INTRODUCTION

After the pioneering study of sympatric western lowland gorillas (Gorilla gorilla gorilla) and tschego chimpanzees (Pan troglodytes troglodytes) in Rio Muni (Equatorial Guinea) by Jones & Sabater Pi (1971), several study sites were established in the 1980s in tropical forests where both chimpanzees and gorillas occur (western lowland gorillas and tschego chimpanzees – Gabon: Tutin & Fernandez, 1984, 1993; Tutin et al., 1991; Tutin, Chapter 5; Central African Republic: Carroll, 1988; Fay, 1988; Remis, 1993, 1994; Congo: Kuroda, 1992; Nishihara, 1992, 1994; Mitani et al., 1993; eastern lowland gorillas (Gorilla gorilla graueri) and long-haired chimpanzees (Pan troglodytes schweinfurthii) eastern Zaïre: Yamagiwa et al., 1992, 1994, Chapter 7).

Recent studies at these sites suggest common features of interspecific relationships between gorillas and chimpanzees. Generally, dietary overlap in plant food is great, ranging from about 50% at Kahuzi-Biega (Yamagiwa et al., 1994) to 60–80% at Lopé (Williamson et al., 1990; Tutin & Fernandez, 1993) and Ndoki (Kuroda, 1992). Overlap in fruit consumption is particularity high (70–90%) and habitat use patterns are similar. Thus, they share a similar niche and ecological competition is likely to occur between these two ape species (Tutin & Fernandez, 1985, 1993; Williamson et al., 1990; Kuroda, 1992). Low population densities of gorillas and chimpanzees in Lopé and Kahuzi-Biega might partly be due to this competition (Tutin & Fernandez, 1985; Yamagiwa et al., 1992).