Seasonal movements in the Sumatran orangutan (Pongo pygmaeus abelii) and consequences for conservation Original Research Article

Preservation of orangutan habitat can only be effective when their range requirements are taken into consideration. Here, the effect of the variation in fruit supply at different altitudes on the seasonal movements of orangutans is investigated. Transects were surveyed every 2 months from 1998 to 2000, at different altitudinal zones in the larger Ketambe area, Indonesia. The density of orangutan and sources of ripe fruit with soft pulp were determined along these transects. Both orangutan density and the availability of fruit varied considerably throughout the survey period at different altitudinal zones. The peaks in fruit production at different altitudes occurred in successive periods. For each altitudinal zone a significant correlation was found between orangutan density and the number of fruit sources, which shows that orangutans follow the production of fruit between these altitudinal zones within the boundaries of their home range. Orangutans with core areas in intermediate and highland areas depend seasonally on fruit production of lowland areas, and vice versa. Thus, conservation of large tracts of primary forest with a range of different altitudes is necessary to preserve the remaining orangutan population.