Grouping behavior of Sumatran orangutans (Pongo abelii) and Tapanuli orangutans (Pongo tapanuliensis) living in forest with low fruit abundance. Issue 5 (18th March 2020)

• Record Type: Journal Article
• Authors: Roth, Tom S.; Rianti, Puji; Fredriksson, Gabriella M.; Wich, Serge A.; Nowak, Matthew G.
• Is Part Of: American journal of primatology. Volume 82:Issue 5(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-03-18
• Subjects: fission–fusion -- food competition -- great ape -- party formation -- sociality -- socioecology -- temporal aggregation; Primates -- Periodicals; Primates -- Périodiques; 599.8
• DOI: 10.1002/ajp.23123
• Languages: English
• ISSNs: 0275-2565

Abstract

Abstract: In contrast to the African great apes, orangutans ( Pongo spp.) are semisolitary: Individuals are often on their own, but form aggregations more often than expected by chance. These temporary aggregations provide social benefits such as mating opportunities. When fruit availability is high, costs of aggregating should be lower, because competition is less pronounced. Therefore, average party size is expected to be higher when fruit availability is high. This hypothesis would also explain why orangutans in highly fruit‐productive habitats on Sumatra are more gregarious than in the usually less productive habitats of Borneo. Here, we describe the aggregation behavior of orangutans in less productive Sumatran habitats (Sikundur and Batang Toru), and compare results with those of previously surveyed field sites. Orangutans in Sikundur were more likely to form parties when fruit availability was higher, but the size of daily parties was not significantly affected by fruit availability. With regard to between‐site comparisons, average party sizes of females and alone time of parous females in Sikundur and Batang Toru were substantially lower than those for two previously surveyed Sumatran sites, and both fall in the range of values for Bornean sites. Our results indicate that the assessment of orangutans on Sumatra as being more social than those on Borneo needs revision. Instead, between‐site differences in sociality seem to reflect differences in average fruit availability. Abstract : Average female party size of orangutans in Sumatran forest with low fruit abundance is more similar to average female party size of Bornean orangutans than to Sumatran orangutans living in forest with high fruit abundance. Research Highlights: Sumatran orangutans in Sikundur were more likely to form parties when fruit availability was higher, but fruit availability did not significantly influence the daily party size on days with party formation. Orangutans in less productive Sumatran forest had a lower average party size than orangutans in highly productive Sumatran forest, and their average party size falls within the range of reported values for Bornean orangutans. Between‐site differences in orangutan sociality may be mediated by between‐site differences in fruit availability, and previously described differences in sociality between orangutans on Sumatra and Borneo may need revision.