The natural history of the chimpanzee (Pan troglodytes verus) at Mt. Assirik, Senegal

Abstract

This study examines the natural history of wild chimpanzees (Pan troglodytes verus) in Senegal, West Africa. This western form of chimpanzee is the least studied of the three geographical races. Ecological studies of chimpanzees have been neglected in favour of behavioural investigations. Those studies which have focussed on ecology have often been distorted by unnatural human intervention. Field studies of chimpanzees are reviewed in terms of their length, the extent of disturbance at the site, and the methods involved. The study area is described: its hot, arid climate and undisturbed state are emphasised. Methods were devised to gain knowledge of the chimpanzees' ecology without interfering with their behaviour or habitat. A detailed description of the types of vegetation is given, and their proportional distribution reveals that there is less forest and woodland at Mt. Assirik than at any other site where chimpanzees have been studied. Chimpanzees use the types of vegetation differentially and this shows seasonal variation. Forest is most used at the end of the dry season. At other times of the year, extensive use is made of woodland. Grassland is used during the wet season. Data from observations of chimpanzees and their nests is used to estimate the population size, range and density. The total number of chimpanzees at Mt. Assirik is estimated as about 25 to 30, density is reckoned as 0.1/km² and their home range as 250 to 300km². The chimpanzees appear to be healthy. Many features of social behaviour, described elsewhere, were confirmed for this subspecies. A high proportion of mixed parties was discovered. This is thought to be an adaptation to an area of open vegetation: its distribution of food, water, and the presence of large carnivores. The chimpanzees are omnivorous. Although mainly frugivorous, they also eat leaves, flowers, seeds, bark, honey, insects and meat. Two species of insect are eaten seasonally, and two types of tool are used to obtain termites and driver ants respectively. Chimpanzees appear to specialise in nocturnal prosimians as mammalian prey. Nests are examined in detail and found to be similar to those made elsewhere. Preferences, for certain species are demonstrated for the first time. Finally, the results of the study are compared with the cultural ecology of a human hunter-gatherer society, the !Kung San of Southern Africa. The comparison is used as a basis for speculation on the behaviour of the ancestral hominids.