Intra- and interspecific variation in vigilance and foraging of two gerbillid rodents, Rhombomys opimus and Psammomys obesus:: the effect of social environment

We studied time budgets in two closely related and ecologically similar gerbillids to test the effect of group size on vigilance and foraging in adult females. Psammomys obesus is strictly solitary, while Rhombomys opimus is mainly social. We compared time budgets of (1) solitary P. obesus females, (2) solitary R. opimus females and (3) R. opimus females living in male-female pairs. Solitary R. opimus females spent more time underground, more time in low-cost vigilant postures, moved more, hoarded food more and fed above ground less than paired females. However, females of both categories spent similar time in high-cost upright postures. These results conform mainly to the group size effect hypothesis, while the high level of high-cost vigilance in paired females can be attributed to within-group vigilance, masking the group size effect. Solitary females of P. obesus and R. opimus showed similar time budget patterns; however, P. obesus allocated more time to high-cost vigilance and less time to feeding. In general, differences in time budgets between heterospecific females of the same social status (solitary or paired) were less pronounced than differences between conspecifics of different status. Thus, variation in activity patterns of females can be largely explained by different social conditions rather than by species affiliation. We discuss the results in terms of predation avoidance strategies in solitary and social species. (C) 2001 The Association for the Study of Animal Behaviour.