Individual vigilance of African herbivores while drinking: the role of immediate predation risk and context

Perceived risk of predation is considered to influence individual prey vigilance, but little is known about the extent to which the context (e.g. distance to cover, group size) and the immediate presence of predators determine individual prey vigilance. We combined behavioural data on individual vigilance of herbivores at waterholes with fine-scale spatiotemporal data from 27 African lions, Panthera leo, fitted with GPS radiocollars to test whether individual prey vigilance increases when predators are in the vicinity and whether this relationship is influenced by group size and presence of other herbivores. The study was conducted on giraffe, Giraffa camelopardalis, greater kudu, Tragelaphus strepsiceros, and plains zebra, Equus quagga, in Hwange National Park, Zimbabwe, between mid-September and mid-November 2008. Kudu was the only species to adjust its vigilance level significantly in the presence of lions, arguably owing to its higher contribution to lion diet in Hwange. Kudu devoted a significantly higher proportion of time to vigilance when lions were in the vicinity. Furthermore, the proportion of time spent on individual vigilance decreased as group size increased during the approach and drinking phases, particularly when lions were in the vicinity. Finally, the presence of other herbivores at the waterhole enabled kudu to spend less time vigilant while drinking. These adjustments involved changes in the length of vigilance bouts, but not in their frequency. Our study suggests that the interplay between the context (group size, presence of other prey) and the immediate presence of predators determines the level of vigilance in prey.