The influence of the behaviour of conspecifics on responses to threat in the Mongolian gerbil, Meriones unguiculatus

Assessing blue wildebeests' vigilance, grouping and foraging responses to perceived predation risk using playback experiments

Two aspects of reactive antipredator behaviour are still unclear for ungulates. First, when there is a direct predation threat, how do prey balance antipredator and social vigilance to learn a predator's location and assess the risk? Second, how do an individual's group and environment affect its responses? We tested the responses of adult females in 101 groups of wildebeest to playbacks of lion roars or car noises in Etosha National Park, Namibia. We analysed how the times they spent in different types categories of vigilance, and their within-group density, were affected by the playbacks and how a range of social and environmental variables affected those responses. Females increased their antipredator vigilance but not their social vigilance, after lion roars but not car noises, suggesting that they mostly relied on their own vigilance rather than social information to try to find the source of the lion roars. Females' antipredator vigilance increased more when they were further from cover and with other prey species, suggesting that both circumstances increased their perception of risk. They 'bunched' more after lion roars than car noises and their bite rates decreased as they bunched. Animals' use of social information about threats is likely to be context-dependent.

Individual variation in vigilance in female eastern grey kangaroos

For herbivores, vigilance usually involves a trade-off with foraging, and a further trade-off between antipredator and social vigilance. Thus individual variation in vigilance may result in significant fitness consequences. We used mixed-effects models to document individual variation in vigilance in wild adult female eastern grey kangaroos (Macropus giganteus) in Queensland, Australia. We tested the effects of group size, presence of adult males and wind on vigilance patterns and variation by analysing 399 five-minute samples on 31 individually identified females who were foraging on the periphery of groups. We determined the proportion of time that individuals spent vigilant, and the proportions of vigilance time spent in social versus antipredator vigilance and in low- versus high-intensity vigilance postures. The proportion of time females spent vigilant decreased with increasing group size but was not affected by wind speed or male presence, while the amount of their vigilance time spent in social vigilance was affected by interactions among the variables. After controlling for these effects, 13 females’ vigilance patterns were significantly different from the population mean. Individual variation was also found in vigilance postures used and the effect of male presence. Understanding individual variation in vigilance is important for understanding individuals’ antipredator and social strategies.

Interactions among social monitoring, anti-predator vigilance and group size in eastern grey kangaroos

Group size is known to affect both the amount of time that prey animals spend in vigilance and the degree to which the vigilance of group members is synchronized. However, the variation in group-size effects reported in the literature is not yet understood. Prey animals exhibit vigilance both to protect themselves against predators and to monitor other group members, and both forms of vigilance presumably influence group-size effects on vigilance. However, our understanding of the patterns of individual investment underlying the time sharing between anti-predator and social vigilance is still limited. We studied patterns of variation in individual vigilance and the synchronization of vigilance with group size in a wild population of eastern grey kangaroos (Macropus giganteus) subject to predation, in particular focusing on peripheral females because we expected that they would exhibit both social and anti-predator vigilance. There was no global effect of group size on individual vigilance. The lack of group-size effect was the result of two compensating effects. The proportion of time individuals spent looking at other group members increased, whereas the proportion of time they spent scanning the environment decreased with group size; as a result, overall vigilance levels did not change with group size. Moreover, a degree of synchrony of vigilance occurred within groups and that degree increased with the proportion of vigilance time peripheral females spent in anti-predator vigilance. Our results highlight the crucial roles of both social and anti-predator components of vigilance in the understanding of the relationship between group size and vigilance, as well as in the synchronization of vigilance among group members.