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.

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.

In species with marked sexual dimorphism, the classic prediction is that the sex which undergoes stronger intrasexual competition ages earlier or quicker. However, more recently, alternative hypotheses have been put forward, showing that this association can be disrupted. Here, we utilize a unique, longitudinal data set of a semi‐captive population of Asian elephants (Elephas maximus), a species with marked male‐biased intrasexual competition, with males being larger and having shorter lifespans, and investigate whether males show earlier and/or faster body mass ageing than females. We found evidence of sex‐specific body mass ageing trajectories: adult males gained weight up to the age of 48 years old, followed by a decrease in body mass until natural death. In contrast, adult females gained body mass with age until a body mass decline in the last year of life. Our study shows sex‐specific ageing patterns, with an earlier onset of body mass declines in males than females, which is consistent with the predictions of the classical theory of ageing.