During a four-year period, more than 6,800 hours of observation were collected on 58 individually recognized wild orangutans in a 35-km2 study area at the Tanjung Puting Reserve, Kalimantan Tengah (Central Indonesian Borneo). As in other areas, although small temporary associations of orangutans recurred, the basic units of populations consisted of (1) adult males, (2) adult females with one or two dependent offspring, and (3) independent immatures (adolescent males and females and subadult males) with units predominantly solitary. However, independent immatures were more gregarious and social than adults. Adolescent females in particular spent more time in groupings than did adult females. Although successfully consorting adult males might spend up to 22% of time observed in association with other orangutans, adult males were the least social of all age/sex classes once consortships were excluded. Ninety percent of their participation in social groupings consisted of sexual consortships. Adult males were totally intolerant of each other; only four encounters between adult males were observed. All involved avoidance or aggression, including one lengthy combat. Adult females were solitary but social and participated in a variety of groupings. However, with the exception of adult females briefly following consorting couples, nonreceptive adult females did not initiate or maintain social contact with orangutans other than adult females. Comparisons with results of studies from other areas indicate that, once samples used in each study are made explicit and aggregations owing to chance encounters at major food sources excluded, orangutan sociality seems relatively consistent over the entire range in which the species is found. Bornean orangutans are not as totally asocial as the first studies in the wild indicated. In fact, it would probably be better to characterize wild orangutans as semisolitary, rather than solitary.
Keywords: Borneo; Pongo pygmaeus pygmaeus; Tanjung Puting; orangutan; sociality.
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