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.

LANGUAGE AS THE DOMAIN OF HOMO

Human beings have traditionally viewed their capacity for language as something special and unique amongst the creatures of the planet. The communication systems of all other animals have been described as ‘non-verbal,’ a term implying the absence of intentionality, symbolic encoding and internal structure (Hinde, 1972; Feldman & Rime, 1991). Because other animals purportedly were unable to produce acts of communication equivalent to language, linguists have assumed that all forms of communication employed by other species are primitive or biologically prewired. Information not generated by an extant emotional state is assumed to be beyond the communicative capacity of animals. Thus, it is thought that animals cannot, for example, intentionally communicate desires or plans to travel to a certain tree or to look for a certain type of food. In fact, even to speak of animals as having ‘plans’ is generally discouraged by the scientific community.

These presuppositions have prevented scientists from interpreting the communication systems of other animals, especially primates, in the same way that they interpret their own communications. The difficulties that result from such limitations are many. They can be realized by imagining what would happen if a scientist tried to decode an unknown human language by looking only at the relationship between the words of a speaker and the behavior of a listener.