Many species receive an antipredator benefit from aggregation such that animals in larger
groups are able to allocate more time to foraging and less to antipredator vigilance. These
beneficial ?group size effects? must be traded-off against the costs of increased competition for
limited resources that may result from aggregation. Certain species, or species living in certain
habitats, may be predisposed to receive greater benefits from aggregation than others. Based on
the results of a study of captive yellow-footed rock-wallabies (Petrogale xanthopus; a
?Vulnerable? macropodid marsupial), we predicted that because rock-wallabies must defend
vital resources (the locations where they shelter by day), there are costs which reduce the
overall antipredator benefits obtained from aggregation while foraging. We tested this
prediction by observing three different species of free-living rock-wallabies as they foraged in
aggregations of different sizes. Allied (P. assimilis) and unadorned (P. inornata) rock-wallabies
received no obvious antipredator benefits from aggregation since there was no effect of group
size on time spent vigilant by individuals. Mareeba rock-wallabies (P. mareeba) may receive
antipredator benefits, since animals tended to forage more and looked less as group size
increased. However, this result was influenced by two observations that had substantial
leverage. Additionally, even if present, the specific shape of this group-size function suggests
that intraspecific competition in P. mareeba increases with group size. As a clade, rockwallabies
appear to have costs which reduce or eliminate antipredator benefits associated with
aggregation. Conservation efforts to recover populations should consider the likely importance
of intraspecific competition for these species, and generalizations about introducing or
translocating social animals socially should rest upon their being demonstrable benefits from
aggregation.
