Optimal Social Group Size in Spotted Hyaenas (Crocuta crocuta): Insights into a Multilevel Society

The spotted hyaena lives in unusually large social groups for a carnivore. Since the infertility trap normally limits the size of social groups in mammals, it seems likely that this species has evolved some way of mitigating the stresses involved. In primates, this usually takes the form of female-female alliances, often embedded in multilevel social systems. I show (1) that the distribution of hyaena clan sizes is multimodal, with a fractal scaling close to 3 and a base unit of 12-15 individuals (3-5 reproductive females) and (2) that fertility is a trade off between the benefits of having more males in the group and the costs incurred by having more females, with 4-5 as the limit on the number of females that can live together without their reproductive rates falling below the demographic replacement rate. I present evidence that females buffer themselves against the infertility trap by forming matrilineal alliances that in turn create a multilevel structure. In this respect, hyaena resemble cercopithecine primates in using social strategies to enable animals to live in larger groups than they would otherwise be able to do.