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. © 2010 The Royal Society

Short-term behavioural responses of impalas in simulated antipredator and social contexts.

Prey animals often have to trade off foraging against vigilance. However, vigilance is costly and individuals are expected to adjust their vigilance and its cost in relation to social cues and their predation risk. To test this, we conducted playback experiments in the field to study how lions' (Panthera leo) roars and male impalas' (Aepyceros melampus) territorial vocalizations affected the vigilance and foraging behaviours as well as movements of female impalas. Our results show that impalas adjusted their activities in different ways depending on the vocalizations broadcast. After lions' roars were played, female impalas increased their vigilance activity (in particular increasing their high-cost vigilance--vigilance without chewing), decreased their bite rates and increased their movements, whereas male impalas' vocalizations caused females to decrease their vigilance (decreasing their low-cost vigilance--vigilance while chewing) and increase their movements without affecting their bite rates. Therefore, it appears that predators' vocalizations stimulate anti-predator behaviours such as vigilance and movement at the expense of foraging, whereas males' vocalizations increase individuals' displacements at the expense of vigilance. Overall, this study shows that both predator and social cues have direct effects on the behaviour of gregarious prey and need to be considered in future studies

How unpredictable is the individual scanning process in socially foraging mammals?

International audienceIn group-forming prey species, theory assumes that individuals within groups should scan independently of one another, with vigilance sequences being relatively unpredictable, making interscan durations highly variable. We attempted to detect any divergence from randomness in the scanning process in three mammalian prey species phylogenetically and geographically separated and exposed to different levels of predation: waterbuck, Kobus ellipsiprymnus defassa, under a high observed predation risk, eastern grey kangaroo, Macropus giganteus, still experiencing occasional predation and European roe deer, Capreolus capreolus, under a very low natural predation risk. Our results revealed that the focal interscan duration increased when the duration of the preceding interscan increased, whatever the studied species and the predation risk that its individuals experienced, and decreased with the preceding scan duration in two species under, respectively, occasional and low predation risks. The exponential distribution was the tested model that fitted the observed distributions of interscan durations least well. We discuss what can trigger non-randomness in scanning, through a non-homogenous Poisson process, at both intra-individual and inter-individual levels, particularly with regard to previous studies that have demonstrated synchronisation of vigilance in such mammals. Our results suggest the need to reconsider any assumption of randomness in scanning in the basic model predicting form and frequency of scanning behaviour by prey species.</p

Effects of playbacks on the use of exclusive vigilance and vigilance while chewing.

<p>Mean proportions of time (± SE) spent by female impalas in (A) exclusive vigilance and (B) vigilance while chewing during the pre- and post-playback periods after their exposure to playbacks of control stimuli, lions’ roars and male impalas’ calls. *, ** and *** indicate significance at the p < 0.05, p < 0.01 and p < 0.001 levels, respectively.</p

Effects of playbacks on females’ behaviour.

<p>Mean (A) proportions of time spent in vigilance (± SE), (B) bite rates (± SE) (numbers of bites per minute during foraging), and (C) step rates (± SE) (numbers of steps per minute) of female impalas exposed to control stimuli, playbacks of lions’ roars and male impalas’ calls during pre- and post-playback periods. *, ** and *** indicate significance at the p < 0.05, p < 0.01 and p < 0.001 levels, respectively.</p

Candidate vigilance models modified from Dehn (1990).

<p>N: group size; N_a: Number of actively vigilant individuals (N - number of juveniles); Pw: Proportion of mothers with an offspring; N_f: Number of all actively vigilant group members (N - number of males + juveniles); ai: parameters estimates.</p

Comparison between the best-fitting candidate models and a null model.

<p>Comparison between the best-fitting candidate models and a null model.</p

Model selection procedure using AICc statistics.

<p>The best-fitting model for each category is presented in bold.</p

Parameters estimates for the selected best-fitting candidate models for a) females with dependant juveniles, b) females without juvenile and c) males.

<p>Parameters estimates for the selected best-fitting candidate models for a) females with dependant juveniles, b) females without juvenile and c) males.</p