ORIGINAL ARTICLE Estrus cycle asynchrony in wild female chimpanzees, Pan troglodytes schweinfurthii Akiko Matsumoto-Oda & Miya Hamai & Hitosige Hayaki & Kazuhiko Hosaka & Kevin D. Hunt & Eiiti Kasuya & Kenji Kawanaka & John C. Mitani & Hiroyuki Takasaki & Yukio Takahata Received: 30 August 2005 / Revised: 8 October 2006 / Accepted: 8 October 2006 / Published online: 10 January 2007 # Springer-Verlag 2007 Abstract Although estrous synchrony has been reported in a number of mammalian species, most often among primates, methodological and analytical problems make it difficult to interpret these results. We developed a novel estrous synchrony index and employed a randomization procedure to analyze long-term observations of female chimpanzee (Pan troglodytes schweinfurthii) estrous cycles at the Mahale Mountains National Park, Tanzania. Our results revealed that female chimpanzees at Mahale avoid synchronizing their estrous periods with each other. We also found that birthrates decreased as the breeding sex ratio increased. We suggest that estrous asynchrony decreases female–female competition for mates. Asynchrony may also reduce the potential for male sexual coercion by nonpreferred mating partners. Keywords Timing of estrus . Mating strategy . Female competition . Sexual conflict . Chimpanzees Introduction Reproductive synchrony is the tendency of individuals to time some part of their reproductive cycle with that of other population members (Findlay and Cooke 1982; Ims 1990). Reproductive synchrony is best understood when its two components, estrous synchrony and breeding synchrony, are distinguished, thus allowing each adaptation to be considered on its own. Estrous synchrony can be defined as the tendency of individuals to undergo the fertile period of the sexual cycle at the same time as other members of the population. The distribution of females in time and space largely determines male–male competition for mates (e.g., Emlen and Oring 1977; Clutton-Brock 1989; Eberle and Kappeler 2002; Setchell and Kappeler 2003; Kappeler and van Behav Ecol Sociobiol (2007) 61:661–668 DOI 10.1007/s00265-006-0287-9 Communicated by S. Alberts A. Matsumoto-Oda (*) Department of Welfare and Culture, Okinawa University, Naha, Okinawa 902-8521, Japan e-mail: matumoto@okinawa-u.ac.jp M. Hamai Primate Research Institute, Kyoto University, Kyoto, Japan H. Hayaki The Graduate School of Humanities and Sciences, Kobe Gakuin University, Kobe, Hyogo, Japan K. Hosaka Faculty of Child Studies, Kamakura Women’ s University, Kamakura, Kanagawa, Japan K. D. Hunt Department of Anthropology, Indiana University, Bloomington, IN, USA E. Kasuya Department of Biology, Kyushu University, Fukuoka, Japan K. Kawanaka : H. Takasaki Department of Biosphere–Geosphere System Science, Okayama University of Science, Okayama, Japan J. C. Mitani Department of Anthropology, University of Michigan, Ann Arbor, MI, USA Y. Takahata School of Policy Studies, Kwansei Gakuin University, Nishinomiya, Hyogo, Japan