INTRODUCTION

Due to their prolonged sexual cycling period and variety of sexual behaviors, there is a prevailing impression that female bonobos are very sexually active. They copulate even during non-reproductive periods (Thompson-Handler 1990; Furuichi 1992; Kano 1992) and use sexual behaviors for various social purposes (Kuroda 1980; Goodall 1986; de Waal 1987; Furuichi 1989; Kano 1989, 1992; Idani 1991; Wrangham 1993; Parish 1994). However, Takahata and others (1996) have pointed out that the copulation rates of female chimpanzees and female bonobos over the swelling cycle, which consists of a swelling phase and a non-swelling phase, are approximately the same, although the copulation rates over the interbirth interval, which consists of a phase in which females show cyclic swelling and a phase in which females do not show cyclic swelling due to lactation or pregnancy, are higher for female bonobos. Although female bonobos copulate most frequently during the swelling phase, even during the non-swelling phase they copulate more often than female chimpanzees (Tutin 1979a; Thompson-Handler et al. 1984; Dahl 1986; Furuichi 1987; Kano 1992; Dixson 1998). Therefore, the findings of Takahata and others logically predict that, during the swelling phase, female bonobos copulate at a lower rate than female chimpanzees. Wrangham (Chapter 15) also points out that adult female chimpanzees copulate especially frequently during the peri-ovulatory period, whereas there is no such data for female bonobos. He suggests that the copulation rates of female chimpanzees and female bonobos differ during the peri-ovulatory period because they incur different costs when living in mixed-sex parties for mating with males.