INTRODUCTION

The view that male–male sexual competition is lower in bonobos than in chimpanzees has recently been proposed (e.g. Ihobe, 1992; Kano, 1992). This view is based on the estimation that more estrous females are available to a male bonobo than to a male chimpanzee. (1) Maximal or semi-maximal perineal tumescence in adult females lasts for more than 20 days for bonobos at Wamba (Kano, 1992), whereas average durations of maximal tumescence for adult female chimpanzees are 9.6 and 12.5 days at Gombe and Mahale, respectively (Nishida & Hiraiwa-Hasegawa, 1987; Takahata et al., Chapter 11). There is no marked difference in cycle length, and in both species most copulations occur when females show sexual swelling (Takahata et al., Chapter 11). Hence, the sexually receptive proportion of each cycle is greater in bonobo females than in their chimpanzee counterparts. (2) Young nulliparous immigrant females of bonobos show continual estrus and the highest copulation rates (Kano, 1989). Although chimpanzee adolescent females also show semi-continuity or irregularity in genital swelling (Tutin & McGrew, 1973), they mate much less frequently with adult males (Goodall, 1968). This implies that male bonobos are attracted to adolescent swellings, while male chimpanzees are not. (3) Females of both species experience estrous cycles after conception, but sexual receptivity in pregnant bonobos appears to last longer.