Primates (2011) 52:373–383 Note on ethics F.N. collected all data from the public viewing area in the zoo where the baboons were accustomed to people being present. Data collection required no direct contact with the animal subjects. Results Sexual behaviour of males and females was similar to that reported for free-ranging hamadryas baboons (Kummer 1968), including initiation of copulation by both sexes and the ‘‘double-foot clasp’’ copulation position. Harassment or interruption of copulating pairs by other females or males never occurred. Overall, we observed 734 copulations (482 during focal sampling, 252 during ad-libitum sampling). Of the copulations, 26.7% (196) were deﬁnitively successful and 29.0% (213) ceased without ejaculation. For 44.3% (325) of the copulations, whether ejaculation occurred was unknown. Series of mounts consisted of a maximum of two unsuccessful copulations and one successful. On 46 occa- sions, two successive successful copulations were recorded without any additional mount between. This indicates that hamadryas males are capable of reaching ejaculation after only one intromission and multiple mounting is not required for ejaculation. Sexual behaviour and swelling states A total of 23 cycles was observed either in part (8) or completely (15). In 75% (198 h) of the total observation time (263 h), at least one female in the colony had a large swelling ([50% of her maximum swelling size). Of the copulations, 85% were observed when females had large swellings, and copulation rate (copulations per hour) was higher during this state of swelling than at times when females had smaller swellings (n = 10, median large = 2.11, median small = 0.13; Wilcoxon matched pairs test: z = 2.80, p = 0.005). We therefore based the following analyses on cases when females had large swellings ([50% of maximum swelling size) and when adult leader males Table 2 Rates and proportions of sexual behaviour in females with a large swelling Number of copulations (n = 614) Decided Successful copulations Synchrony Female vocalization Male aggression Initiated by males 347 614 614 491 489 377 were involved, which reduces the number of analysed copulations to 614. Copulation of females with large swellings Of the 614 copulations, 182 (29.6%) resulted in ejacula- tion, 165 (26.9%) were unsuccessful, and in 267 (43.5%) cases we were unable to decide whether the copulation was successful. Of the 347 copulations for which we identiﬁed success or lack of it, 52.4% were successful (Table 2). In 19.4% of the 614 cases, at least one addi- tional female of the same OMU also had a swelling on the day the respective copulation was observed (syn- chronous). In 26.1% of copulations, females uttered copulation calls. In 491 cases we were able to observe whether or not the male was aggressive toward the female before copulation, and found they were in 13.0% of instances only. Of 489 cases in which we were able to observe whether males or females initiated the copulation, we found males initiated 57.3%. An average copulation lasted 6.2 s ± 1.9 s (mean ± SD, n = 487; range 2–12 s), during which the male on average performed 9.7 ± 3.3 pelvic thrusts (mean ± SD, n = 504; range 1–20 thrusts). Both variables were strongly correlated (n = 484; r = 0.77; p \ 0.001). Multilevel linear regression analysis showed that suc- cessful copulation was signiﬁcantly longer than unsuc- cessful copulation (Fig. 2; mean difference = 1.26 s (95% bCI 0.87–1.65, z = 6.3, p \ 0.0001). Variation of copulation rate and successful copulation Copulation rates per female ranged from 0.52 to 5.25 copulations per hour (n = 10, median = 2.03 copulations per hour, Table 3) and rates varied signiﬁcantly among individual females (G-test, df = 9, G = 67, p \ 0.001). This difference could not be explained by variation in the socionomic sex ratio, because we found no correlation between the sex ratio within OMUs and copulation rates (Spearman: n = 10, rs = -0.33, p = 0.35). There was, however, a negative correlation between the socionomic sex ratio and proportion of successful copulation (Spear- man: n = 10, rs = -0.69, p \ 0.05), suggesting that Yes 182 119 160 64 280 % Yes (range between females) 52.4 (5.3–71.6) 19.4 (0.0–66.7) 26.1 (0.0–79.1) 13.0 (0.0–31.4) 57.3 (8.3–70.0) 123