Primates (2011) 52:373–383 staying close to the male or to other, mostly related, females in the single-male group. Hamadryas females are assumed not to be related and their social network is relatively weak (Kummer 1968; Swedell 2006), leading to a reduced tendency to form a cohesive group with other females, which may impose additional monitoring and herding costs on the male. In ge- ladas (Theropithecus gelada), which have a multi-level system superﬁcially similar to that of hamadryas baboons, females within an OMU are related and have a strong ten- dency to remain as a cohesive group, even if the leader male changes (Grueter and Zinner 2004; le Roux et al. 2011). If the time and energy costs for monitoring and herding females for hamadryas leader males are actually higher than in other species, and if these costs increase with increasing number of females within an OMU, it seems reasonable that males in smaller OMUs (those with fewer females) can copulate successfully with the same female more often than males in bigger OMUs. In our study females in bigger OMUs tended to have fewer successful copulations, with the odds of successful copulation decreasing by 0.60 with each unit increase in sex ratio (Fig. 4). Study of AIC-related model assessment criteria did not provide any noteworthy support for additional factors affecting the probability of successful copulation. Neither the occurrence of female copulation calls, sex of the cop- ulation initiator, nor male aggression toward females sig- niﬁcantly affected the probability that copulation was successful. Although copulation calls may contribute to the success of copulations in other species (Barbary macaques, Pfef- ferle et al. 2008), we did not observe this in our study (see also Swedell 2006). In our study, copulation calls only occurred in a small number of copulations (26.1% of all cases) in comparison with observations of chacma baboons, for which female copulatory vocalizations are much more frequent (84%: Saayman 1970, 97%: O’Con- nell and Cowlishaw 1994). It therefore seems that copu- lation calls have, at best, only a minor effect on male copulatory behaviour in hamadryas baboons, which is in agreement with the two hypothesized immediate functions of copulation calls: to encourage mating attempts by other males and to increase mate guarding by the consort male (Pradhan et al. 2006). In hamadryas baboons, chances of copulation with other males are very rare because of already intensive mate guarding by the leader male. Female baboons present to males particularly often when they have swellings, to show their willingness to mate (Saayman 1970; Hrdy and Whitten 1987). Therefore, we might expect males to ejaculate more often when females initiate copulations, because female proceptivity may indicate a high probability of conception. On the other hand, males might initiate more successful copulations 381 when females are receptive compared with non-reproduc- tive times so they use their energy devoted to gamete production efﬁciently. However, neither of these hypothe- ses was supported here, because the sex of the initiator of copulation did not affect the likelihood of copulation success. Aggression toward females, for example holding, pull- ing, and biting, is an important part of the hamadryas baboon males’ herding behaviour (Kummer 1968; Swedell and Schreier 2009) and could also be involved in a sexual context. However, we found no evidence that pre-copula- tory male aggression increased the probability that copu- lation was successful. In fact, this form of male aggression was observed in only 13% of all copulation. Our second main question deals with the occurrence of extra-group copulation in hamadryas baboons. The ability of females to choose mates is supposed to have a positive effect on female ﬁtness (Andersson 1994). In hamadryas baboon, the leader male and his females build stable pair bonds, sustained by the aggressive herding behaviour of the male. This leaves other adult males little chance to copulate (Kummer et al. 1974; Sigg et al. 1982) and thus limits female opportunity for mate choice. Juvenile and adoles- cent males, however, sometimes copulate with females (Kummer 1968; Sigg et al. 1982; Swedell 2006) and the adolescents are able to inseminate them (Zinner et al. 2006). We observed several extra-group copulations with juvenile and subadult males. However, such copulation was infrequent and rarely successful. All extra-group copula- tion was initiated by females, showing that females were willing to interact sexually with these males. Most extra- group copulation involved females from the OMU with the lowest sex ratio, which might be an indication that male control is not as efﬁcient in relatively large OMUs. How- ever, we found no effect of oestrus synchrony on either the occurrence or success of extra-group copulation, although the small sample size means that conclusions about the effect of sex ratio and synchrony on extra-group copulation are preliminary. Hamadryas baboons have been regarded as multiple mount ejaculators (MME) since Kummer (1968: 39) stated that: ‘‘True copulation among hamadryas consists of a series of mountings occurring at intervals of three to eight minutes. Ejaculation occurs only after several such mountings.’’ Several authors adopted Kummer’s descrip- tion (Dewsbury 1972, 1989; Dixson 1998), although sub- sequent observations showed that hamadryas males are capable of ejaculation with only single mounts (Nystrom 1992). Our data conﬁrm this ﬁnding and raise the question of whether a simple dichotomy exists between SME (single-mount ejaculator) and MME species. Whether copulation in hamadryas baboons is success- ful—i.e. whether it leads to ejaculation—depends primarily 123