Predation risk effects on intense and routine vigilance of Burchell's zebra and blue wildebeest

Abstract

Prey increase vigilance to maximize predator detection, but this comes at the expense of foraging depending on the vigilance type: (1) intense vigilance, when all feeding processes are ceased, and (2) routine vigilance, when animals continue chewing (i.e. lower foraging cost). Few studies have distinguished between vigilance types when examining the effects of predation risk and, in the absence of a commonly accepted conceptual framework, the variables used to define predation risk vary greatly between studies. We investigated the relative importance of four predation risk categories (risky place assessed at the landscape and habitat level, vegetation characteristics at foraging site level, prey characteristics and resource availability) for the time spent on intense and routine vigilance by Burchell's zebra, Equus quagga burchellii, and blue wildebeest, Connochaetes taurinus taurinus, under predation risk from reintroduced predators, lions, Panthera leo. The risk categories each represented a different predation risk component and included metrics that defined the component at multiple scales. Intense vigilance responses were scale dependent, with zebra responding to a risky place at the landscape level and wildebeest to vegetation characteristics at a foraging area scale. Yet both species were able to adjust and balance time spent on vigilance types. Prey characteristics reduced the intense vigilance of wildebeest as herd size increased. Both species maintained similar levels of intense vigilance between seasons, despite lower resource availability, and thus higher foraging costs, in the dry season. However, the reduction in grass quality probably resulted in the increase in routine vigilance by both species during the dry season, as more time was needed to chew grasses with high fibre content. Our findings suggest different underlying mechanisms for the two types of vigilance behaviour, which were influenced by the ecology of the species, and demonstrate the importance of distinguishing vigilance types in predation risk studies.