Yellow-footed rock-wallaby group size effects reflect a trade-off.

As group size increases, individuals of many species modify the time alloca ted to anti-predator vigilance and foraging.. Group size effects can result from a reduction in predation risk or from an increase in competition as a function of aggregation. Anti-predator models of vigilance and foraging gr oup size effects both predict a non-linear relationship between group size and time allocation. Linear relationships between group size and time alloc ation may reflect the modification of such relationships by intraspecific i nterference competition for limited resources, which would reveal a fundame ntal cost of sociality. We studied the degree to which group size effects i n the yellow-footed rock-wallaby (Petrogale xanthopus, a macropodid marsupi al) were non-linear. Like several other macropods, yellow-footed rock-walla bies foraged more and looked less as group size increased. Variation in vig ilance was best explained by the number of conspecifics within 10 m-a dista nce substantially less than the 30-50 in often used to quantify group size in macropodids. Linear regressions explained more variation than non-linear ones, suggesting that wallabies traded-off the benefits of aggregation wit h the costs of competition. Moreover, dominant yellow-foots looked less and tended to forage more than subordinate animals. We hypothesize that compet ition may be relatively more important in the life-histories of yellow-foot ed rock-wallabies than those of other macropodid marsupials.