Predation risk and herd position influence the proportional use of antipredator and social vigilance by impala

Abstract

Vigilance is just one of the many fitness-enhancing activities that animals do each day. However, as vigilance is used for both predator detection and obtaining social information, individuals must decide how to divide their time between these two vigilance types. Yet, it is unclear (1) how prey species living in the centre and edge of groups prioritize their use of these different vigilance types and (2) how this prioritization varies with increasing risk. To explore this, we focused on the degree to which impala, Aepyceros melampus, a herd-living antelope, adjusted their antipredator (looking out from the herd) and social (looking at herd members) vigilance across three sites with different predator guilds and predator densities. We found that as predator types and densities increased, herd size increased, but that impala did not adjust the total time they spent vigilant (antipredator plus social vigilance). Thus, they did not sacrifice other fitness-enhancing activities (e.g. foraging) to increase vigilance. Yet, overall, within the herds, edge individuals displayed greater total vigilance (antipredator plus social) and showed a greater proportional use of antipredator vigilance compared to central individuals. With increasing predator numbers, edge individuals maintained and ultimately increased their proportional use of antipredator vigilance. In contrast, central individuals reduced their use of antipredator vigilance and increased social vigilance. As suggested elsewhere, this adjustment might have been related to these individuals trying to increase foraging efficiency and cohesion benefits. However, they may have also monitored conspecifics to detect threats, while obtaining both short- (e.g. less time spent in a vulnerable head-down position) and long-term (e.g. reduced daily foraging time allowing more time for less vulnerable activities) antipredator benefits. Ultimately, our results highlight that herd position and predator differences can influence overall vigilance levels and how individuals adjust their use of antipredator and social vigilance.