Behavioural Trade-Offs in Impala: The Influence of Predators and Social Structure on Foraging and Vigilance

Abstract

Group-dwelling mammalian herbivores (e.g., impala) often have to balance between the amount of time spent foraging and being vigilant. Vigilance behaviour (scanning for predators) helps reduce predation risk while prey are vulnerable during foraging events. Impalas typically occur in either territorial herds containing dominant males, females and juveniles, or bachelor herds that contain only subordinate males. To assess how factors such as sex, social structure and the presence or absence of predators may affect impala behaviour, we examined the vigilance and foraging behaviour of impalas, both inside a game reserve with limited large predators as well as in the Kruger National Park, in eastern South Africa, with many predators present. In the low-predator environment, dominant males in territorial herds spent more time vigilant than females and juveniles, and males in territorial herds spent more time vigilant than males in bachelor herds. When comparing vigilance behaviour in areas with and without predators, females and bachelor males exhibited no difference in their behaviour between locations. Surprisingly, dominant males in territorial herds were more vigilant in low-predator environments than in high-predator environments (Kruger), which may suggest that vigilance by dominant males is directed towards competitor males rather than predators.