Inter-group competition and stable group sizes

The stability of the sizes of foraging groups of animals is reviewed with a special emphasis on the effect of inter-group competition. A simple model, in which cooperation, within-group and inter-group competition are explicitly accounted for, shows that when resources are scarce and patchy, inter-group competition tends to keep animals within their groups, even when these groups are overpopulated. The model's conclusions are in accord with empirical observations indicating that animals tend to forage in groups that are larger than the expected optimal size.