Dichotomies in perceived predation risk of drinking wallabies in response to predatory crocodiles

Most advances in our understanding of predation risk involve foraging animals, yet animals also trade off predation risk with other essential activities. Drinking under the risk of predation may elicit similar prey behaviours to those in foraging animals, but has been little studied. Agile wallabies, Macropus agilis, manipulate their environment by excavating drinking holes just out of reach of predatory saltwater crocodiles. Paradoxically, however, some wallabies drink from the river's edge despite the immediate availability of the lower-risk drinking holes. We studied the behaviour of wallabies with multiple drinking options to test a series of predictions for animals drinking under the risk of predation. More wallabies drank during the day than at night, regardless of drinking site. However, night-drinking wallabies were much more likely to use the excavated holes than the 'riskier' river. This spatiotemporal dichotomy in perceived predation risk may reflect a mismatch in nocturnal sensory capabilities between predator and prey, and would provide sufficient motivation for wallabies to create low-risk drinking holes. Drinking duration, our surrogate for harvest, was not influenced by drinking site type (hole versus river), drinking time or social context (solitary versus group), suggesting that some animals may not forego drinking during high-risk periods, as predicted and observed for some foraging animals. Compared to wallabies in groups, solitary wallabies were less likely to drink from the river, especially at night, and were more vigilant. Although evidence is limited, our research suggests that vigilance may be ineffective for nocturnally drinking wallabies in the face of this concealed, ambush predator.