378 females in OMUs with a more female-biased sex ratio experienced lower copulation success. We did not ﬁnd the expected effect of oestrus synchrony (operational sex ratio) on copulation success. However, sample size for testing the effect was rather small. Only 19.4% of copulation occurred when the mating female and an additional female of the same OMU were simulta- neously oestrous. These cases concerned only 4 females of 2 OMUs. In all cases only one additional female in the respective OMUs was simultaneously in oestrus. Determinants of successful copulation The proportion of successful copulations ranged from 5% to 72% for females (n = 10, median = 49%, Table 3) and varied signiﬁcantly among individuals (G-test: df = 9, Fig. 2 Average duration (s) of unsuccessful and successful copula- tion and of copulation of unknown outcome (mean ± SD) Primates (2011) 52:373–383 G = 124, p \ 0.001). The single-variable multilevel logistic regression models used to predict copulation suc- cess are compared in Table 4. Relative swelling size has the lowest AIC value and, therefore, best approximates the data set. The odds of a successful copulation increase by 1.029 (95% bca CI 1.001–1.049) with each 1% unit increase in swelling size (Fig. 3). However, the second best model, with sex ratio as a predictor, has only a slightly larger D AICi of 0.68 and is 1.4 times less likely to be the best model. There is almost no support for any other model to be a best model. Socionomic sex ratio correlates with OMU to a high degree, because the sex ratio within an OMU was constant during our observation period. The differences between the two variables are because sex ratio can be treated as a continuous variable whereas OMU is a nominal (categor- ical) variable. The apparent effect may simply be a spuri- ous effect because of its correlation with OMU (or male ID, as OMU is confounded with male ID) and we therefore calculated the AIC of a model including OMU. This model has a D AICi of 3.38 and is less likely to be the best model by a factor of 5.4. Comparison of the AIC between models including either OMU or sex ratio shows that the AICi of the model with sex ratio is smaller by a factor of 2.7. The model with OMU is a factor of 3.9 less likely to be a better model than that with sex ratio as a predictor. Therefore, socionomic sex ratio predicts success of ejacu- lation better than OMU (and/or male ID). In the second step of our model selection analysis, we calculated the AIC and AIC-related measures for 5 models with two variables, each of which used relative swelling size and one of the 5 remaining variables and compared these models with each other and with the best single- variable model (relative swelling size) (Table 5). The Table 3 Individual copulation frequencies and copulation success of females (relative swelling size [50%) Because females were the focus of the study, we could not calculate the copulation rate for males a Females which did not show any sign of a swelling, because of pregnancy or lactational amenorrhea. They were excluded from the analysis b Females which experienced extra-group copulation 123 Female Sex OMU Sex ratio (females in OMU) Observed number of copulations (decided copulations) Copulations/h Proportion of successful copulations (%) Af1 Af2a Bf1 Bf2 Cf1 Cf2 Cf3 Df1b Df2 Df3b Df4a Df5a Df6b f f f f f f f f f f f f f A A B B C C C D D D D D D 2 2 2 2 3 3 3 6 6 6 6 6 6 153 (81) 103 (50) 61 (48) 10 (7) 36 (25) 80 (51) 48 (23) 59 (20) 35 (23) 29 (19) 3.08 2.37 5.25 0.52 1.42 1.65 2.50 2.72 1.69 1.00 71.6 48.0 70.8 42.9 60.0 49.0 52.2 15.0 30.4 5.3