Grouping behavior of Sumatran orangutans (Pongo abelii) and Tapanuli orangutans (Pongo tapanuliensis) living in forest with low fruit abundance

Abstract

In contrast to the African great apes, orangutans (Pongo spp.) are semisolitary: In-dividuals are often on their own, but form aggregations more often than expected bychance. These temporary aggregations provide social benefits such as mating op-portunities. When fruit availability is high, costs of aggregating should be lower,because competition is less pronounced. Therefore, average party size is expected tobe higher when fruit availability is high. This hypothesis would also explain whyorangutans in highly fruit‐productive habitats on Sumatra are more gregarious thanin the usually less productive habitats of Borneo. Here, we describe the aggregationbehavior of orangutans in less productive Sumatran habitats (Sikundur and BatangToru), and compare results with those of previously surveyed field sites. Orangutansin Sikundur were more likely to form parties when fruit availability was higher, butthe size of daily parties was not significantly affected by fruit availability. Withregard to between‐site comparisons, average party sizes of females and alone timeof parous females in Sikundur and Batang Toru were substantially lower than thosefor two previously surveyed Sumatran sites, and both fall in the range of values forBornean sites. Our results indicate that the assessment of orangutans on Sumatra asbeing more social than those on Borneo needs revision. Instead, between‐sitedifferences in sociality seem to reflect differences in average fruit availability.