As group size increases, individuals of many species modify the time allocated 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 group size effects both predict a non-linear relationship between group size and time allocation. Linear relationships between group size and time allocation may reflect the modification of such relationships by intraspecific interference competition for limited resources, which would reveal a fundamental cost of sociality. We studied the degree to which group size effects in the yellow-footed rock-wallaby (Petrogale xanthopus, a macropodid marsupial) were non-linear. Like several other macropods, yellow-footed rock-wallabies foraged more and looked less as group size increased. Variation in vigilance was best explained by the number of conspecifics within 10 m - a distance substantially less than the 30-50 m 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 with the costs of competition. Moreover, dominant yellow-foots looked less and tended to forage more than subordinate animals. We hypothesize that competition may be relatively more important in the life-histories of yellow-footed rock-wallabies than those of other macropodid marsupials.