Biology subjects

Dantzer, B.

Publications and source records attributed to Dantzer, B..

29 records · Page 2Linked to original sources

bioRxiv2018

Seed masting causes fluctuations in optimum litter size and lag load in a seed predator

The episodic production of large seed crops by some perennial plants, is referred to as masting and is known to increase seed escape by alternately starving and swamping seed predators. These pulses of resources, however, 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. Lag loads in seed predators could result from mast-induced shifts in optimum phenotypes that exceed the capacity of seed predators to adaptively track optimum phenotypes through phenotypic plasticity. Alternatively, masting could generate mismatches in selection across generations, where adaptation to the parental environment leads to maladaptation in the offspring environment. Here we measured natural selection on female North American red squirrels (Tamiasciurus hudsonicus) across 28 years and five white spruce (Picea glauca) masting events. Red squirrel litter sizes were similar to optimum litter sizes during non-mast years, but were well below optimum litter sizes during resource-rich mast years. Mast events, therefore caused selection for larger litters ({beta} = 0.25) and a lag load (L = 0.25) on red squirrels during mast years. Furthermore, we found that the annual fitness of spruce trees was negatively related to the local density of squirrels during mast years, indicating that the observed lag load on squirrels enhanced the number of spruce cones escaping squirrel predation. Although, the frequency of mast events and the demography of red squirrels were such that 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. Variable selection on litter size caused by white spruce mast events, therefore, induced a lag load on the population of red squirrels that was not affected by whether individual squirrels were born during mast (matching) or non-mast (mismatching) conditions.

evolutionary biology↗

bioRxiv2018

Stress activity is not predictive of coping style in North American red squirrels

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.\n\nSignificance StatementIndividuals 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.

animal behavior and cognition↗

bioRxiv2018

Is physiological stress state reflected in acoustic structure of vocalizations? An experimental test in wild North American red squirrels

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, very few studies have experimentally examined the acoustic expression of short-term stress state. The transmission of such information about physiological state could have broad implications, potentially allowing other individuals to modify their behavior or life history traits in response to this public information. 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. Our results provide mixed evidence that rattles show 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 received control treatments and unmanipulated (unfed) squirrels. Our results indicate that short-term stress does affect the acoustic structure of vocalizations, but elevated circulating GC levels are not solely responsible for such changes.

animal behavior and cognition↗

bioRxiv2018

Red squirrels mitigate costs of territory defence through social plasticity

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) were 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. Long-term 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 was 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 can reduce the costs of territoriality by appropriately adjusting behaviour in response to changes in their social environment. 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.

animal behavior and cognition↗

bioRxiv2018

Red squirrel territorial vocalizations deter intrusions by conspecific rivals

In many species, territory defense 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 energy 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 owners presence after its removal by broadcasting the owners rattle from a loudspeaker at the center of the territory once every seven minutes. During the control treatment the territory was left in silence after 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 vocalizations alone are not sufficient without other cues of the territory owner.

animal behavior and cognition↗

bioRxiv2018

Individual variation in the dear enemy phenomenon via territorial vocalizations in red squirrels

Territoriality arises when the benefits of resources exceed the costs of defending them. The dear enemy phenomenon, where familiar territorial neighbours refrain from intruding on one another and mutually reduce their defensive efforts, allows for reduction of these costs but requires discrimination between conspecifics. We hypothesized that territorial vocalizations in red squirrels (Tamiasciurus hudsonicus) are used for this discrimination. We performed a speaker replacement experiment where red squirrels (n = 41) were temporarily removed from their territories and replaced with a speaker broadcasting their own call, an unfamiliar call, or silence. Contrary to our prediction, there were no differences in overall intrusion risk among our three playbacks, but the identity of intruders did vary. Existing variation in familiarity within territorial neighbourhoods should be considered, rather than the binary classification of familiar or stranger, when studying dear enemy effects. We also discuss the variable importance of silence in acoustic territorial populations.

animal behavior and cognition↗

bioRxiv2018

Indirect effects on fitness between individuals that have never met via an extended phenotype

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 which mean the environments others experience are altered. North American red squirrels (Tamiasciurus hudsonicus) guard food hoards, an extended phenotype that typically outlives the individual and is almost always inherited by non relatives. Hoarding by previous owners can therefore influence subsequent owners. We found that red squirrels bred earlier and had higher lifetime fitness if the previous 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 others resource dependent traits and fitness without meeting via extended phenotypes, and so the past can influence contemporary population dynamics.

ecology↗

bioRxiv2018

Indirect effects of territorial neighbors on the timing of spring breeding may counteract changes in selection in North American red squirrels

Organisms can affect one anothers phenotypes when they socially interact. Indirect genetic effects occur when an individuals 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 variance parameters and the strength of selection at low and high population densities. Social effects of neighbours on the date of spring breeding were weak at low, but stronger at high population densities. Indirect phenotypic effects accounted for a larger amount of variation in the date of breeding than direct differences among-individuals, although the genetic component to these indirect effects was not statistically significant. Nevertheless, the estimated effect size was large enough to suggest that indirect genetic effects could alter evolutionary change, resulting in less change at high densities despite stronger selection. Despite the difficulty in estimating them precisely, indirect genetic effects have clear potential to alter evolutionary trajectories in any natural systems where organisms interact.

evolutionary biology↗

bioRxiv2018

Maternal stress increases cooperative behaviour of offspring in wild meerkats

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. We show that in Kalahari meerkats (Suricata suricatta), the circulating glucocorticoid (GC) 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 GC-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 grew more slowly 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 GCs as they aged, which could explain the observed increases in cooperative behaviour. Because early life growth is a crucial determinant of fitness in female meerkats, our results indicate that GC-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.

animal behavior and cognition↗

bioRxiv2018

Experimental increases in glucocorticoids alter function of the neuroendocrine stress axis in wild red squirrels without negatively impacting survival and reproduction

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.

zoology↗

bioRxiv2017

Multilevel And Sex-Specific Selection On Competitive Traits In North American Red Squirrels

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.\n\nData archival: the data set is archived on Dryad (info XXX), with a five-year embargo from the date of publication.

evolutionary biology↗