The behaviour of 48 chimpanzees (Pan troglodytes verus) was studied over 27
months in Liberia, West Africa. The chimpanzees were first studied while they were
housed in groups in enclosures in a medical research institute, and then after their release
onto a 9.7 ha offshore island.
When the chimpanzees were observed in captivity, data on social behaviour were
collected with the use of check sheets and ad libitum notes. Data were collected on
aggression, social grooming, social play, sexual behaviour, and individual spacing. After
release onto the island, data on both social and subsistence behaviour were collected with
the use of ad libitum notes.
Both changes in social behaviour and in the development of subsistence behaviour
were observed following release of the chimpanzees onto the island. Rates of aggression
decreased following release, whereas rates of social grooming increased. Rates of social
play decreased overall, but this was due to a decrease in social play by adults.
Stereotyped or abnormal behaviour shown by some subjects declined. Subsistence
behaviours which were observed following release were foraging for naturally occurring
foods (leaves, fruits, seeds, and nuts), ant-eating, and tool-use for nut-cracking. Some
subjects were also seen building sleeping-nests in trees. The chimpanzees also split into
subgroups (including consortships) which showed similar trends in size and composition
to those observed in wild populations of chimpanzees.
Some techniques found to be useful during the release process are discussed, and
the study is compared to previous primate release projects.