Orangutans are the only great apes found outside of Africa. At present, they occur only on the two large Sunda-shelf islands of Sumatra and Borneo. Most researchers recognize two separate subspecies, Pongo pygmaeus abelii in Sumatra and P.p. pygmaeus in Borneo.1,2 Relative to other primates, they have a variety of unusual features. These large-bodied frugivores are among the most solitary of anthropoid primates.3–5 They are also highly dimorphic, with the average body mass (78 kg) of males being more than twice that of females (36 kg).6 Despite its large body size the red ape has more specialized morphological adaptations for arboreality than do the African great apes. Researchers generally recognize sexually mature male orangutans with two physically distinct morphs, a phenomenon described as bimaturism. Males of the relatively smaller morph are known for forcing copulations with adult females.5,7–9.
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