The environmental roots of the relationship between glucocorticoids and body mass 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 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. ### Competing Interest Statement The authors have declared no competing interest.