Primates (2011) 52:373–383 Methods Study site, animals and data collection Observations were conducted at the hamadryas baboon colony of the Zoologischer Garten in Berlin, Germany from August to December 2005. The baboons were living 2 in an outdoor enclosure (ca. 400 m ) with access to an indoor enclosure. Food was provided daily in the indoor part of the enclosure, and water was available permanently in a well outdoors. The outdoor enclosure consisted of an artiﬁcial rock with several places to hide. Around the rock, the ground consisted of sand banks with several trunks enriching the site. The social organization and social structure of the col- ony was determined by analysis of dyadic grooming interactions. The colony had the typical hamadryas social system, as described for free-ranging and captive popula- tions (Kummer 1968; Colmenares et al. 1994). It consisted of four OMUs with four adult males, 13 adult females, two subadult males, and several infants of both sexes. Because no information on the baboons’ exact ages was available, animals were categorized into age classes by size and fur colour according to Sigg et al. (1982). We focussed our observations on females, using focal animal sampling (Altman 1974) to record the behaviour of receptive females (indicated by perineal swellings). When only one female was swollen, we observed her for as long as possible. If two or more females were swollen simul- taneously, we systematically sampled each in turn. Thus, focal observation periods lasted from 6 min to 5 h (mean = 60 min, SD = 36 min), depending on the female’s reproductive status, sexual activity, and visibility. We recorded the occurrence, duration, and success of copulation, number of pelvic thrusts per copulation, and occurrence of female copulation calls. We also recorded all 375 acts of male aggression to females in the context of cop- ulation and noted whether males or females initiated cop- ulation. The data reported here come from 262.8 h of observation. Three out of 13 females in the colony were excluded from the observation and analysis because they did not develop sexual swellings during the observation period, owing to pregnancy or lactational amenorrhoea. In addition to focal sampling, all sexual interactions noticed by the observer were recorded ad libitum. Deﬁnitions The most important distinction was made between suc- cessful (indicated by the presence of ejaculate) and unsuccessful copulation. Cases for which we could not determine copulation success within 2 min were termed ‘‘unknown outcome’’ copulation and were excluded from analysis. Deﬁnitions of the variables describing copulations are given in Table 1. We were not able to record all variables for all copulation, because of lack of visibility. We excluded incomplete cases from the respective analyses. As a result, sample sizes differ, depending on the variable considered. All calculations of copulation rates (copula- tions per hour) were corrected for observation time. Estimation of relative swelling size For baboons, the probability of ovulation is highest on the last day of maximum swelling and the ﬁrst day of detu- mescence (Gillman and Gilbert 1946; Hendrickx and Kraemer 1969; Wildt et al. 1977). The size of the ano- genital swelling thus indicates the likelihood of ovulation and, hence, the probability of conception, and males are expected to mate preferentially during periods of maximum swelling (Hrdy and Whitten 1987; Wallis 1992; Nunn Table 1 Deﬁnitions of variables Variable Copulation success Copulation duration Pelvic thrusts Copulation calls (call) Overt male aggression (agg) Initiation of copulation (ini) Sex ratio Synchronya (syn) Swelling size Deﬁnition Ejaculation, indicated by clearly visible clues of fresh sperm on the male’s penis or the female’s perineum after copulation. When the outcome of a copulation could not be determined within 2 min after the copulation, the respective copulation was termed ‘‘unknown outcome’’ copulation Time (s) from insertion of the male’s penis until its extraction from the female’s vagina Number of pelvic thrusts of the male during copulation Female utters context-speciﬁc call while copulating Biting, pulling, or grabbing of the female by the male before copulation Initiation by the female via presenting or by the male via approaching, pressing, or pulling of the female Number of adult females in an OMU When more than one female of the same OMU had a large swelling ([50% of her maximum swelling size) on the same day See text a Synchrony here does not imply any female strategy; it merely refers to the fact that two or more females are simultaneously oestrous 123