Hamadryas baboons are known for their complex, multi-level social structure consisting of troops, bands, and one-male units (OMUs) [Kummer, 1968. Social organization of hamadryas baboons. Chicago: The University of Chicago Press. 189p]. Abegglen [1984. On socialization in hamadryas baboons: a field study. Lewisburg, PA: Bucknell University Press. 207p.] observed a fourth level of social structure comprising several OMUs that rested near one another on sleeping cliffs, traveled most closely together during daily foraging, and sometimes traveled as subgroups independently from the rest of the band. Abegglen called these associations "clans" and suggested that they consisted of related males. Here we confirm the existence of clans in a second wild hamadryas population, a band of about 200 baboons at the Filoha site in lowland Ethiopia. During all-day follows from December 1997 through September 1998 and March 2005 through February 2006, data were collected on activity patterns, social interactions, nearest neighbors, band fissions, and takeovers. Association indices were computed for each dyad of leader males, and results of cluster analyses indicated that in each of the two observation periods this band comprised two large clans ranging in size from 7 to 13 OMUs. All band fissions occurred along clan lines, and most takeovers involved the transfer of females within the same clan. Our results support the notion that clans provide an additional level of flexibility to deal with the sparse distribution of resources in hamadryas habitats. The large clan sizes at Filoha may simply be the largest size that the band can split into and still obtain enough food during periods of food scarcity. Our results also suggest that both male and female relationships play a role in the social cohesion of clans and that males exchange females within clans but not between them.

Multilevel societies (MLSs)-stable nuclear social units within a larger collective encompassing multiple nested social levels-occur in several mammalian lineages. Their architectural complexity and size impose specific demands on their members requiring adaptive solutions in multiple domains. The functional significance of MLSs lies in their members being equipped to reap the benefits of multiple group sizes. Here we propose a unifying terminology and operational definition of MLS. To identify new avenues for integrative research, we synthesise current literature on the selective pressures underlying the evolution of MLSs and their implications for cognition, intersexual conflict, and sexual selection. Mapping the drivers and consequences of MLS provides a reference point for the social evolution of many taxa including our own species.

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