Hamadryas baboons (Papio hamadryas hamadryas) range throughout the semi-arid regions of the Horn of Africa, including parts of Ethiopia, Sudan, Somalia, Djibouti, and Eritrea, and the southwestern part of the Arabian peninsula in Yemen and Saudi Arabia. Called "desert baboons" to distinguish them ecologically from other Papio baboons ("savanna baboons" and "mountain baboons"), hamadryas are unique among primates with regard to their complex, multi-level social system and their extreme male-dominated society, both of which have been viewed as adaptations to a harsh semi-desert environment.

Three main levels of organization characterize hamadryas baboon society. Troops are large aggregations (usually over 100 individuals) that assemble at sleeping sites but do not otherwise function as cohesive social groups. Each troop includes one or more bands, whose members travel together during the day and coordinate their movements. The band is probably the social grouping analogous to the "troop" of other papionin monkeys. Adult members of different bands rarely interact (and when they do interact, they usually do so agonistically), and, even when two or more bands share a sleeping cliff, each band travels and forages separately from other bands during the day. Within each band are a number of one-male units (OMU's). Each OMU consists of one adult "leader" male, one or more adult females, their offspring (up to the age of 2-3 years), and sometimes one or more "follower" males. Also within bands are "solitary" males, males who are neither leaders of OMUís nor are attached to an OMU as a follower male. Solitary males and older juvenile males move freely within the band, do not associate regularly with any one OMU, and interact mainly with other solitary males and juveniles.

Cohesion of hamadryas one-male units is maintained by aggressive herding behavior of leader males, who threaten and bite females that lag behind, wander too far away, or interact with individuals outside the unit. Leader males appear to have nearly exclusive sexual access to the females in their unit, as non-leader males have been reported to copulate only rarely. Non-leader males rarely threaten or show aggression toward leader males over estrous females or attempt to copulate with another male's females. Rather, adult males appear to "respect" other males' "possession" of females and are socially inhibited from interacting with females when they already "belong to" another male of the same band, regardless of the relative dominance rank between the two males. When a leader male loses his females, the recipient is often a younger follower male of his own unit. Older leader males may, in fact, voluntarily relinquish their females to younger followers, with whom they have developed a friendly relationship and to whom they might be related.

Abegglen (1984) observed a fourth level of social organization between that of bands and one-male units. In Band I near Erer Gota, Ethiopia, Abegglen noticed that certain males, both solitary and with females, associated more frequently with one another than they did with other males. Abegglen saw obvious physical resemblances among these males, proposed that they were close kin, and called their associations "clans." (Unfortunately, Abegglen's proposition was never confirmed with genetic data because samples were never obtained from these males.) Abegglen reported that OMU's within each of these clans were spatially distinct from OMU's in other clans, both during daily travel and on the sleeping cliffs. Males tended not to transfer out of their natal clan, and males of the same clan tended to have affiliative social relationships with one another, characterized by grooming among solitary males and formalized, stereotypical "notifying" behavior (whereby one male quickly approaches, looks at, presents to, and then leaves another male) among leader males and between leader males and their followers. The clan structure has also been observed by Stolba (1979) in other bands near Erer Gota.

Overall, patterns of hamadryas behavior appears to be largely determined by competition, cooperation, and negotiation among males. Female behavior is less obvious and has not traditionally been thought to contribute greatly to hamadryas social organization. Both studies from captivity and ongoing research at Filoha, however, suggest that social relationships among females and female choice may both play a significant, if subtle, role in hamadryas society.

For Further Information:

Abegglen, J-J (1984) On Socialization in Hamadryas Baboons. London: Associated University Presses.

Kummer, H (1968) Social Organization of Hamadryas Baboons: A Field Study. Chicago: University of Chicago Press.

Kummer, H (1995) In Quest of the Sacred Baboon. Princeton, NJ: Princeton University Press.