382 on whether females have large swellings, i.e. whether the probability of ovulation and conception is high. A second important effect seems to be the number of females in an OMU, with decreasing probability of success as the num- ber of females increases. Our results suggest that male hamadryas baboons concentrate their successful copulation on times when the probability of ovulation, and hence conception, is high, i.e. when females have their largest swellings. Furthermore, males in larger OMUs with many females may face a time and/or energy-allocation problem in controlling all their females and at the same time spending time/energy on successful copulations. Acknowledgments The authors are very grateful to the Zoologi- scher Garten Berlin for giving us the opportunity to conduct this study. We especially thank Dr P. Rahn from the Zoologischer Garten for his kind support, and Brandon Wheeler and Peter Henzi for constructive comments on an earlier version of the paper. 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