An experimental study of behavioural group size effects in tammar wallabies, Macropus eugenii

As animals aggregate with others, the time they allot to social and nonsoci al activities changes. Antipredator models of vigilance and foraging group size effects both predict a nonlinear relationship between group size and t he time allocated to behaviour. Group size effects were experimentally stud ied in captive adult female tammar wallabies, a small macropodid marsupial, by increasing group size from 1 to 10. Tammars foraged more, looked less, groomed more, engaged in more aggressive interactions and moved about less as group size increased. Nonlinear regression models explained more variati on in the time allocated to foraging, looking, locomotion and affiliative b ehaviour than linear models. Variation in self-grooming and aggression was better explained by linear models. Wallabies lay down significantly more, a nd walked significantly less, as group size increased: these relationships were significantly nonlinear. Thus, changes in perceived predation risk, wh ich are characterized by nonlinear relationships, explain tammar wallaby gr oup size effects for most activities. These results support the assertion t hat predation has played an important role in macropodid social evolution. Moreover, the findings suggest that conservation biologists should pay part icular attention to group size when translocating or reintroducing endanger ed macropodids. (C) 1999 The Association for the Study of Animal Behaviour.