Female-female competition in Bornean orangutans

The mostly solitary ranging of orangutans and the large areas over which they traverse have hampered quantification of Bornean orangutan ranging patterns and feeding competition. Because of their semisolitary existence, female orangutans have few competitive interactions among themselves. However, contest and scramble types of competition occur, and researchers consider both to be important for the species. Using 9 yr of data and > 22,300 h of observation of adult female orangutans in Gunung Palung National Park in Indonesian Borneo, we examined both forms of competition. Based on our analyses, we have 4 conclusions: 1) Adult female orangutans have highly overlapping home ranges, and thus there is potential for scramble competition to impose a cost. 2) Adult female orangutans actively avoid each other, suggesting that scramble competition indeed imposes a cost. 3) Adult females have distinct core areas that overlap to a lesser degree than home ranges do. 4) Analyses of contest competition reveal a slight spatial component to female competition for the first time. Preliminary evidence for core area defense and passive range exclusion may be among the mechanisms responsible for maintaining distinct adult female core areas.