation in mammalian somatic cells. Our work demonstrates the utility of historic specimens for methylation analyses, as with genomic analyses; however, studies will need to accommodate the large variance in the quantity of data produced by older specimens.

Phenotypic plasticity—one individual’s capacity for phenotypic variation under different environments—is critical for organisms facing fluctuating conditions within their lifetime. North American red squirrels (Tamiasciurus hudsonicus) experience drastic among-year fluctuations in conspecific density. This shapes juvenile competition over vacant territories and overwinter survival. To help young cope with competition at high densities, mothers can increase offspring growth rates via a glucocorticoid-mediated maternal effect. However, this effect is only adaptive under high densities, and faster growth often comes at a cost to longevity. While experiments have demonstrated that red squirrels can adjust hormones in response to fluctuating density, the degree to which mothers differ in their ability to regulate glucocorticoids across changing densities remains unknown—little is known about within-individual plasticity in endocrine traits relative to among-individual variation. Findings from our reaction norm approach revealed significant individual variation not only in a female red squirrel’s mean endocrine phenotype, but also in endocrine plasticity in response to changes in local density. Future work on the proximate and ultimate drivers of variation in the plasticity of endocrine traits and maternal effects is needed, particularly in free-living animals experiencing fluctuating environments.

SummaryOne of the outstanding questions in evolutionary biology is the extent to which mutually beneficial interactions and kin-selection can facilitate the evolution of cooperation by mitigating conflict between interacting organisms. The indirect fitness benefits gained from associating with kin are an important pathway to conflict resolution [1], but conflict can also be resolved if individuals gain direct benefits from cooperating with one another (e.g. mutualism or reciprocity) [2]. Owing to the kin-structured nature of many animal societies, it has been difficult for previous research to assess the relative importance of these mechanisms [3–5]. However, one area that might allow for the relative roles of kin-selection and mutualistic benefits to be disentangled is in the resolution of conflict over territorial space [6]. While much research has focused on group-living species, the question of how cooperation can first be favoured in solitary, territorial species remains a key question. Using 22 years of data from a population of North American red squirrels, we assessed how kinship and familiarity with neighbours affected fitness in a territorial mammal. While living near kin did not enhance fitness, familiarity with neighbours increased survival and annual reproductive success. These fitness benefits were strong enough to compensate for the effects of aging later in life, with potential consequences for the evolution of senescence. We suggest that such substantial fitness benefits provide the opportunity for the evolution of cooperation between adversarial neighbours, offering insight into the role that mutually beneficial behaviours might play in facilitating and stabilizing social systems.Graphical Abstract

Individual natal dispersal behaviour is often difficult to predict as it can be influenced by multiple extrinsic and intrinsic factors. Individual differences in personality have been shown to be an important correlate of dispersal behaviour. However, the relationships between personality traits and dispersal are often inconsistent within and across studies and the causes of these discrepancies are often unknown. Here we sought to determine how individual differences in activity and aggression, as measured in an open-field trial, were related to natal dispersal distance in a wild population of North American red squirrels (Tamiasciurus hudsonicus). For 14 cohorts, while individual aggression consistently had no association with dispersal distance, the association between activity and dispersal fluctuated through time, mediated by population density. The environmental-dependence of the relationship between personality and dispersal in this population is indicative of the importance of considering external conditions when predicting dispersal behaviour.

Summary One of the outstanding questions in evolutionary biology is the extent to which mutually beneficial interactions and kin selection can facilitate the evolution of cooperation by mitigating conflict between interacting organisms. The indirect fitness benefits gained from associating with kin are an important pathway to conflict resolution, 1 but conflict can also be resolved if individuals gain direct benefits from cooperating with one another (e.g., mutualism or reciprocity). 2 Because of the kin-structured nature of many animal societies, it has been difficult for previous research to assess the relative importance of these mechanisms. 3 , 4 , 5 However, one area that might allow for the relative roles of kin selection and mutualistic benefits to be disentangled is in the resolution of conflict over territorial space. 6 Although much research has focused on group-living species, the question of how cooperation can first be favored in solitary, territorial species remains a key question. Using 22 years of data from a population of North American red squirrels, we assessed how kinship and familiarity with neighbors affected fitness in a territorial mammal. Although living near kin did not enhance fitness, familiarity with neighbors increased survival and annual reproductive success. These fitness benefits were strong enough to compensate for the effects of aging later in life and have potential consequences for the evolution of senescence. We suggest that such substantial fitness benefits provide the opportunity for the evolution of cooperation between adversarial neighbors, offering insight into the role that mutually beneficial behaviors might play in facilitating and stabilizing social systems.

There is tremendous diversity in ageing rates and lifespan not only among taxa but within species, and particularly between the sexes. Women often live longer than men, and considerable research on this topic has revealed some of the potential biological, psychological and cultural causes of sex differences in human ageing and lifespan. However, sex differences in lifespan are widespread in nonhuman animals suggesting biology plays a prominent role in variation in ageing and lifespan. Recently, evolutionary biologists have borrowed techniques from biomedicine to identify whether similar mechanisms causing or contributing to variation in ageing and lifespan in humans and laboratory animals also operate in wild animals. Telomeres are repetitive noncoding DNA sequences capping the ends of chromosomes that are important for chromosomal stability but that can shorten during normal cell division and exposure to stress. Telomere shortening is hypothesized to directly contribute to the ageing process as once telomeres shorten to some length, the cells stop dividing and die. Men tend to have shorter telomeres and faster rates of telomere attrition with age than women, suggesting one possible biological cause of sex differences in lifespan. In this issue of Molecular Ecology, Watson et al. (2017) show that telomere lengths in wild Soay sheep are similar between females and males near the beginning of life but quickly diverge with age because males but not females showed reduced telomere lengths at older ages. The authors further show that some of the observed sex difference in telomere lengths in old age may be due to male investment in horn growth earlier in life, suggesting that sexually dimorphic allocation to traits involved in sexual selection might underlie sex differences in telomere attrition. This study provides a rare example of how biological mechanisms potentially contributing to sex differences in lifespan in humans may also operate in free-living animals. However, future studies using a longitudinal approach are necessary to confirm these observations and identify the ultimate and proximate causes of any sex differences in telomere lengths. Collaborations between evolutionary biologists and gerontologists are especially needed to identify whether telomere lengths have a causal role in ageing, particularly in natural conditions, and whether this directly contributes to sex differences in lifespan.

The phenotype of parents can have long-lasting effects on the development of offspring as well as on their behaviour, physiology, and morphology as adults. In some cases, these changes may increase offspring fitness but, in others, they can elevate parental fitness at a cost to the fitness of their offspring. Here, we show that in Kalahari meerkats (Suricata suricatta) the circulating glucocorticoid hormones of pregnant females affect the growth and cooperative behaviour of their offspring. We performed a 3-year experiment in wild meerkats to test the hypothesis that stress-mediated maternal effects reduce the potential for offspring to reproduce directly and therefore cause them to exhibit more cooperative behaviour. Daughters (but not sons) born to mothers treated with cortisol during pregnancy were smaller early in life and exhibited significantly more of two types of cooperative behaviour (pup rearing and feeding) once they were adults compared to offspring from control mothers. They also had lower measures of stress hormones as they aged, which could explain the observed increases in cooperative behaviour. Because early growth is a crucial determinant of fitness in female meerkats, our results indicate that stress-mediated maternal effects may reduce the fitness of offspring but may elevate parental fitness as a consequence of increasing the cooperative behaviour of their daughters. Significance Statement Maternal characteristics can have a profound influence on offspring, which are referred to as maternal effects. It is widely assumed that selfish maternal effects, where mothers modify offspring to benefit their own fitness and decrease the fitness of offspring, are rare but this assumption is rarely tested. We show that a stress-mediated maternal effect in meerkats altered the growth, behaviour, and stress physiology of daughters in such a way that benefitted mothers at the expense of their daughters. Daughters from stressed mothers grew more slowly early in life, which should reduce their future reproductive opportunities, but exhibited more cooperative behaviour that should directly benefit their mother. This highlights the important role of sociality in the evolution of maternal effects.

Glucocorticoids (GCs) are involved in the regulation of an animal’s energetic state. Under stressful situations, they are part of the neuroendocrine response to cope with environmental challenges. Animals react to aversive stimuli also through behavioral responses, defined as coping styles. Both in captive and wild populations, individuals differ in their behavior along a proactive–reactive continuum. Proactive animals exhibit a bold, active-explorative and social personality, whereas reactive ones are shy, less active-explorative and less social. Here, we test the hypothesis that personality traits and physiological responses to stressors covary, with more proactive individuals having a less pronounced GC stress response. In wild populations of invasive gray squirrels Sciurus carolinensis, we measured fecal glucocorticoid metabolites (FGMs), an integrated measure of circulating GCs, and 3 personality traits (activity, sociability, and exploration) derived from open field test (OFT) and mirror image stimulation (MIS) test. Gray squirrels had higher FGMs in Autumn than in Winter and males with scrotal testes had higher FGMs than nonbreeding males. Personality varied with body mass and population density. Squirrels expressed more activity-exploration at higher than at lower density and heavier squirrels had higher scores for activity-exploration than animals that weighed less. Variation in FGM concentrations was not correlated with the expression of the 3 personality traits. Hence, our results do not support a strong association between the behavioral and physiological stress responses but show that in wild populations, where animals experience varying environmental conditions, the GC endocrine response and the expression of personality are uncorrelated traits among individuals.

It is well established that circulating maternal stress hormones (glucocorticoids, GCs) can alter offspring phenotype. There is also a growing body of empirical work, within ecology and evolution, indicating that maternal GCs link the environment experienced by the mother during gestation with changes in offspring phenotype. These changes are considered to be adaptive if the maternal environment matches the offspring's environment and maladaptive if it does not. While these ideas are conceptually sound, we lack a testable framework that can be used to investigate the fitness costs and benefits of altered offspring phenotypes across relevant future environments. We present error management theory as the foundation for a framework that can be used to assess the adaptive potential of maternal stress hormones on offspring phenotype across relevant postnatal scenarios. To encourage rigorous testing of our framework, we provide field‐testable hypotheses regarding the potential adaptive role of maternal stress across a diverse array of taxa and life histories, as well as suggestions regarding how our framework might provide insight into past, present, and future research. This perspective provides an informed lens through which to design and interpret experiments on the effects of maternal stress, provides a framework for predicting and testing variation in maternal stress across and within taxa, and also highlights how rapid environmental change that induces maternal stress may lead to evolutionary traps.

Hormones such as glucocorticoids (colloquially referred to as “stress hormones”) have important effects on animal behavior and life history traits, yet most of this understanding has come through correlative studies. While experimental studies offer the ability to assign causality, there are important methodological concerns that are often not considered when manipulating hormones, including glucocorticoids, in wild animals. In this study, we examined how experimental elevations of cortisol concentrations in wild North American red squirrels (Tamiasciurus hudsonicus) affected their hypothalamic–pituitary–adrenal (HPA) axis reactivity, and life history traits including body mass, litter survival, and adult survival. The effects of exogenous cortisol on plasma cortisol concentrations depended on the time between treatment consumption and blood sampling. In the first nine hours after consumption of exogenous cortisol, individuals had significantly higher true baseline plasma cortisol concentrations, but adrenal gland function was impaired as indicated by their dampened response to capture and handling and to injections of adrenocorticotropic hormone compared to controls. Approximately 24 hours after consumption of exogenous cortisol, individuals had much lower plasma cortisol concentrations than controls, but adrenal function was restored. Corticosteroid binding globulin (CBG) concentrations were also significantly reduced in squirrels treated with cortisol. Despite these profound shifts in the functionality of the HPA axis, squirrel body mass, offspring survival, and adult survival were unaffected by experimental increases in cortisol concentrations. Our results highlight that even short-term experimental increases in glucocorticoids can affect adrenal gland functioning and CBG concentrations but without other side-effects.

Invasive alien species can cause extinction of native species through processes including predation, interspecific competition for resources or disease‐mediated competition. Increases in stress hormones in vertebrates may be associated with these processes and contribute to the decline in survival or reproduction of the native species. Eurasian red squirrels (Sciurus vulgaris) have gone extinct across much of the British Isles and parts of Northern Italy following the introduction of North American invasive grey squirrels (Sciurus carolinensis). We extracted glucocorticoid metabolites from faecal samples to measure whether the presence of the invasive species causes an increase in physiological stress in individuals of the native species. We show that native red squirrels in seven sites where they co‐occurred with invasive grey squirrels had glucocorticoid concentrations that were three times higher than those in five sites without the invasive species. Moreover, in a longitudinal study, stress hormones in native red squirrels increased after colonisation by grey squirrels. When we experimentally reduced the abundance of the invasive grey squirrels, the concentration of faecal glucocorticoid metabolites in co‐occurring red squirrels decreased significantly between pre‐ and postremoval periods. Hence, we found that the invasive species acts as a stressor which significantly increases the concentrations of glucocorticoids in the native species. Given that sustained elevations in glucocorticoids could reduce body growth and reproductive rate, our results are consistent with previous studies where the co‐occurrence of the invasive grey squirrel was associated with smaller size and lower reproductive output in red squirrels.

In cooperative breeders, aggression from dominant breeders directed at subordinates may raise subordinate stress hormone (glucocorticoid) concentrations. This may benefit dominants by suppressing subordinate reproduction but it is uncertain whether aggression from dominants can elevate subordinate cooperative behaviour, or how resulting changes in subordinate glucocorticoid concentrations affect their cooperative behaviour. We show here that the effects of manipulating glucocorticoid concentrations in wild meerkats (Suricata suricatta) on cooperative behaviour varied between cooperative activities as well as between the sexes. Subordinates of both sexes treated with a glucocorticoid receptor antagonist (mifepristone) exhibited significantly more pup protection behaviour (babysitting) compared to those treated with glucocorticoids (cortisol) or controls. Females treated with mifepristone had a higher probability of exhibiting pup food provisioning (pup-feeding) compared to those treated with cortisol. In males, there were no treatment effects on the probability of pup-feeding, but those treated with cortisol gave a higher proportion of the food they found to pups than those treated with mifepristone. Using 19 years of behavioural data, we also show that dominant females did not increase the frequency with which they directed aggression at subordinates at times when the need for assistance was highest. Our results suggest that it is unlikely that dominant females manipulate the cooperative behaviour of subordinates through the effects of aggression on their glucocorticoid levels and that the function of aggression directed at subordinates is probably to reduce the probability they will breed.

SYNOPSIS Hormonal pleiotropy occurs when a part of the endocrine system (e.g., hormone concentrations) influences the expression of two or more phenotypes. Although hormonal pleiotropy may have similar evolutionary consequences as genetic pleiotropy, most conceptual and empirical work on its putative evolutionary consequences to date has focused on identifying whether the different components of an endocrine axis (titer, receptor expression, etc.) that affect trait expression are themselves able to evolve independently from one another. This is important because if these different components evolve together, the expression of two traits affected by the same hormone may be yoked and evolve non-independently. Here, we first describe methodological approaches used to identify how hormonal pleiotropy could cause the co-evolution of performance and life history traits. We then focus on a similar but less studied concept about how hormonal pleiotropy can affect phenotypic responses to selection. If the expression of two traits is affected by the same hormone, the magnitude of the phenotypic response to selection may be exacerbated or retarded compared to the absence of this hormonal pleiotropy. We use classical concepts from quantitative genetics to discuss an approach for identifying whether hormonal pleiotropy has such evolutionary consequences using data collected from longitudinal studies of wild animals. We develop a simple quantitative genetics model to derive predictions about the conditions under which hormonal pleiotropy would affect the response to selection. We focus on performance and life history traits and how the effects of hormonal pleiotropy on the evolution of these traits depend upon the genetic correlations between the hormone and traits as well as the direction and strength of selection on the two traits. Finally, we review the literature for examples that have estimated these model parameters to characterize the studies that have or have not found support for these model predictions.

The age trajectory of reproductive performance of many iteroparous species features an early - life increase in performance followed by a late - life senescent decline. The largest contribution of lifetime reproductive success is therefore gained at the age at which reproductive performance peaks. Using long term data on North American red squirrels we show that the environmental conditions individuals encountered could cause variation among individuals in the “height” and timing of this peak, contributing to life history variation and fitness in this population that experiences irregular resource pulses. As expected, high peak effort was positively associated with lifetime reproductive output up to a high level of annual effort. Furthermore, individuals that matched their peak reproductive effort to an anticipated resource pulse gained substantial fitness benefits through recruiting more offspring over their lifetime. Individual variation in peak reproductive effort thus has strong potential to shape life history evolution by facilitating adaptation to fluctuating environments.

Individuals often interact more closely with some members of the population (e.g. offspring, siblings or group members) than they do with other individuals. This structuring of interactions can lead to multilevel natural selection, where traits expressed at the group-level influence fitness alongside individual-level traits. Such multilevel selection can alter evolutionary trajectories, yet is rarely quantified in the wild, especially for species that do not interact in clearly demarcated groups. We quantified multilevel natural selection on two traits, postnatal growth rate and birth date, in a population of North American red squirrels (Tamiasciurus hudsonicus). The strongest level of selection was typically within-acoustic social neighbourhoods (within 130m of the nest), where growing faster and being born earlier than nearby litters was key, while selection on growth rate was also apparent both within-litters and within-study areas. Higher population densities increased the strength of selection for earlier breeding, but did not influence selection on growth rates. Females experienced especially strong selection on growth rate at the within-litter level, possibly linked to the biased bequeathal of the maternal territory to daughters. Our results demonstrate the importance of considering multilevel and sex-specific selection in wild species, including those that are territorial and sexually monomorphic.Data archival: the data set is archived on Dryad (info XXX), with a five-year embargo from the date of publication.

Maternal stress can prenatally influence offspring phenotypes and there are an increasing number of ecological studies that are bringing to bear biomedical findings to natural systems. This is resulting in a shift from the perspective that maternal stress is unanimously costly, to one in which maternal stress may be beneficial to offspring. However, this adaptive perspective is in its infancy with much progress to still be made in understanding the role of maternal stress in natural systems. Our aim is to emphasize the importance of the ecological and evolutionary context within which adaptive hypotheses of maternal stress can be evaluated. We present five primary research areas where we think future research can make substantial progress: (1) understanding maternal and offspring control mechanisms that modulate exposure between maternal stress and subsequent offspring phenotype response; (2) understanding the dynamic nature of the interaction between mothers and their environment; (3) integrating offspring phenotypic responses and measuring both maternal and offspring fitness outcomes under real-life (either free-living or semi-natural) conditions; (4) empirically testing these fitness outcomes across relevant spatial and temporal environmental contexts (both pre- and post-natal environments); (5) examining the role of maternal stress effects in human-altered environments-i.e., do they limit or enhance fitness. To make progress, it is critical to understand the role of maternal stress in an ecological context and to do that, we must integrate across physiology, behavior, genetics, and evolution.

Variation in maternal behaviour can have profound consequences on offspring phenotype, survival, reproductive success, as well as maternal fitness. Understanding the mechanisms that underlie variation in maternal care can provide us with a more thorough understanding of the evolution of parental care. In some taxa, the effects of steroid hormones such as testosterone on paternal care have been well studied. However, patterns of female androgens in natural populations are rarely documented and relationships between maternal androgens and behaviour remain poorly studied. In this study, we first validated an enzyme immunoassay to measure faecal androgen metabolites (FAM) in territorial North American red squirrels, Tamiasciurus hudsonicus. We validated this assay by demonstrating that (1) our antibody reacts with testosterone metabolites, (2) reproductive females have significantly higher FAM than nonreproductive females, and (3) patterns of FAM were largely mirrored in plasma androgen levels. Second, we tested the hypothesis that androgen concentrations drive behavioural trade-offs in free-ranging breeding female red squirrels during gestation and lactation using 3 years of FAM data and 10 years of behavioural observations. FAM increased after conception and parturition, peaked during mid-lactation around the time of juvenile emergence, and then declined during the remainder of lactation and after weaning. Around the peak of FAM during mid-lactation, nest use was lowest, while territory defence and time spent foraging were at their highest levels. These associations between maternal androgens and behaviour support the hypothesis that androgens may play an important role in mediating maternal behaviour in free-ranging animals.

Fisher's principle explains that population sex ratio in sexually reproducing organisms is maintained at 1 : 1 owing to negative frequency-dependent selection, such that individuals of the rare sex realize greater reproductive opportunity than individuals of the more common sex until equilibrium is reached. If biasing offspring sex ratio towards the rare sex is adaptive, individuals that do so should have more grandoffspring. In a wild population of North American red squirrels (Tamiasciurus hudsonicus) that experiences fluctuations in resource abundance and population density, we show that overall across 26 years, the secondary sex ratio was 1 : 1; however, stretches of years during which adult sex ratio was biased did not yield offspring sex ratios biased towards the rare sex. Females that had litters biased towards the rare sex did not have more grandoffspring. Critically, the adult sex ratio was not temporally autocorrelated across years, thus the population sex ratio experienced by parents was independent of the population sex ratio experienced by their offspring at their primiparity. Expected fitness benefits of biasing offspring sex ratio may be masked or negated by fluctuating environments across years, which limit the predictive value of the current sex ratio.

In many cooperatively breeding animal societies, breeders outlive non-breeding subordinates, despite investing heavily in reproduction [1-3]. In eusocial insects, the extended lifespans of breeders arise from specialized slowed aging profiles [1], prompting suggestions that reproduction and dominance similarly defer aging in cooperatively breeding vertebrates, too [4-6]. Although lacking the permanent castes of eusocial insects, breeders of vertebrate societies could delay aging via phenotypic plasticity (similar rank-related changes occur in growth, neuroendocrinology, and behavior [7-10]), and such plastic deferment of aging may reveal novel targets for preventing aging-related diseases [11]. Here, we investigate whether breeding dominants exhibit extended longevity and delayed age-related physiological declines in wild cooperatively breeding meerkats. We show that dominants outlive subordinates but exhibit faster telomere attrition (a marker of cellular senescence and hallmark of aging [12]) and that in dominants (but not subordinates), rapid telomere attrition is associated with mortality. Our findings further suggest that, rather than resulting from specialized aging profiles, differences in longevity between dominants and subordinates are driven by subordinate dispersal forays, which become exponentially more frequent with age and increase subordinate mortality. These results highlight the need to critically examine the causes of rank-related longevity contrasts in other cooperatively breeding vertebrates, including social mole-rats, where they are currently attributed to specialized aging profiles in dominants [4].

Despite considerable interest in the forces shaping the relationship between brain size and cognitive abilities, it remains controversial whether larger-brained animals are, indeed, better problem-solvers. Recently, several comparative studies have revealed correlations between brain size and traits thought to require advanced cognitive abilities, such as innovation, behavioral flexibility, invasion success, and self-control. However, the general assumption that animals with larger brains have superior cognitive abilities has been heavily criticized, primarily because of the lack of experimental support for it. Here, we designed an experiment to inquire whether specific neuroanatomical or socioecological measures predict success at solving a novel technical problem among species in the mammalian order Carnivora. We presented puzzle boxes, baited with food and scaled to accommodate body size, to members of 39 carnivore species from nine families housed in multiple North American zoos. We found that species with larger brains relative to their body mass were more successful at opening the boxes. In a subset of species, we also used virtual brain endocasts to measure volumes of four gross brain regions and show that some of these regions improve model prediction of success at opening the boxes when included with total brain size and body mass. Socioecological variables, including measures of social complexity and manual dexterity, failed to predict success at opening the boxes. Our results, thus, fail to support the social brain hypothesis but provide important empirical support for the relationship between relative brain size and the ability to solve this novel technical problem.

The causes and consequences of physiological stress in wildlife are of great interest in a wide range of biological disciplines including understanding how environmental changes affect species fate and persistence. In some areas, the Eurasian red squirrel ( Sciurus vulgaris ) faces local extinction because of the invasive Eastern gray squirrel ( Sciurus carolinensis ). Whether or not physiological stress is induced by the presence of invasive species or contributes to local declines in Eurasian red squirrels is unknown. Here, we develop an assay that can be used to quantify physiological stress in fecal samples from Eurasian red squirrels to eventually address these questions. We captured free-living squirrels (6 females, 11 males) and placed them into captivity for 48h. Fecal glucocorticoid metabolite (FGM) concentrations in female and male squirrels were significantly higher 24 and 32h after initial captivity with a lag time to peak excretion ranging from 24 to 36h. We measured FGM concentrations in free-living squirrels (37 females, 45 males) over a 3-year period. Lactating free-living squirrels had higher FGM concentrations compared to nonbreeding or pregnant squirrels but there were no differences in FGM concentrations in males with scrotal and abdominal testes. Free-living squirrels had the highest FGM concentrations in the winter and lowest in the summer (winter > autumn > summer). Squirrels kept in captivity for 4–48h had significantly higher FGM concentrations than free-living squirrels (111 fecal samples from 82 squirrels). FGM concentrations in captive but not wild squirrels were significantly repeatable. We found no sex differences and no association with body mass in FGM concentrations in captive or free-living squirrels. Our results indicate that this assay can accurately quantify physiological stress in Eurasian red squirrels, which may be useful for future studies to document how the invasive Eastern gray squirrel contributes to local extinction.