Social groups and the myth of the ‘one-male unit’

Long before any detailed information on the social organization of wild mandrills or drills was available, a number of authors had commented on their relatively large group sizes as compared to those of other rainforest cercopithecines (Jeannin, 1936; Malbrant and Maclatchy, 1949; Sanderson, 1940). Malbrant and Maclatchy, for example, thought that mandrill groups might contain as many as 50 individuals, and that drills occur in grandes hordes bruyantes. Struhsaker (1969) recorded counts of nine to 55 (mean= 23.3) for 12 drill groups in Cameroon, but noted that ‘most of these were incomplete’.

The earliest field study of any magnitude was conducted by Gartlan (1970) on drills of the Southern Bakundu Forest Reserve in western Cameroon. He counted drill groups ranging in size from 14 to 179 individuals; at least three adult males and 21 adult females were included in the largest group. Gartlan also encountered two solitary adult males. Subsequent fieldwork in Gabon (Jouventin, 1975a) and in Southern Cameroon (in the Campo Reserve: Hoshino et al., 1984) also produced some useful data on mandrill group size and composition. Jouventin (1975a) proposed that the basic social unit in mandrills consists of one large adult male with five to ten females (with or without infants) and ten juveniles. He thought that a number of these small social units, or ‘harems’, sometimes unite to form large troops of up to 200 mandrills, while ‘excess adult males live a solitary existence in the forest’. Hoshino et al. (1984) counted groups of between 15 and 95 mandrills at Campo Reserve. They concluded that two types of grouping occur in mandrills, ‘one-male and multimale groups’, but they observed solitary males as well on nine occasions. Hoshino et al. also regarded the one-male groups, which consisted of one large male and 14 other individuals on average, as permanent units that then coalesced from time to time to form larger multimale bands. The implication was, as proposed by Jouventin (1975a), that mandrill one-male units were, in general terms, equivalent to those that occur in polygynous species such as the hamadryas baboon or the gelada.