Being “hangry”: food depletion and its cascading effects on social behaviour

Evolutionary theory suggests two alternative ways in which competitive interactions could vary in response to different levels of food abundance. Competition theory suggests that aggression should be greater when resource availability is lower, as an evolutionary stable strategy to access food. Alternatively, energy allocated to aggressive interactions should increase when the available spectrum of food resources is wider, in turn allowing a greater selection. We tested these hypotheses on a group-living herbivore, the Apennine chamois, Rupicapra pyrenaica ornata. We compared social, aggressive and vigilance behaviour and relevant endocrine correlates across three areas: two 'poorer' areas, i.e. with a lower availability of nutritious pasture, and a 'richer' one. In the richer area, we observed: (1) the largest group size/greatest proportion of young individuals in groups; (2) the lowest rate/intensity of aggression between individuals, at feeding; (3) the lowest duration of vigilance and proportion of 'costly' vigilance, i.e. postures performed without chewing food; and (4) the lowest levels of testosterone and cortisol metabolites, suggesting a lower endogenous aggressiveness/stress response. Our findings agree with the competition theory, suggesting a role of food depletion in increasing aggression between foraging individuals, as an evolutionary stable strategy, with cascading effects on group phenology, vigilance and stress.