Geographic variation in orangutan behavior and biology

Extensive field data are available for three (out of four) Pongo taxa: the Sumatran P. abelii and the Bornean P. pygmaeus wurmbii of west and central Kalimantan and P. p. morio of east Kalimantan and Sabah. The data show a strong west–east gradient in morphology, behavioral ecology, and life history. From west to east, relative jaw robusticity and tooth enamel thickness increase, the frequency of reliance on non-fruit fallback foods—in particular inner bark of trees—increases dramatically, female day journey length and home range size decreases, the frequency of fat mobilization (and probably deposition) increases (although this has not yet been measured in P. p. morio), brain size decreases, sensitivity to selective logging decreases, average density decreases, and interbirth interval decreases. Social organization shows a similar west–east gradient, with Sumatran orangutans exhibiting a greater degree of sociality by a number of measures, although variation within Borneo is less clear. On Borneo, there may be less developmental arrest, male long calls are slower and have fewer pulses per call, consortships tend to be shorter, and a higher proportion of matings are forced. Geographic variation in orangutan features is probably produced through a combination of plastic developmental responses, genetic differences and cultural processes. The chapter offers a new hypothesis for the adaptive significance of these differences, based on the observed reduction in mean level of fruit production and increased incidence of periods of extreme scarcity from west to east. We highlight important remaining questions.