You're stressing me out! Effect of interspecific competition from red deer on roe deer physiological stress response

Cortisol is the main glucocorticoid hormone commonly measured to assess adrenocortical activity in vertebrates in response to potential stressors. Competition and habitat quality may act as chronic stressors leading to an increase in cortisol accumulation. Moreover, chronic cortisol concentrations may seriously hamper the reproductive potential, health status and body condition of a species. Using hair collected from the rump of 88 adult roadkill roe deer Capreolus capreolus in the north-eastern Italy, we aimed to assess the effect of both ungulate competitor population densities and habitat suitability on the long-term physiological stress response of roe deer, using hair cortisol as a reference parameter. Our findings revealed that roe deer showed higher cortisol concentrations in response to higher red deer Cervus elaphus population density, most likely as a consequence of direct interspecific competition. No significant cortisol changes were however recorded in response to both wild boar Sus scrofa and roe deer population densities, probably as a result of reduced competition because of abundant feeding resources, different species population densities and spatial/temporal avoidance among species. Moreover, the absence of a significant effect as far as habitat suitability is concerned, suggest that roe deer are well adapted to live even in poor quality habitats most likely as long as they are close to better quality habitats which provide access to food resources and refuge sites. Our results highlight the importance of hair cortisol as a suitable parameter to assess physiological chronic stress response towards different environmental/ecological conditions.