Life History, Activity Period and Lemur Social Systems

Extant lemuriform and anthropoid primates differ dramatically in social behavior, grouping patterns, social structure, and sexual dimorphism In this paper, we analyze and contrast grouping and bonding patterns among lemurs, and suggest determinants of social organization that explain many of these differences. Lemurs exhibit three main grouping patterns: solitary individuals, bonded pairs, and larger groups. Transitions between pairs and larger groups occur within several species and even within populations in some of Eulemur species and in Varecia. Larger groups of these species have an equal adult sex ratio, and spatial associations and social behavior suggest the existence of male-female pair bonds. Larger groups of the consistently diurnal Lemur and Propithecus also show equal adult sex ratios, but no evidence for pair-bonding. These differences among lemurs are likely due to variation in the activity periods of the cathemeral species, whose groups are hypothesized to break up into pairs during periods of nocturnal activity. The pair bond is therefore the fundamental umt of many lemur groups. This is a special case of the permanent male-female associations found among the great majority of primate species. We suggest that the evolution of permanent male-female associations is favored by selection for infanticide prevention, because the male can effectively protect the females against infanticide attempts by other males. Where females live alone permanently or part of their time (the cathemeral species), this association takes the form of a pair bond. Where they live in groups, it takes the form of permanently bisexual groups. This hypothesis leads us to predict no male-female association in species with absentee maternal care. This prediction is confirmed: the interspecific variation in infant care (carry vs hide) covaries perfectly with grouping pattern among prosimians, as well as among other primate radiations. We conclude that activity period and type of infant care determine grouping and bonding patterns in both prosimian and anthropoid societies. Both cathemeral activity periods and infant carrying among primates were probably made possible by increased precociality.