Yellow-footed rock-wallaby (Petrogale xanthopus) group size effects reflect a trade-off

As group size increases, individuals of many species modify the time allocated to antipredator vigilance and foraging. Group size effects could result from a reduction in predation risk or by an increase in competition as a function of aggregation. Antipredator models of vigilance and foraging group size effects predict a nonlinear relationship and would illustrate a fundamental cost of sociality. We studied the degree to which yellow-footed rock-wallaby (Petrogale zanthopus, a macropodid marsupial) group size effects were nonlinear. Like several other macropods, yellow-footed rock-wallabies foraged more and looked less as group size increased.Variation in vigilence was best explained by the number of conspecifics within 10m~ a distance substantially less than the 30 ^ 50 m often used to quantify group size in macropodids. Linear regressions explained more variation than nonlinear ones, suggesting that wallabies traded off th benefits of aggregation with the costs of competition. Moreover dominant yellow-foots looked less and tended to forage more than subordinant animals. Unlike tammar wallabies (Macropus eugenii), which show strong nonlinear group size effects, yellow-footed rock-wallabies defend caves and crevices as refuges. This important natural history distinction may be responsible for the trade-off.