374 emerge, for instance, in one-male groups if the operational sex-ratio is strongly female-biased by oestrus synchrony. However, quantitative data on sexual behaviour and copulatory patterns are sparse or non-existent for many primate species. This includes hamadryas baboons (Papio hamadryas), despite decades of study and extensive data on their social system (Kummer 1968, 1995; Swedell 2002a, b; Schreier and Swedell 2009). Hamadryas baboons live in a multi-level society, of which the basic entity is the one- male unit (OMU), consisting of one adult male and one to several females and their offspring (Kummer 1968; Abegglen 1984; Schreier and Swedell 2009). Ascending social levels include the clan (an association of OMUs possibly linked by male kinship), the band (the ecological unit), and the troop (a temporary aggregation of bands at a sleeping site). Attached to some OMUs are one or two follower males. Also within a band one ﬁnds a number of solitary males that are not attached to a particular OMU (Pines et al. 2011). OMU leader males try to maintain exclusive social and sexual access to a set of females and they herd their females aggressively by use of neck bites and threats. Consequently, almost all mating takes place within OMUs, classifying the hamadryas mating system as polygynous and monandrous (Kummer 1968; Stammbach 1987; Swedell 2006). Although Kummer et al. (1974; cf. Sigg et al. 1982) argued that the potential for female mate choice is small, because of the ability of leader males to monopolize females in their OMUs, recent studies show that not all infants are offspring of OMU-leaders, which means that successful extra-group copulation occurs (Yamane et al. 2003; Zinner et al. 2006). Sexual interac- tions with subadult males, presumably capable of siring infants, have been observed in the wild (Kummer 1968; Swedell 2006). Oestrus synchrony can lead to female–female com- petition for mating and conception but can also affect the ability of females to copulate with males other than their OMU leaders, thus increasing a female’s chances for mate choice. This may be because leader males might have time and energy-allocation problems when forced to herd several females at the same time (Zinner et al. 1994). Furthermore, in contrast with males of other species with a single male multi-female systems (e.g. many Asian colobines, Kirkpatrick 2011; most guenons, Jaffe 2011; gorillas, Robbins 2011) copulation success in male hamadryas baboons may not only be affected by the operational sex-ratio within the one-male unit but also by the socionomic sex-ratio. As a consequence of the multi- level organisation of hamadryas society, hamadryas leader males live permanently in close spatial proximity to a number of adult male competitors, i.e. other OMU-lead- ers, follower males, and solitary males which do not have 123 Primates (2011) 52:373–383 any females (Kummer 1968; Swedell 2006; Pines et al. 2011). At the same time, hamadryas females seem to have a lower tendency to stay close to their males than in other species living in one-male units (e.g. gorillas, Stewart and Harcourt 2007), forcing leader males to monitor and, if necessary, herd their females, even when the females are not sexually receptive. A hamadryas male may be caught in a situation where he has to consort a receptive female while at the same time he has to control and herd his other females. The sexual behaviour of free-ranging hamadryas baboons was ﬁrst described by Kummer (1968). Sexual interactions can be initiated either when females present to males or when males approach and touch the females with their hands to make them stand up or press or pull females into a copulatory positions. Copulation is characterized by mounts during which males grasp the females’ ankles with their feet (‘‘double-foot clasp position’’) and by intromis- sion of the penis (Kummer 1968). Females sometimes give copulation calls (Swedell 2006). In many species, proceptive females are responsible for initiating most copulation (Hrdy and Whitten 1987; Hashimoto and Furuichi 2006). Furthermore, female baboons, similar to some other catarrhine primates, provide males with some information about the probability of ovulation through anogenital swellings. In other baboon taxa, the probability of ovulation is highest on the last 1 or 2 days of maximum swelling before detumescence (Gill- man and Gilbert 1946; Hendrickx and Kraemer 1969; Wildt et al. 1977; Shaikh et al. 1982). Male chacma baboons seem to be more attracted to females with bigger swellings (Girolami and Bielert 1987). The pattern of copulation in hamadryas baboons has been classiﬁed as ‘‘multiple mount-to-ejaculation’’ (MME) or ‘‘multiple brief intromission’’, in which ejaculation takes place only after a series of mounts and intromissions (Dewsbury 1972, 1989; Dixson 1998). Kummer (1968) assumed that single extra-group copulation is reproduc- tively insigniﬁcant, because he found that single mounts did not lead to ejaculation, although more recent research calls this into question (Yamane et al. 2003; Zinner et al. 2006). In our paper we report a quantitative analysis of hamadryas baboon sexual behaviour. In particular, we address the factors that determine whether a copulation leads to an ejaculation. Such factors include female reproductive condition, with swelling size used as a proxy for fecundity (probability of ovulation), socionomic and operational sex-ratios within OMUs, and male and female behaviour before copulation. Furthermore we were inter- ested in the frequency of extra-group copulation in this species, because OMU leader males are thought to have reproductive monopolies.