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, as well as 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 (Kummer 1968, 1971; Swedell & Plummer 2012).
Four 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. Within each band are two or more clans, associations among males that are detectable using spatial association data but otherwise not usually an easily observable social unit (Abegglen 1984; Schreier & Swedell 2009). Finally, within clans are a number of one-male units (OMUs).
Hamadryas society centers on one-male units (OMUs), a.k.a. 'harems', consisting of a 'leader male' and a small group of females (Kummer 1968). The cohesive force maintaining this structure is the behavior of the leaders, who coerce females into OMUs by threatening and biting them (Swedell & Schreier 2006). Despite this coercion and its costs (Swedell et al 2014), females do appear to pursue their own social and sexual agenda to the extent that they can. For example, females maintain social relationships with other females (Swedell 2002, 2006); they may be able to influence male takeovers (Bachman & Kummer 1980; Swedell 2000, 2006); and they may use strategies to reduce infanticide by males (Swedell & Tesfaye 2003; Swedell & Saunders 2006; Swedell et al., 2014).
Each OMU consists of one adult leader male, one or more adult females, their dependent offspring, 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 (Kummer 1968; Swedell 2006). Leader males appear to have nearly exclusive sexual access to the females in their unit, as non-leader males 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 (Kummer et al., 1974) until the leader male shows signs of weakness or the balance of power shifts between the two males, leading to a takeover attempt (Kummer 1968; Swedell & Schreier 2009; Pines et al., 2011). When a leader male loses his females, the recipient may be a younger follower male of his own unit, a solitary male, or another leader male (Pines et al., 2011). Female acquisition strategies vary between these categories of males, with solitary and follower males pursuing different strategies in their pursuit of their first females (Pines et al., 2011).
all text and photos copyright © Larissa Swedell