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Female advertisement of reproductive state and receptivity has the potential to play a large role in the mating systems of many taxa, but investigations of this phenomenon are underrepresented in the literature. North American red squirrels (Tamiasciurus hudsonicus) are highly territorial and engage in scramble competition mating, with males converging from spatially disparate territories to engage in mating chases. Given the narrow estrus window exhibited in this species, the ubiquitous use of vocalizations to advertise territory ownership, and the high synchronicity of males arriving from distant territories, we hypothesized that female vocalizations contain cues relating to their estrous state. To test this hypothesis, we examined the spectral and temporal properties of female territorial rattle vocalizations collected from females of known reproductive condition over 3 years. While we found no distinct changes associated with estrus specifically, we did identify significant changes in the spectral characteristics of rattles relating to both female body mass and reproductive state relative to parturition. To the best of our knowledge, this is the first evidence of changes in vocal characteristics associated with late pregnancy in a nonhuman mammal. [ABSTRACT FROM AUTHOR]

Many studies assume that it is beneficial for individuals of a species to be heavier, or have a higher body condition index (BCI), without accounting for the physiological relevance of variation in the composition of different body tissues. We hypothesized that the relationship between BCI and masses of physiologically important tissues (fat and lean) would be conditional on annual patterns of energy acquisition and expenditure. We studied three species with contrasting ecologies in their respective natural ranges: an obligate hibernator (Columbian ground squirrel, Urocitellus columbianus), a facultative hibernator (black-tailed prairie dog, Cynomys ludovicianus), and a food-caching non-hibernator (North American red squirrel, Tamiasciurus hudsonicus). We measured fat and lean mass in adults of both sexes using quantitative magnetic resonance (QMR). We measured body mass and two measures of skeletal structure (zygomatic width and right hind foot length) to develop sex- and species-specific BCIs, and tested the utility of BCI to predict body composition in each species. Body condition indices were more consistently, and more strongly correlated, with lean mass than fat mass. The indices were most positively correlated with fat when fat was expected to be very high (pre-hibernation prairie dogs). In all cases, however, BCI was never better than body mass alone in predicting fat or lean mass. While the accuracy of BCI in estimating fat varied across the natural histories and annual energetic patterns of the species considered, measuring body mass alone was as effective, or superior in capturing sufficient variation in fat and lean in most cases. [ABSTRACT FROM AUTHOR]

While cooperative interactions among kin are a key building block in the societies of group-living species, their importance for species with more variable social environments is unclear. North American red squirrels (Tamiasciurus hudsonicus) defend individual territories in dynamic neighbourhoods and are known to benefit from living among familiar conspecifics, but not relatives. However, kin-directed behaviours may be restricted to specific genealogical relationships or strongly mediated by geographical distance, masking their influence at broader scales. Using distance between territories as a proxy for the ability of individuals to interact, we estimated the influence of primary kin (parents, offspring, siblings) on the annual survival and reproductive success of red squirrels. This approach revealed associations between fitness and access to kin, but only for certain genealogical relationships and fitness components. For example, females had enhanced annual survival when living closer to their daughters, though the reverse was not true. Most surprising was the finding that males had higher annual reproductive success when living closer to their father, suggesting possible recognition and cooperation among fathers and sons. Together, these findings point to unexpected nuance in the fitness consequences of kinship dynamics for a species that is territorial and largely solitary. [ABSTRACT FROM AUTHOR]

Territories are typically defined as spatially exclusive areas that are defended against conspecifics. Given the spatial nature of territoriality, it is inherently density dependent, but the economics of territoriality also depend on the distribution and abundance of defended resources. Our objectives were to assess the effects of changing population density and food availability on individually based territorial phenotypes.We developed a novel analytical framework that bridges spatially explicit territories with social network analysis to model density‐dependent territorial phenotypes. Using the outputs from our data pipeline, we modelled plasticity in territory size and territory intrusion rates in a long‐term study population of North American red squirrels Tamiasciurus hudsonicus. Red squirrels defend year‐round territories around a central hoard (midden) of white spruce Picea glauca cones. Importantly, white spruce is a masting species that produces large cone crops every 4–7 years (i.e. mast years) in our study area interspersed with non‐mast years when few cones are produced. In the spring following mast years, populations are approximately double in size, but are lower in the spring of non‐mast years.We predicted that territory size and intrusion rates would decrease as resource abundance, and consequently population density, increased. By contrast, as resource abundance decreased via depletion, and therefore density decreased, territories should increase in size and intrusions should also increase.As we expected, individual territory size and territorial intrusions were negatively density dependent, such that increased density after mast years resulted in smaller territories and fewer intrusions. When considering between‐individual variation in plasticity across a density gradient, individuals responded differently to changes in population density within their lifetime.Our results show that territory size and intrusion rates display negative density dependence. When food becomes available in the autumn of a mast year and density in spring of the following year increases, territories shrink in size to effectively a small area around the midden. While our findings for red squirrels are unique compared to other systems, they serve as a reminder that the direction and strength of fundamental ecological relationships can depend on the nature of the system. [ABSTRACT FROM AUTHOR]

Reproduction is an energetically expensive activity for both sexes. However, if males and females differ in their annual timing of reproduction, such that peak investment for one sex occurs during a more resource-limited period, there is an opportunity for sex-specific selection to act on the acquisition of energetic resources. Both male and female North American red squirrels (Tamiasciurus hudsonicus) cache conifer cones, although males typically have larger caches than females. Peak energetic investment in reproduction occurs for males during the mating season in winter and early spring (when squirrels rely almost exclusively on cached resources) and for females during lactation (which can align with fresh food availability). We provide evidence that suggests sex differences in cache size are likely driven by a stronger positive connection between cached resources and components of fitness for males than for females. Specifically, males with larger caches have greater siring success than males with smaller caches, whereas for females, only early breeding females experience a positive effect of cache size on the number of recruits produced. We also show that males sire pups and females give birth earlier in the year if they have larger caches compared with squirrels of the same sex with smaller caches. Sexual selection can thus extend beyond traits directly connected to mating behavior, and can act on traits related to acquiring resources needed to fuel reproduction that are expressed months or years in advance of breeding efforts. Significance statement: The timing and energetic requirements of reproduction are often different for males and females which could drive different life history strategies between the sexes. Red squirrels in the southwest Yukon, Canada cache white spruce cones in autumn, which serve as their primary food source until fresh food becomes available during summer. We measured the size of squirrel caches and subsequently monitored their reproduction to test whether the influence of stored food differs for males and females. We confirmed that males have larger caches than females and show that this is likely driven by a stronger positive association of cache size and components of fitness for males than for females. These findings highlight that the strength of selection can differ on the homologous traits of males and females leading to sex-specific extended phenotypes. [ABSTRACT FROM AUTHOR]

The gut microbiome impacts host health and fitness, in part through the diversification of gut metabolic function and pathogen protection. Elevations in glucocorticoids (GCs) appear to reduce gut microbiome diversity in experimental studies, suggesting that a loss of microbial diversity may be a negative consequence of increased GCs. However, given that ecological factors like food availability and population density may independently influence both GCs and microbial diversity, understanding how these factors structure the GC-microbiome relationship is crucial to interpreting its significance in wild populations. Here, we used an ecological framework to investigate the relationship between GCs and gut microbiome diversity in wild North American red squirrels (Tamiasciurus hudsonicus). As expected, higher GCs predicted lower gut microbiome diversity and an increase in metabolic taxa. Surprisingly, but in line with prior empirical studies on wild animals, gastrointestinal pathogens decreased as GCs increased. Both dietary heterogeneity and an upcoming food pulse exhibited direct effects on gut microbiome diversity, whereas conspecific density and reproductive activity impacted diversity indirectly via changes in host GCs. Our results provide evidence of a gut–brain axis in wild red squirrels and highlight the importance of situating the GC-gut microbiome relationship within an ecological framework. [ABSTRACT FROM AUTHOR]

When resources are limited, mean fitness is constrained and competition can cause genes and phenotypes to enhance an individual's own fitness while reducing the fitness of their competitors. Negative social effects on fitness have the potential to constrain adaptation, but the interplay between ecological opportunity and social constraints on adaptation remains poorly studied in nature. Here, we tested for evidence of phenotypic social effects on annual fitness (survival and reproductive success) in a long-term study of wild North American red squirrels (Tamiasciurus hudsonicus) under conditions of both resource limitation and super-abundant food resources. When resources were limited, populations remained stable or declined, and there were strong negative social effects on annual survival and reproductive success. That is, mean fitness was constrained and individuals had lower fitness when other nearby individuals had higher fitness. In contrast, when food resources were super-abundant, populations grew and social constraints on reproductive success were greatly reduced or eliminated. Unlike reproductive success, social constraints on survival were not significantly reduced when food resources were super-abundant. These findings suggest resource-dependent social constraints on a component of fitness, which have important potential implications for evolution and adaptation. [ABSTRACT FROM AUTHOR]

Long‐term studies of wild animals provide the opportunity to investigate how phenotypic plasticity is used to cope with environmental fluctuations and how the relationships between phenotypes and fitness can be dependent upon the ecological context.Many previous studies have only investigated life‐history plasticity in response to changes in temperature, yet wild animals often experience multiple environmental fluctuations simultaneously. This requires field experiments to decouple which ecological factor induces plasticity in fitness‐relevant traits to better understand their population‐level responses to those environmental fluctuations.For the past 32 years, we have conducted a long‐term integrative study of individually marked North American red squirrels Tamiasciurus hudsonicus Erxleben in the Yukon, Canada. We have used multi‐year field experiments to examine the physiological and life‐history responses of individual red squirrels to fluctuations in food abundance and conspecific density.Our long‐term observational study and field experiments show that squirrels can anticipate increases in food availability and density, thereby decoupling the usual pattern where animals respond to, rather than anticipate, an ecological change.As in many other study systems, ecological factors that can induce plasticity (such as food and density) covary. However, our field experiments that manipulate food availability and social cues of density (frequency of territorial vocalizations) indicate that increases in social (acoustic) cues of density in the absence of additional food can induce similar life‐history plasticity, as does experimental food supplementation.Changes in the levels of metabolic hormones (glucocorticoids) in response to variation in food and density are one mechanism that seems to induce this adaptive life‐history plasticity.Although we have not yet investigated the energetic response of squirrels to elevated density or its association with life‐history plasticity, energetics research in red squirrels has overturned several standard pillars of knowledge in physiological ecology.We show how a tractable model species combined with integrative studies can reveal how animals cope with resource fluctuations through life‐history plasticity. [ABSTRACT FROM AUTHOR]

Organisms survive environmental variation by combining homeostatic regulation of critical states with allostatic variation of other traits, and species differences in these responses can contribute to coexistence in temporally variable environments.In this paper, we simultaneously record variation in three functional traits—body temperature (Tb), heart rate and activity—in relation to three forms of environmental variation—air temperature (Ta), photoperiod and experimentally manipulated resource levels—in free‐ranging snowshoe hares and North American red squirrels to characterize distinctions in homeotherm responses to the extreme conditions of northern boreal winters.Hares and squirrels differed in the level and precision of Tb regulation, but also in the allostatic pathways necessary to maintain thermal homeostasis. Hares demonstrated a stronger metabolic pathway (through heart rate variation reflective of the thermogenesis), while squirrels demonstrated a stronger behavioural pathway (through activity variation that minimizes cold exposure).As intermediate‐sized, winter‐active homeotherms, hares and squirrels share many functional attributes, yet, through the integrated monitoring of multiple functional traits in response to shared environmental variation, our study reveals many pairwise species differences in homeostatic and allostatic traits, that both define and are defined by the natural history, functional niches and coexistence of sympatric species. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]

Juvenile survival to first breeding is a key life‐history stage for all taxa. Survival through this period can be particularly challenging when it coincides with harsh environmental conditions such as a winter climate or food scarcity, leading to highly variable cohort survival. However, the small size and dispersive nature of juveniles generally make studying their survival more difficult.In territorial species, a key life‐history event is the acquisition of a territory. A territory is expected to enhance survival, but how it does so is not often identified. We tested how the timing of territory acquisition influenced the winter survival of juvenile North American red squirrels Tamiasciurus hudsonicus, hereafter red squirrels, and how the timing of this event mediated the sources of mortality. We hypothesized that securing a territory prior to when food resources become available would reduce juvenile susceptibility to predation and climatic factors overwinter.Using 27 years of data on the survival of individually marked juvenile red squirrels, we tested how the timing of territory acquisition influenced survival, whether the population density of red squirrel predators and mean temperature overwinter were related to individual survival probability, and if territory ownership mediated these effects.Juvenile red squirrel survival was lower in the years of high predator abundance and in colder winters. Autumn territory owners were less susceptible to lynx Lynx canadensis and possibly mustelid Mustela and Martes spp., predation. Autumn territory owners had lower survival in colder winters, but surprisingly non‐owners had higher survival in cold winters.Our results show how the timing of a life‐history event like territory acquisition can directly affect survival and also mediate the effects of biotic and abiotic factors later in life. This engenders a better understanding of the fitness consequences of the timing of key life‐history events. [ABSTRACT FROM AUTHOR]

Parental investment theory predicts that observed levels of parental care afforded to offspring are set by the benefits (to offspring quality and survival) relative to the costs (to parental survival or future reproduction). Although difficult to document in mammals, there is often substantial individual variation in the amount of parental care within species. We measured the impact of individual variation in maternal care ("attentiveness" towards offspring or maternal motivation) on offspring growth and survival in a wild population of North American red squirrels (Tamiasciurus hudsonicus). We used latency to return to pups following a nest intrusion as a measure of maternal attentiveness to pups. We found this behavior to be repeatable within individuals suggesting this behavior is a personality trait or a "maternal style." In this population, postnatal growth rate is important for pup overwinter survival. Pups from large litters grew faster if they had a highly attentive mother, indicating that maternal care can mitigate the trade-off between litter size and offspring growth and potentially improve survival of pups. Additionally, more attentive mothers had slightly higher lifetime reproductive success than less attentive mothers. These results highlight important fitness effects of having a highly attentive mother and show that maternal care can alter a fundamental life history trade-off between offspring quantity and quality. Significance statement: It pays to be attentive to your pups as a squirrel mom. In a long-term study of a wild population of North American red squirrels, we tested whether maternal behavior was predictive of growth rate and survival of offspring. To do this, we observed maternal attentiveness towards offspring by recording the time until the mother returned to her pups following a nest intrusion by researchers. We found repeatable individual variation in maternal attentiveness suggesting consistent "maternal styles" among individuals. Mothers who returned faster to pups following a nest intrusion produced faster growing pups and were able to produce larger fast-growing litters. Over their entire lifetime, attentive mothers also had more offspring survive to adulthood. [ABSTRACT FROM AUTHOR]

Dispersal is nearly universal; yet, which sex tends to disperse more and their success thereafter depends on the fitness consequences of dispersal. We asked if lifetime fitness differed between residents and immigrants (successful between‐population dispersers) and their offspring using 29 years of monitoring from North American red squirrels (Tamiasciurus hudsonicus) in Canada. Compared to residents, immigrant females had 23% lower lifetime breeding success (LBS), while immigrant males had 29% higher LBS. Male immigration and female residency were favoured. Offspring born to immigrants had 15–43% lower LBS than offspring born to residents. We conclude that immigration benefitted males, but not females, which appeared to be making the best of a bad lot. Our results are in line with male‐biased dispersal being driven by local mate competition and local resource enhancement, while the intergenerational cost to immigration is a new complication in explaining the drivers of sex‐biased dispersal. [ABSTRACT FROM AUTHOR]

The episodic production of large seed crops by some perennial plants (masting) is known to increase seed escape by alternately starving and swamping seed predators. These pulses of resources might also act as an agent of selection on the life histories of seed predators, which could indirectly enhance seed escape by inducing an evolutionary load on seed predator populations. We measured natural selection on litter size of female North American red squirrels (Tamiasciurus hudsonicus) across 28 years and five white spruce (Picea glauca) masting events. Observed litter sizes were similar to optimum litter sizes during nonmast years but were well below optimum litter sizes during mast years. Mast events therefore caused selection for larger litters ( β ′ = 0.25) and a lag load (L = 0.25) on red squirrels during mast years. Reduced juvenile recruitment associated with this lag load increased the number of spruce cones escaping squirrel predation. Although offspring and parents often experienced opposite environments with respect to the mast, we found no effect of environmental mismatches across generations on either offspring survival or population growth. Instead, squirrels plastically increased litter sizes in anticipation of mast events, which partially, although not completely, reduced the lag load resulting from this change in food availability. These results therefore suggest that in addition to ecological and behavioral effects on seed predators, mast seed production can further enhance seed escape by inducing maladaptation in seed predators through fluctuations in optimal trait values. [ABSTRACT FROM AUTHOR]

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 9 h 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 h 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. [ABSTRACT FROM AUTHOR]

Acoustic signaling is an important means by which animals communicate both stable and labile characteristics. Although it is widely appreciated that vocalizations can convey information on labile state, such as fear and aggression, fewer studies have experimentally examined the acoustic expression of stress state. The transmission of such public information about physiological state could have broad implications, potentially influencing the behavior and life-history traits of neighbors. North American red squirrels (Tamiasciurus hudsonicus) produce vocalizations known as rattles that advertise territorial ownership. We examined the influence of changes in physiological stress state on rattle acoustic structure through the application of a stressor (trapping and handling the squirrels) and by provisioning squirrels with exogenous glucocorticoids (GCs). We characterized the acoustic structure of rattles emitted by these squirrels by measuring rattle duration, mean frequency, and entropy. We found evidence that rattles do indeed exhibit a "stress signature." When squirrels were trapped and handled, they produced rattles that were longer in duration with a higher frequency and increased entropy. However, squirrels that were administered exogenous GCs had similar rattle duration, frequency, and entropy as squirrels that were fed control treatments and unfed squirrels. Our results indicate that short-term stress does affect the acoustic structure of vocalizations, but elevated circulating GC levels do not mediate such changes. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Interactions between organisms are ubiquitous and have important consequences for phenotypes and fitness. Individuals can even influence those they never meet, if they have extended phenotypes that alter the environments others experience. North American red squirrels (Tamiasciurus hudsonicus) guard food hoards, an extended phenotype that typically outlives the individual and is usually subsequently acquired by non‐relatives. Hoarding by previous owners can, therefore, influence subsequent owners. We found that red squirrels breed earlier and had higher lifetime fitness if the previous hoard owner was a male. This was driven by hoarding behaviour, as males and mid‐aged squirrels had the largest hoards, and these effects persisted across owners, such that if the previous owner was male or died in mid‐age, subsequent occupants had larger hoards. Individuals can, therefore, influence each other's resource‐dependent traits and fitness without ever meeting, such that the past can influence contemporary population dynamics through extended phenotypes. [ABSTRACT FROM AUTHOR]

The miniaturization and affordability of new technology is driving a biologging revolution in wildlife ecology with use of animal‐borne data logging devices. Among many new biologging technologies, accelerometers are emerging as key tools for continuously recording animal behavior. Yet a critical, but under‐acknowledged consideration in biologging is the trade‐off between sampling rate and sampling duration, created by battery‐ (or memory‐) related sampling constraints. This is especially acute among small animals, causing most researchers to sample at high rates for very limited durations. Here, we show that high accuracy in behavioral classification is achievable when pairing low‐frequency acceleration recordings with temperature. We conducted 84 hr of direct behavioral observations on 67 free‐ranging red squirrels (200–300 g) that were fitted with accelerometers (2 g) recording tri‐axial acceleration and temperature at 1 Hz. We then used a random forest algorithm and a manually created decision tree, with variable sampling window lengths, to associate observed behavior with logger recorded acceleration and temperature. Finally, we assessed the accuracy of these different classifications using an additional 60 hr of behavioral observations, not used in the initial classification. The accuracy of the manually created decision tree classification using observational data varied from 70.6% to 91.6% depending on the complexity of the tree, with increasing accuracy as complexity decreased. Short duration behavior like running had lower accuracy than long‐duration behavior like feeding. The random forest algorithm offered similarly high overall accuracy, but the manual decision tree afforded the flexibility to create a hierarchical tree, and to adjust sampling window length for behavioral states with varying durations. Low frequency biologging of acceleration and temperature allows accurate behavioral classification of small animals over multi‐month sampling durations. Nevertheless, low sampling rates impose several important limitations, especially related to assessing the classification accuracy of short duration behavior. An under‐acknowledged consideration in biologging is the trade‐off between sampling rate and sampling duration, created by battery‐ (or memory‐) related sampling constraints. Since this is especially acute among small animals, we show that use of low frequency sampling rates on accelerometers can yield high accuracy and provide advice on how to apply this technique to other animals. Using this technique allows for long‐duration deployments that continuous record behaviour revealing seasonal patterns in activity. [ABSTRACT FROM AUTHOR]

Abstract: Phenological shifts are the most widely reported ecological responses to climate change, but the requirements to distinguish their causes (i.e. phenotypic plasticity vs. microevolution) are rarely met. To do so, we analysed almost two decades of parturition data from a wild population of North American red squirrels (Tamiasciurus hudsonicus). Although an observed advance in parturition date during the first decade provided putative support for climate change‐driven microevolution, a closer look revealed a more complex pattern. Parturition date was heritable [h2 = 0.14 (0.07–0.21 (HPD interval)] and under phenotypic selection [β = −0.14 ± 0.06 (SE)] across the full study duration. However, the early advance reversed in the second decade. Further, selection did not act on the genetic contribution to variation in parturition date, and observed changes in predicted breeding values did not exceed those expected due to genetic drift. Instead, individuals responded plastically to environmental variation, and high food [white spruce (Picea glauca) seed] production in the first decade appears to have produced a plastic advance. In addition, there was little evidence of climate change affecting the advance, as there was neither a significant influence of spring temperature on parturition date or evidence of a change in spring temperatures across the study duration. Heritable traits not responding to selection in accordance with quantitative genetic predictions have long presented a puzzle to evolutionary ecologists. Our results on red squirrels provide empirical support for one potential solution: phenotypic selection arising from an environmental, as opposed to genetic, covariance between the phenotypic trait and annual fitness. [ABSTRACT FROM AUTHOR]

In many species, territory advertisement is thought to be one of the primary functions of acoustic communication. North American red squirrels are a territorial species in which 'rattles' have long been thought to be the principal signal communicating territory ownership. These vocalizations have been assumed to deter intruders, thus reducing energetic costs and the risk of injury associated with direct aggressive interactions. However, this hypothesis has not been directly tested. Here we used a speaker occupation experiment to test whether red squirrel rattles function to deter conspecific rivals. We studied 29 male squirrels and removed each individual from his territory twice in a paired design. During the experimental treatment, we simulated the owner's presence after its removal by broadcasting the owner's rattle from a loudspeaker at the centre of the territory once every 7 min. During the control treatment, the territory was left in silence following the temporary removal of the owner. We found that the presence of a speaker replacement reduced the probability of intrusion by 34% and increased the latency to first intrusion by 7%, providing support for the hypothesis that rattles play an active role in reducing intrusion risk. However, intrusions were not completely averted by the speaker replacement, indicating that for some individuals vocalizations alone are not a sufficient deterrent without other cues of the territory owner. [ABSTRACT FROM AUTHOR]

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 neighborhoods (within 130 m 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. [ABSTRACT FROM AUTHOR]

Time allocation by lactating mammals is a reconciliation of often opposing nutritional and thermal demands of both the offspring and mother. Here we test the hypothesis that nest attendance patterns of lactating red squirrels (Tamiasciurus hudsonicus) vary with environmental and biological traits that relate to the thermoregulation of mothers and their offspring. We used temperature dataloggers to continuously record nest attendance and activity of free-ranging females with neonatal (n = 45) and preemergent (n = 53) litters. Lactating red squirrels concentrated activity out of the nest around the warmest parts of winter days and the coldest parts of summer days. Both younger and lighter litters had mothers that spent more time in the nest and shorter periods of time out of the nest. Females matched timing of activity within a day to the hourly air temperatures closest to their thermal neutral zone, serving to reduce individual and offspring thermoregulatory costs associated with activity. Nest attendance patterns appear to be constrained by the thermal, and possibly nutritional, requirements of the litter with females fine-tuning behavior to match constantly changing environmental and biological conditions, consistent with reduced energetic costs. [ABSTRACT FROM AUTHOR]

Successful reproduction in a seasonal environment can be accomplished with resources that are stored before use (“capital resources”) or resources that are used immediately (“income resources”). Research examining capital versus income resource usage during reproduction has primarily focused on assigning species to positions along a capital–income gradient. Here, we examine the causes and reproductive consequences of among and within-year variation in hoarded capital versus income resource usage by female North American red squirrels ( Tamiasciurus hudsonicus) during mid-lactation in a highly seasonal environment. Among years, the proportion of feeding events that were on capital resources (PROPCAP) averaged 39 % during the yearly median mid-lactation periods, but ranged widely between 2 and 100 %. In years with earlier parturition dates, females primarily used hoarded capital resources during mid-lactation, whereas in years with later parturition dates, females primarily used income resources during mid-lactation. Within years, PROPCAP during mid-lactation tended to be greater in early-breeding females than in late-breeding females. Rates of water flux in females during mid-lactation provided further evidence that late-breeding females used more water-rich income resources. The proportion of litters that were partially or completely lost, and the litter mass that lactating females supported, was not influenced by the large among-year differences in hoarded capital resource usage. Red squirrels appear to delay reproduction following years with low cone production to time peak reproductive demands to be late enough to be supported by income resources that only become available later in the season. In conclusion, our results offer a rare example of the capacity of a food-hoarding mammal to support reproduction exploiting a wide range of capital and income resources. [ABSTRACT FROM AUTHOR]

In territorial species, competitive asymmetries can result in the uneven distribution of food resources as high-quality individuals force young or subordinates to occupy smaller or lower-quality sites, or both. However, spatiotemporal variation in the production of resources also can influence an individual's ability to monopolize resources and, consequently, affect survival. We examined how spatial and temporal variation in food supply affects the distribution of resources among territorial food-hoarding red squirrels (Tamiasciurus hudsonicus)b y mapping cone production over 62.5 ha of white spruce (Picea glauca) forest and measuring the interrelationships between territory size, cone supply, and survival during 4 years of low cone abundance in Yukon, Canada. Territory size and cone production within a territory varied 10- and 520-fold, respectively, with juvenile squirrels occupying smaller territories with fewer cones. Because of small-scale heterogeneity in the distribution of cones, territory size explained low to moderate amounts of the variation in territory-wide cone production within (rs = 0.49-0.69) and across (rs = 0.31) years. Furthermore, spatial heterogeneity in cone production varied across years such that territory location was not useful for predicting relative food availability from one year to the next. Perhaps as a consequence of this heterogeneity, the number of cones within an individual's hoard was only weakly correlated with territory size (rs = 0.42) and not correlated with territory-wide cone production. Hoard size better predicted overwinter survival than did territory size, whereas cone production on a territory was not predictive of survival. We suggest that caution be used in equating territory size with food supply because small-scale spatial heterogeneity can weaken this relationship, particularly in food-hoarders. Spatiotemporal variation in food abundance, in turn, may cause high-quality territories in one year to be low-quality territories in other years, which will limit the ability of individuals to assess the long-term quality of territories at the time of settlement. [ABSTRACT FROM AUTHOR]

In short-term hoarding rodents, reproduction and hoarding often overlap and reproductive status can alter hoarding behavior. In long-term hoarders these activities are often separated seasonally. Therefore, annual and sex-specific variation in the timing of reproduction relative to the hoarding season may influence variation in the hoarding behavior of long-term hoarders. North American red squirrels (Tamiasciurus hudsonicus) clip and hoard white spruce (Picea glauca) cones each autumn, but have variation in reproductive timing such that in some years the hoarding season overlaps with reproductive activity. Over 4 years with naturally varying cone availability we quantified reproductive timing and hoarding behavior. Males completed mating 22-90 days before the onset of hoarding, whereas large numbers (45-86%) of females during large cone crop years weaned litters after hoarding commenced. Despite these differences in reproductive timing, across all years females clipped more cones than did males, whereas males hoarded more cones and had a higher propensity to larder- hoard than did females. Interestingly, in the years with large cone crops, females that weaned their litters later (during the hoarding season) did not have reduced hoarding or reproductive performance (offspring growth and recruitment) than females that weaned their litters earlier. In the year with the most extensive overlap between female reproduction and hoarding, lactating females did not differ from males in the amount of time they allocated to hoarding. In this year, the daily energy expenditure of lactating females during the hoarding season did not differ from that of males engaged in hoarding or from that of lactating females prior to the hoarding season. Hence, under high-resource conditions the competing demands of lactation and hoarding can both be sustained, allowing red squirrels flexibility in the separation between reproduction and hoarding. [ABSTRACT FROM AUTHOR]

1. Despite the central importance of the rate of energy expenditure in the lives of animals, the major drivers of within-species variation in energy expenditure remain uncertain, largely because most intraspecific studies focus on one or only a few potential determinants of expenditure. 2. Here, we examine the determinants of daily energy expenditure (DEE) in free-ranging female North American red squirrels ( Tamiasciurus hudsonicus Erxleben) occupying a highly seasonal environment. By relating variation in 260 measurements of DEE from 176 individuals to key sources of seasonal (reproductive and foraging stages), environmental (resources and air temperature) and individual (body mass and individual identity) variation, our comprehensive analysis examines the relative importance of DEE predictors that have been more commonly examined in isolation. 3. Red squirrels demonstrated extensive variation in DEE with 5th (177 kJ per day) and 95th (660 kJ per day) percentile DEE levels that would correspond to mammals on an interspecific scale ranging in mass from 148 to 1120 g. 4. Seasonal stage differences accounted for most variation in DEE, with high expenditure during lactation and autumn hoarding, and very low expenditure during winter. Contrary to interspecific studies, energy expenditure increased with increasing ambient temperature and it was weakly related to body mass in all seasons except for winter. High resource availability was associated with reduced energy expenditure in winter, but elevated expenditure during lactation and hoarding. 5. Collectively, these results highlight substantial intraspecific variation in energy expenditure, most of which can be explained by a combination of seasonal stages and environmental conditions, and fundamental differences in the importance and direction of determinants of energy expenditure when examined at the intra- versus the interspecific level. [ABSTRACT FROM AUTHOR]

Predator satiation resulting from inter-annual reproductive synchrony has been widely documented in masting plants, but how reproductive synchrony within a year influences seed escape is poorly understood. We evaluated whether the intra-annual reproductive synchrony of individual white spruce trees (Picea glauca) increased seed escape from their primary predispersal seed predator, North American red squirrels (Tamiasciurus hudsonicus). Trees with cones that matured synchronously relative to those of other trees within red squirrel territories were significantly more likely to escape squirrel predation in years with both low and superabundant levels of cone production, generating a significantly positive linear selection differential for increasing intra-annual reproductive synchrony. Thus, this masting plant escapes seed predation in numbers through inter-annual synchrony in seed production and in time through intra-annual synchrony of seed availability. [ABSTRACT FROM AUTHOR]

While many animals defend territories to secure resources such as food and mates, little is known about why territory owners of the same species vary in their territorial defence behaviour. We tested whether potential intruder pressure, defence of offspring, resource-holding potential or aggressiveness of the individual territory owner best explained intraspecific differences in territorial defence in a wild population of North American red squirrels (Tamiasciurus hudsonicus). We assessed territorial defence behaviour of individual red squirrels by recording whether or not they produced territorial vocalizations, known as rattles, both in response to a territorial playback and during natural observation sessions without an experimental stimulus. We compared the relative fit of four a priori models to explain territorial defence intensity in red squirrels and found that rattling behaviour in red squirrels under natural conditions was best explained by the intruder pressure hypothesis. Red squirrels were more likely to vocalize if they were surrounded by a higher density of conspecifics on neighbouring territories, indicating that they adjust territorial defence in response to potential intruder pressure. However, vocalization responses of red squirrels to the playback were not affected by local density, which was reflected in similar support for the four a priori models. The differing effects of local density on red squirrel vocalization rate during natural observations and following playbacks indicates that the effects of local density on the territorial behaviour of red squirrels depends on the particular context in which this behaviour is expressed. [ABSTRACT FROM AUTHOR]

Abstract: A growing number of longitudinal studies in free-ranging animals are measuring fecal steroid hormone metabolite concentrations (FHM). Free-ranging animals can exhibit major seasonal changes in their diet, yet we know relatively little about how diet affects FHM. We experimentally manipulated the diets of female and male North American red squirrels (Tamiasciurus hudsonicus) to determine how diet affected fecal cortisol (FCM) and androgen (FAM) metabolite concentrations. We measured FCM using an enzyme immunoassay (EIA) that we have previously validated and measured FAM using an assay we have previously validated for use in females and validate for males herein. We validated our EIA to measure FAM in males by identifying that 44.5±0.05% of recovered radiolabeled testosterone was excreted in the feces, our EIA antibody detected the fecal testosterone metabolites, and males with scrotal testes had significantly higher FAM (3.02±0.06lnng/g dry feces) than those with abdominal testes (2.73±0.06). We initially fed all squirrels the same diet, but then switched one group of squirrels to a diet consisting of conifer seed (n =4 squirrels) whereas the other group was switched to peanut butter (n =7). FCM and FAM in squirrels fed conifer seed significantly increased from 0 to 94h after their diets were changed. FCM in squirrels fed peanut butter significantly declined, whereas FAM declined but not significantly. This demonstrates that change in dietary fiber consumption (peanut butter versus conifer seed) or even slight differences in diet (conifer versus sunflower seeds) can strongly influence FHM. [Copyright &y& Elsevier]

The article presents a study of multiple male mating (MMM) behaviour of female North American red squirrels (Tamiasciurus hudsonicus).It provides details of the behavioural method of observing the copulation of these free-ranging squirrels to estimate the heritability of MMM using the Markov chain Monte Carlo model. It explores the significance of the research in terms of the evolution of multiple male mating.

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. [ABSTRACT FROM AUTHOR]

Mast seeding involves the episodic and synchronous production of large seed crops by perennial plants. The predator satiation hypothesis proposes that mast seeding maximizes seed escape because seed predators consume a decreasing proportion of available seeds with increasing seed production. However, the seed escape benefits of masting depend not only on whether predators are satiated at high levels of seed production, but also on the shape of their functional response (type II vs. type III), and the actual proportion of available seeds that they consume at different levels of seed production. North American red squirrels (Tamiasciurus hudsonicus) are the primary vertebrate predator of white spruce (Picea glauca) mast seed crops in many boreal regions because they hoard unopened cones in underground locations, preempting the normal sequence of cone opening, seed dispersal, and seed germination. We document the functional response of cone-hoarding by red squirrels across three non-mast years and one mast year by estimating the number of cones present in the territories of individual red squirrels and the proportion of these cones that they hoarded each autumn. Even though red squirrels are not constrained by the ingestive and on-body (fat reserves) energy reserve limitations experienced by animals that consume seeds directly, most squirrels hoarded <10% of the cones present on their territories under mast conditions. Cone availability during non-mast years also reached levels that satiated the hoarding activity of red squirrels; however, this occurred only on the highest-quality territories. Squirrels switched to mushroom-hoarding when cone production was low and mushrooms were abundant. This resulted in type III functional response whereby the proportional harvest of cones was highest at levels of cone availability that were intermediate within non-mast years. Overall, more cones escaped squirrel cone-hoarding during a mast event than when cone production was low in non-mast years, which supports the predator satiation hypothesis. However, the highly variable seed escape in non-mast years may help to explain why all spruce cone production is not concentrated into fewer, larger, mast years. [ABSTRACT FROM AUTHOR]

Abstract: Patterns in stress hormone (glucocorticoid: GC) levels and their relationship to reproductive condition in natural populations are rarely investigated. In this study, we (1) validate an enzyme-immunoassay to measure fecal cortisol metabolite (FCM) levels in North American red squirrels (Tamiasciurus hudsonicus), and (2) examine relationships between FCM levels and reproductive condition in a free-ranging red squirrel population. Injected radiolabeled cortisol was entirely metabolized and excreted in both the urine (mean±SE; 70.3±0.02%) and feces (29.7±0.02%), with a lag time to peak excretion in the feces of 10.9±2.3h. Our antibody reacted with several cortisol metabolites, and an adrenocorticotropic injection significantly increased FCM levels above baseline levels at 8h post-injection. Relative to baseline levels, manipulation by handling also tended to increase FCM levels at 8h post-manipulation, but this difference was not significant. FCM levels did not differ significantly between samples frozen immediately and 5h after collection. Reproductive condition significantly affected FCM levels in free-ranging females (pregnant>lactating>post-lactating>non-breeding) but not males (scrotal testes vs. abdominal testes). Among females with known parturition dates, FCM levels increased during gestation, peaked at parturition, and declined during lactation. The difference between pregnant and lactating females was therefore dependent upon when the fecal samples were obtained during these periods, suggesting caution in categorizing reproductive stages. This study demonstrates the utility of fecal hormone metabolite assays to document patterns of glucocorticoid levels in free-ranging animals. [Copyright &y& Elsevier]

1. Resting metabolic rate (RMR) varies considerably among and within species. Two central questions in physiological ecology are whether values of RMR are repeatable and whether an association exists between RMR and fitness. 2. First, we investigated the repeatability of RMR in food hoarding, juvenile, North American red squirrels ( Tamiasciurus hudsonicus Erxleben). Second, we explored links between RMR and survival. A low RMR may enhance survival if it reduces winter expenditure costs and/or allows more energy to be allocated towards autumn food hoarding. Alternately, a high RMR may enhance survival if it enables juveniles to hoard more food by increasing the throughput of energy available for investment in hoarding activities. 3. Resting metabolic rate adjusted for body mass, was repeatable in both males and females ( r = 0·77) over a short-term (mean 24·3 days) but only among females ( r = 0·72) over a long-term interval (mean 192 days). 4. Heavier juveniles and those with a lower RMR relative to their body mass were more likely to survive over-winter. Multiple selection models found significant selection for a decreased RMR (β′ = −0·56 ± 0·16) and increased mass (β′ = 0·69 ± 0·17). Survivors also tended to have more food stored within their hoard. 5. A low RMR relative to body mass and large body mass may have allowed individuals to minimize the expenditure costs related to a larger body mass, while maximizing thermal inertia. [ABSTRACT FROM AUTHOR]

1. Environmental conditions experienced early in life may have long-lasting effects on individual performance, thereby creating ‘silver-spoon effects’. 2. We used 15 years of data from a North American red squirrel ( Tamiasciurus hudsonicus Erxleben) population to investigate influences of food availability, density and spring temperature experienced early in life on reproduction and survival of female squirrels during adulthood. 3. We found that spring temperature and food availability did not affect female survival after 1 year of age, whereas higher squirrel densities led to lower survival, thereby affecting longevity and lifetime fitness. 4. In addition, both food availability experienced between birth and weaning, and spring temperature in the year of birth, had long-lasting positive effects on female reproductive success. These results emphasize the critical effect environmental conditions during the early life stages can have on the lifetime performance of small mammals. 5. These long-term effects of early food and temperature were apparent only once we controlled for conditions experienced during adulthood. This suggests that silver-spoon effects can be masked when conditions experienced early in life are correlated to some environmental conditions experienced later in life. 6. The general importance of silver-spoon effects for adult demographic performance might therefore be underestimated, and taking adult environment into account appears to be necessary when studying long-term cohort effects. [ABSTRACT FROM AUTHOR]

Measures the potential for evolution of nestling growth rates in a natural population of North American red squirrels or Tamiasciurus hudsonicus. Influences of heritable maternal effects on the potential for evolution; Estimation of the sources of variation in growth in body mass and growth in body size of the squirrels; Sources of bias in the direct-indirect genetic covariance estimate for growth in body mass.

The article presents a study which applies the heat dissipation limit hypothesis on lactating red squirrels in southwestern Yukon. Researchers examine whether the insulation of nests used by lactating females is less under conditions when heat production within the nest is high and conditions limit heat dissipation. Key findings indicate that squirrels occupy less insulated nests in response to the conditions that increase heat load. Researchers also found that heat dissipation is a vital issue for lactating squirrels, even in high latitude environments.

Individuals vary in their behavioral and physiological responses to environmental changes. These behavioral responses are often described as "coping styles" along a proactive-reactive continuum. Studies in laboratory populations often, but not always, find that behavioral responses and physiological responses to stressors covary, where more proactive (more aggressive and active) individuals have a lower physiological stress response, specifically as measured by hypothalamic-pituitary-adrenal (HPA) axis activity. These studies support the possibility of hormonal pleiotropy underlying the presentation of behaviors that make up the proactive-reactive phenotype. However, recent research in wild populations is equivocal, with some studies reporting the same pattern as found in many controlled laboratory studies, whereas others do not. We tested the hypothesis that physiological and behavioral stress responses are correlated in wild adult North American red squirrels (Tamiasciurus hudsonicus). We used fecal cortisol metabolites (FCMs) as a non-invasive, integrated estimate of circulating glucocorticoids for our measurement of HPA axis activity. We found that FCM concentrations were not correlated with three measures of behavioral coping styles (activity, aggression, and docility) among individuals. This does not support the hypothesis that hormonal pleiotropy underlies a proactive-reactive continuum of coping styles. Instead, our results support the "two-tier" hypothesis that behavioral and physiological stress responses are independent and uncorrelated traits among individuals in wild populations that experience naturally varying environments rather than controlled environments. If also found in other studies, this may alter our predictions about the evolutionary consequences of behavioral and endocrine coping styles in free-living animals. Significance statement: Individuals vary in how they respond to stressors through behavior and physiology, but we find the two responses are independent in wild animals. Many laboratory studies find links between the behavioral and physiological stress responses; however, studies conducted with wild populations are less conclusive. In wild North American red squirrels, independence between the physiological response and behavioral response may allow adaptive responses to a changing environment without pleiotropic constraint. [ABSTRACT FROM AUTHOR]

A wide range of species have been found to differentiate kin from nonkin. However, the ability to recognize kin, or the costs and benefits of discriminating kin from nonkin may depend on particular extrinsic environmental or intrinsic physiological conditions, resulting in context-dependent kin discrimination. North American red squirrels, Tamiasciurus hudsonicus , have previously been shown to be capable of discriminating the territorial calls of kin and nonkin. More recently, post hoc analysis of existing data suggested that kin discrimination might depend on the state of the caller (i.e. acute stress). Here we tested this hypothesis with independently collected data from a repeated-measures playback experiment and found no evidence in support of acute stress mediating kin discrimination. While squirrels were shown to register and respond to playbacks, exposure to a trapping/handling stressor in the caller had no effect on the receiver's response to the calls of kin and nonkin. Furthermore, we found no overall discrimination between the calls of kin and nonkin. These independently collected data instead suggest that the previous post hoc finding consistent with context-dependent kin discrimination was likely a spurious relationship associated with the data from which the post hoc hypothesis was generated rather than representing a real biological relationship. Our findings from this study emphasize the importance of testing post hoc hypotheses with independent experiments. • We test whether acute stress enhances red squirrel kin discrimination of rattle calls. • We found no evidence of stress-mediated or general kin discrimination. • These findings falsify a previously posited hypothesis in the literature. • Kin discrimination in red squirrels may be more closely linked to food resources. [ABSTRACT FROM AUTHOR]

The pace-of-life syndrome hypothesis posits that personality traits (i.e. consistent individual differences in behaviour) are linked to life history and fitness. Specifically, fast-paced individuals are predicted to be proactive (i.e. active and aggressive) with an earlier age at first reproduction, a shorter life span and higher fecundity than slow-paced individuals. Environmental conditions and sex differences may be important in maintaining behavioural and life history variation in populations and may influence the covariance of personality with life history or lifetime fitness. However, these effects are rarely tested together. We investigated whether the occurrence of a resource pulse (called a mast year) during adulthood altered the associations between personality and life history traits or lifetime offspring production in adult North American red squirrels, Tamiasciurus hudsonicus. Despite accounting for environmental context during adulthood, we found no evidence of an overall pace-of-life syndrome in this population as personality was not associated with age at first reproduction or longevity in either sex. Males and females had similar activity levels, but females were more aggressive, potentially due to the fitness benefits of protecting their offspring from predation. In all females, regardless of mast experience, there was no association between activity and lifetime pup production, but there was a positive association between aggression and lifetime pup production. In males that experienced a mast, there was a positive association between lifetime pup production and both activity and aggression. In males that did not experience a mast, there was no association between activity and lifetime pup production, but there was a negative association between aggression and lifetime pup production. Lifetime recruit production (number of adult offspring recruited into the breeding population) in either sex was not influenced by activity or aggression regardless of mast experience. Overall, our results suggest that the infrequent occurrence of mast years may contribute to maintaining variation in personality traits in red squirrels. • We found no evidence of an association between personality and life history. • Aggression was associated with higher lifetime fitness in females. • In males the association between fitness and personality was context specific. [ABSTRACT FROM AUTHOR]

For territorial species, the ability to be behaviourally plastic in response to changes in their social environment may be beneficial by allowing individuals to mitigate conflict with conspecifics and reduce the costs of territoriality. Here we investigated whether North American red squirrels, Tamiasciurus hudsonicus , are able to minimize costs of territory defence by adjusting behaviour in response to the familiarity of neighbouring conspecifics. Since red squirrels living in familiar neighbourhoods face reduced intrusion risk, we predicted that increasing familiarity among territorial neighbours would allow squirrels to spend less time on territorial defence and more time in the nest. Longitudinal behavioural data (1995–2004) collected from the same squirrels across several different social environments indicated that red squirrels reduced rates of territorial vocalizations and increased nest use in response to increasing familiarity with neighbours. In contrast, cross-sectional data (2015–2016), which provided observations from each individual in a single social environment, did not provide evidence of this plasticity. Post hoc analyses revealed that evidence of social plasticity in this system is primarily due to within-individual changes in behaviour, which we were unable to estimate in the cross-sectional data. Our results demonstrate that red squirrels respond to changes in their social environment by adjusting their behaviour in a manner that reduces the costs of territoriality. However, our results also suggest that estimating plasticity by comparing behaviour among individuals (i.e. cross-sectional analyses) may not always be reliable. Our ability to detect these effects may therefore depend on having data with multiple observations from the same individuals across different social environments. • Social plasticity may allow species to minimize costs of territory defence. • Red squirrels with familiar neighbours reduced rates of territorial rattle calls. • Squirrels with familiar neighbours also increased time spent in nest. • Squirrels showed social competence by reducing defence in low-risk environments. • Detecting plasticity depended on measuring within-individual changes in behaviour. [ABSTRACT FROM AUTHOR]

Interactions with conspecifics are an important aspect of an individual's environment. Although it is well known that the presence of conspecifics can have important effects on behaviour, in general it is also now acknowledged that the composition of the social environment can vary, and that this variation may have profound effects on individual behaviour and fitness. Using a wild population of North American red squirrels, Tamiasciurus hudsonicus , we investigated the importance of the composition of the social environment in a territorial species by assessing whether the risk of intrusion faced by territory owners varied with the degree of relatedness and familiarity in their social neighbourhoods. To test this, we conducted temporary removals of territory owners and observed the time until intrusion and the identity of intruding individuals. We found that individuals in neighbourhoods with low average familiarity faced a higher risk of intrusion and that unfamiliar neighbours were more likely to intrude. Surprisingly, we found that related neighbours also posed a higher risk of intrusion. The results from our study suggest that familiarity with neighbours may be an ecologically and evolutionarily relevant measure of the social environment, even in a species considered to be ‘asocial’. Future studies should consider the potential importance of the social environment, which has heretofore been mostly overlooked, as a relevant selective pressure in asocial, territorial species. [ABSTRACT FROM AUTHOR]

Evolutionary endocrinology aims to understand how natural selection shapes endocrine systems and the degree to which endocrine systems themselves can induce phenotypic responses to environmental changes. Such responses may be specialized in that they reflect past selection for responsiveness only to those ecological factors that ultimately influence natural selection. Alternatively, endocrine responses may be broad and generalized, allowing organisms to cope with a variety of environmental changes simultaneously. Here, we empirically tested whether the endocrine response of female North American red squirrels (Tamiasciurus hudsonicus) was specialized or generalized. We first quantified the direction and magnitude of natural selection acting on three female life history traits (parturition date, litter size, offspring postnatal growth rate) during 32 years of fluctuations in four potential ecological agents of selection (food availability, conspecific density, predator abundance, and temperature). Only three of the four variables (food, density, and predators) affected patterns of natural selection on female life history traits. We then quantified fecal glucocorticoid metabolites (FGMs) across 7 years and found that all four environmental variables, regardless of their effects on patterns of selection, were associated with glucocorticoid production. Our results provide support for a generalized, rather than specific, glucocorticoid response to environmental change that can integrate across multiple co-occurring environmental stressors. • Hormonal responses to environmental change can be generalized or specialized. • In highly fluctuating environments, a specialized hormonal response may maximize fitness. • Food, density, and predators, but not temperature, affect selection on female life history traits. • Female squirrels exhibit glucocorticoid responses to all four environmental factors. • Squirrels mount a generalized, rather than specific, response to environmental change. [ABSTRACT FROM AUTHOR]

The ability to discriminate among individuals, or among classes of individuals, can provide animals with important fitness benefits. Although several mechanisms for discrimination are possible, most require animals to show stable phenotypic variation that reflects their identity or their membership in a particular class (e.g. sex, mate, kin). For territorial animals that rarely interact physically, vocalizations could serve as long-distance signals that facilitate discrimination. In this study, we tested whether the territorial rattle vocalizations of North American red squirrels, Tamiasciurus hudsonicus , are repeatable, and whether they could hence provide the basis for multiple types of discrimination. We measured four structural features from two rattles from each of 76 marked squirrels. All four features were repeatable, which is consistent with territorial rattles being individually distinctive. We then conducted a playback experiment to determine whether squirrels use rattles for discrimination. Specifically, we tested whether squirrels discriminate between the rattles of neighbours and non-neighbours, and kin (coefficient of relatedness, r ≥ 0.25) and non-kin ( r < 0.125). Following a 2 × 2 factorial design, we broadcast a rattle from a non-neighbouring nonkin individual to 15 subjects, from a neighbouring nonkin individual to 14 subjects, from a non-neighbouring kin individual to 11 subjects, and from a neighbouring kin individual to 13 subjects. Subjects did not discriminate between the rattles of neighbours and non-neighbours, but did respond differently to the rattles of kin and nonkin. Specifically, squirrels were significantly more likely to produce a rattle of their own in response to the broadcasted rattles of nonkin versus the broadcasted rattles of kin. This result demonstrates that red squirrels can use territorial vocalizations for kin discrimination. It also suggests that they are more tolerant of territorial intrusions by kin. [ABSTRACT FROM AUTHOR]

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. 3–5 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. • In red squirrels, fitness is affected by relationships with territory neighbors • Neighbor familiarity enhanced reproduction and survival whereas living near kin did not • The fitness benefits of familiarity were strong enough to offset the effects of aging • Mutually beneficial interactions might play a central role in conflict resolution Siracusa et al. investigate whether neighbors affect the fitness of territorial red squirrels and show that familiarity with neighbors, not kinship, increases reproduction and survival. They find these effects are strongest in the senescent period, with potential to offset age-related declines in survivial and reproductive success. [ABSTRACT FROM AUTHOR]

In fluctuating environments, mothers may enhance the fitness of their offspring by adjusting offspring phenotypes to match the environment they will experience at independence. In free-ranging red squirrels, natural selection on offspring postnatal growth rates varies according to population density, with selection favoring faster-growing offspring under high-density conditions. We show that exposing mothers to high-density cues, accomplished via playbacks of territorial vocalizations, led to increased offspring growth rates in the absence of additional food resources. Experimental elevation of actual and perceived density induced higher maternal glucocorticoid levels, and females with naturally or experimentally increased glucocorticoids produced offspring that grew faster than controls. Therefore, social cues reflecting population density were sufficient to elicit increased offspring growth through an adaptive hormone-mediated maternal effect. [ABSTRACT FROM AUTHOR]