4 - Dispersal patterns, group structure, and reproductive parameters of eastern lowland gorillas at Kahuzi in the absence of infanticide

Summary

Introduction

Gorillas are classified into three subspecies (Gorilla gorilla gorilla, G. g. graueri, G. g. beringei) and are distributed in two widely separated forest habitats. Mitochondrial DNA sequence analyses have found a clear distinction between the western subspecies (g. gorilla) and the two eastern subspecies (g. graueri and g. beringei) that is equivalent to a species-level distinction (Ruvolo et al., 1994; Garner & Ryder, 1996). Recent studies have shown marked differences in ecological features among these three subspecies, such as diet and ranging. Mountain gorillas are terrestrial folivores, while western lowland gorillas regularly feed on fruits and insects, and eastern lowland gorillas seasonally show frugivorous characteristics even in montane forests (Watts, 1984; Tutin & Fernandez, 1992, 1993; Yamagiwa et al., 1994, 1996; Kuroda et al., 1996; Doran McNeilage, this volume; McNeilage, this volume). More temporary and permanent shifts in ranging occur in western and eastern lowland gorillas and lead to their larger annual home range than that of mountain gorillas (Casimir & Butenandt, 1973; Remis, 1994; Tutin, 1996; Watts, 1998). This is presumably caused by the abundance or availability of high-quality foods (e.g. fruits) and may possibly influence the social organization of gorillas (Doran & McNeilage, 1998, this volume). Indeed, smaller-sized groups are expected under stronger within-group competition for foods (Wrangham, 1979). The fluid social units showing frequent subgrouping reported for western lowland gorillas at Ndoki (Mitani, 1992) and at Bai Hokou (Remis, 1994; Goldsmith, 1999) may be caused by the sparse distribution of fruits.