FOOD SEASONALITY AND SOCIOECOLOGY IN PAN : ARE WEST AFRICAN CHIMPANZEES ANOTHER BONOBO?

Comparative feeding ecology of African apes has recently been intensively in- vestigated principally for testing the THV (terrestrial herbaceous vegetation) hypothesis. The hypothesis argues that peaceful behavioral nature observed in bonobo (Pan paniscus) com- pared to chimpanzee (P. troglodytes) is derived from presence of suffi cient THV as fallback foods during lean periods, resulting from habitat segregation from more herbivorous gorillas. There was some supportive evidence for the hypothesis such as presence of feeding competi- tion for fi brous foods between chimpanzees and gorillas in sympatry. However, many cast doubt on the function of THV to maintain female cohesiveness. Overall, the ecological and social causality theorized in the THV hypothesis appears sound, but THV itself unlikely plays a major role, although critical data, bonobo ecology in particular, are still missing. Observed behavioral variation among chimpanzee subspecies suggests that West African chimpanzees are behaviorally more peaceful than East African subspecies. Intensive comparison of some socio-ecological parameters between Bossou and Kibale chimpanzees supported this idea. Data suggested that, also in this case, THV consumption is unlikely a key factor. Because West African chimpanzees, like bonobos, have probably been segregated from gorilla habi- tat for considerable amount of time in their evolutionary history, collective infl uence from coexisting with gorillas, not a competition over a single food source, must be responsible for socio-ecological differentiation observed among bonobo, Western chimpanzees, and Eastern chimpanzees.