Age, seasonality, and correlates of aggression in female Apennine chamois

Intrinsic and environmental stressors, such as age and seasonality, may influence social behavior and endocrine levels in gregarious foragers, but little is known about how season and age affect both behavioral and physiological responses. We evaluated seasonal/age variation of aggression and vigilance, and seasonal/age variation of endocrine levels (fecal cortisol and testosterone metabolites), in a gregarious herbivore, the Apennine chamois Rupicapra pyrenaica ornata. We examined a period of decreasing resource abundance and maternal care from July to October, a key period for nursing, weaning, and early growth of offspring. Aggression rate, vigilance, and cortisol levels decreased throughout July–October, while aggression intensity showed the reverse. Aggression intensity peaked when chamois were on the most nutritious vegetation patches. Dominance increased with age, and prime-aged females (4–8 years old) showed higher cortisol and testosterone levels and were involved in aggressive interactions more often than subadult or older females. Our findings suggest that, in early summer, when nursing effort is the greatest, selection of nutritious food patches leads to frequent encounters between female chamois, enhancing aggression rate, vigilance, and endogenous stress response. The progressive decrease in food abundance throughout July–October triggers competition for scarce resources and increased intensity of aggression. Most likely, the energetic demands of lactation and offspring guarding were key determinants of behavioral and physiological stress of female chamois. Our results suggest a multi-factorial compromise between reproductive state and stress levels, in a group-living species.