ゴリラの生活様式に見られる地域差について

ヴィルンガ火山群とカフジ山の比較から

抄録

Socio-ecological characters of gorillas in the Kahuzi and the Virunga regions were compared, in relation to environmental factors. A lower population density was estimated for Kahuzi than Virunga. A complex mosaic of primary and secondary montane forests, bamboo forest, and Cyperus swamp in Kahuzi provided a greater variety of gorillas' food items than the relatively simple vegetation of Virunga. Although no difference was detected in the day journey length between both areas, gorillas in Kahuzi tended to range in wider areas, feeding mainly on the leaves or bark of several species of vine, while gorillas in Virunga ranged over small areas feeding on various parts of several herb species. Seasonal food items such as bamboo shoots or fruits stimulated gorillas to shift their ranges between primary and secondary vegetation in Kahuzi. These conditions were possibly responsible for the different frequency of inter-unit encounters between the Kahuzi (low) and Virunga (high) gorillas. Ranging of the Kahuzi gorillas might be affected by the amount and distribution of food resources, while ranging of the Virunga gorillas tended to be affected by social factors at inter-unit encounters. Social structure and its maintenance mode based on female transfer between social units and on male emigration from their natal groups, were almost the same for both areas. There was no difference in group size, age-sex composition per population or per group between the Kahuzi populations and the 1959-60 Virunga population reported by Schaller (1963), but were recognized significant differences between the latter and the 1982 Virunga population. It is suggested that a reduction of National Park size and human disturbance such as cattle raising or poaching might have initiated this social change within the Virunga population in the early 1970's: The reduction of habitat brought the higher density of social units, which increased number of inter-unit encounters in several isolated areas. Inter-unit interactions stimulated male emigration or female transfer to produce lone males or newly-formed groups, and thus decreased group size. Lone males and newly-formed groups increased agonistic interactions between social units, which probably prevented maturing males from participating in violent interactions between prime adult males, and possibly has recently led them to form multi-male groups or all-male groups. Evidence of infanticide observed only in Virunga also implies that competition among male gorillas has recently increased there. These factors suggest the social and behavioral flexibility of gorillas to environmental changes in the montane forest.