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Great ape life-history data are especially relevant for tests of the predictions of life-history theory and to establish firmly the derived features of human life history and therefore the changes that took place during hominin evolution. This chapter compares what is known about life history data on Sumatran and Bornean orangutans. The results indicate that interbirth intervals are longer for Sumatran than Bornean orangutans. In addition, interbirth intervals on Borneo appear to decrease with a west–east gradient. The chapter proposes that these differences might be related to fruit availability differences between and within the islands of Sumatra and Borneo. As mortality data are at present not available from Borneo we compared mortality rates of captive Sumatran and Bornean orangutans. No differences for captive Sumatran and Bornean orangutans were found, however. Interbirth intervals between Sumatran and Bornean orangutans were also not found, but overall interbirth intervals were significantly shorter in captivity. We discuss these results in comparison with other hominoids.

Orangutans have the longest interbirth interval of any mammal, with existing data suggesting that these intervals may be significantly longer in Sumatra than in Borneo. This finding presents a paradox because our models of reproductive ecology suggest that the higher habitat quality of Sumatra should lead to shorter interbirth intervals. This chapter explores this intriguing difference between Sumatran and Bornean orangutans by detailing the available evidence on how orangutan reproduction is influenced by ecology and life history. Data are evaluated in light of the models, mechanisms and hypotheses by which energetics influence reproduction in apes and humans. The chapter makes recommendations for future research that will lead to a more thorough understanding of orangutan reproductive ecology. New hypotheses about acute vs cumulative effects of energy on ovarian function, the magnitude of the shift in energy intake, and the role of developmental plasticity in orangutan reproductive functioning are also presented.

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.