INTRODUCTION

Chimpanzees (Pan troglodytes) and bonobos (P. paniscus) have a similar social structure (patrilineal multi-male and multi-female social unit; Itani, 1985). However, bonobos exhibit quite different copulatory activities from other non-human primates, including chimpanzees: (1) bonobos copulate more often than other anthropoid apes (Kano, 1989); (2) bonobos copulate in varied postures (Savage-Rumbaugh & Wilkerson, 1978); (3) copulationlike behavior often occurs in other social contexts (Kano, 1980; de Waal, 1989); (4) female bonobos exhibit lengthy estrus (Dahl, 1986); (5) suckling mothers resume sexual cycling within a year after giving birth (Kano, 1992). Some authors (e.g. Blount, 1990) have discussed these features in connection with the evolution of the sexuality of human females.

However, how different is the sexual behavior of bonobos from that of chimpanzees in the wild? Here, we compare the copulatory behavior of these two species, based on previous reports and on our field studies.

BACKGROUND

In the Mahale Mountains National Park, Tanzania, the chimpanzees of M group have been observed since 1965. The group contained about 100 individuals in 1981 (see table 3 in Nishida et al., 1990). The adult sex ratio (10 : 39) was significantly skewed toward females compared with the expected ratio of 1 : 1 (Chi-squared = 17.2, df = 1, p < 0.001; for discussion of this tendency, see Hiraiwa-Hasegawa et al., 1984; Nishida et al., 1990).