Wildlife populations are increasingly exposed to human-induced modifications of their habitats. To cope with anthropogenic stressors, animals can adjust their behaviour—for example, by shifting their activity to more sheltered habitats, or becoming more nocturnal. However, whether use of spatial and temporal adjustments in behaviour may regulate the endocrine response is poorly documented. Here, we analyzed faecal cortisol metabolites (FCMs) of wild roe deer (Capreolus capreolus) living in a human-dominated agro-ecosystem. Using Global Positioning System monitoring of 116 individuals, we assessed their spatial behaviour and tested whether proximity to anthropogenic structures (linear distance to built-up areas) and the use of refuge habitats (woodland and hedges) influenced FCM levels. In accordance with our predictions, individuals ranging closer to anthropogenic structures during daytime had higher FCM levels, but this relationship was buffered as use of refuge habitat increased. In addition, this link between proximity to anthropogenic structures and FCM levels disappeared when we analyzed spatial behaviour at night. Finally, FCM levels were higher when the ambient temperature was lower, and during years of low resource availability. Our results demonstrate that the stress levels of large mammals may be strongly influenced by their proximity to anthropogenic activities, but that these effects may be buffered by behavioural adjustments in terms of space use and circadian rhythm. Whereas most studies have focused on the influence of environmental heterogeneity, our analysis highlights the need to also consider the fine-scale spatial response of individuals when studying the hormonal response of wild animals to human disturbance. We emphasize the potential to mitigate this hormonal stress response, and its potential negative consequences on population dynamics, through the preservation or restoration of patches of refuge habitat in close proximity to human infrastructure.

While evidence that telomere length is associated with health and mortality in humans and birds is accumulating, a large body of research is currently seeking to identify factors that modulate telomere dynamics. We tested the hypothesis that high levels of glucocorticoids in individuals under environmental stress should accelerate telomere shortening in two wild populations of roe deer (Capreolus capreolus) living in different ecological contexts. From two consecutive annual sampling sessions, we found that individuals with faster rates of telomere shortening had higher concentrations of fecal glucocorticoid metabolites, suggesting a functional link between glucocorticoid levels and telomere attrition rate. This relationship was consistent for both sexes and populations. This finding paves the way for further studies of the fitness consequences of exposure to environmental stressors in wild vertebrates.

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Environmental fluctuations force animals to adjust glucocorticoids (GCs) secretion and release to current conditions. GCs are a widely used proxy of an individual stress level. While short-term elevation in GCs is arguably beneficial for fitness components, previous studies have documented that the relationship between long-term baseline GCs elevation and fitness components is largely inconsistent. Using longitudinal data on roe deer (Capreolus capreolus) from two populations with markedly different environmental contexts, we tested whether baseline GC levels negatively correlate with body mass - a trait positively associated with demographic individual performance - on the short- to long-term. In support, higher baseline GC concentrations were associated to lighter body mass, both measured during the same capture event, in juvenile and adult males of both populations. We also found evidence of a stronger association between higher GC levels and lower body mass for males born in poor years. GCs were not related to body mass neither on the long-term, nor in females of neither population. We propose that the sex-specific association between GCs and body mass are consistent between populations and regulated by environmental conditions at a finer-scale, and are driven by sex-specific life histories. The sex- and context-dependent responses of stress highlight the complexity of studying stress in the wild.

The use of fecal corticosteroid metabolites (FCMs) has proven to be well suited to evaluate adrenocortical activity, a major component of the stress response, particularly in wildlife. As with any tools, confounding factors and drawbacks must be carefully considered. Among them, sample preservation and storage are of particular importance, as they can affect stability of FCMs and lead to biased results and interpretations. Arguably, immediate freezing of fecal samples upon collection is the best practice to preserve FCM integrity, however, for logistical reasons, this condition is rarely feasible in the field. It is generally argued that temporary storage of samples at low above-zero temperature is an acceptable way of preserving samples in the field before freezing them for long-term storage.However, to our knowledge, there is no empirical study that demonstrates the stability of fecal metabolites in samples stored at +4°C. In this study, we collected a fresh fecal sample from 20 captive roe deer, each of which was homogenized and split into three subsamples (60 subsamples in total) to investigate the effects on FCMs levels of temporary storage at +4°C for 24 h and 48 h before freezing versus immediate freezing at −20°C after feces collection. Compared to immediate

Environmental fluctuations force animals to adjust glucocorticoids (GCs) secretion and release to current conditions. GCs are a widely used proxy of an individual stress level. While short‐term elevation in GCs is arguably beneficial for fitness components, previous studies have documented that the relationship between long‐term baseline GCs elevation and fitness components can vary according to ecological and individual factors and according to the life‐history of the species studied. Using longitudinal data on roe deer Capreolus capreolus from two populations facing markedly different environmental contexts, we tested whether baseline GC levels negatively correlate with body mass – a trait positively associated with demographic individual performance – on the short‐ to long‐term. In support, higher baseline GC concentrations were associated to lighter body mass, both measured during the same capture event, in adults of both populations. Overall, we showed that despite the marked environmental and demographic differences between populations and despite the between‐sex differences in life history (i.e. reproductive tactics), the relationship between body mass and GCs is consistent across environmental contexts, but might differ according to the life history stage of an individual. This work opens promising perspectives to further explore the relationship between GC and fitness‐related traits according to life history stages in free‐ranging mammals across seasonal and environmental contexts. The timing and context‐dependence of GC levels highlight the complexity of studying stress responses in the wild.