Foraging in a social antelope: Effects of group size on foraging choices and resource perception in impala

1. Many studies of foraging in social species have identified benefits for individuals from foraging in groups, but few have demonstrated that there are also costs. We wanted to evaluate the effects of the group on individual foraging behaviour in a social antelope, and the way it affects resource perception by the forager when foraging in different group sizes. 2. We monitored the foraging behaviour of impala (Aepyceros melampus Lichtenstein) in Acacia scrubland in highveld Zimbabwe. We investigated the influence of group size on the foraging decisions of focal individuals, and the process by which impala select bushes to feed on, and how many bites to take. 3. We demonstrated that group size had a major influence in the foraging decisions made by the impala, as it determined the number of bites taken from a selected bush by an individual. The cost of foraging in groups was higher when feeding on small bushes, i.e. the potential number of bites to be taken by an individual from low biomass bushes decreased faster with increasing group size than for high biomass bushes. 4. Impala appeared to be frequency-independent 'predators' with strong preferences for high profitability 'prey' types and seem to select bushes ('prey') on a potential gain basis (i.e. potential number of bites to be gained), and not on a strict biomass basis. This result implies that they were taking the group size into account when choosing the bushes. 5. We suggest that impala scanned the Acacia field at the bite level, the field being a mosaic of patches of different bite densities, and that the perception of the resource by individuals in a groups is strongly affected by the number of individuals foraging together.