Male orang-utan bimaturism and reproductive success at Camp Leakey in Tanjung Puting National Park, Indonesia
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Unlike most mammals, male orang-utans exhibit bimaturism, in that mature individuals express one of two distinct morphological forms. Socially subordinate, ‘unflanged’ males are comparable to females in their size and facial morphology, while socially dominant ‘flanged’ males exhibit extreme sexual dimorphism and secondary sexual characteristics, primarily in the form of cheek pads (‘flanges’). Although male ornaments in other species are often phenotypically plastic, such characteristics in orang-utans are irreversible—and, given that both morphs are sexually mature and can father offspring—their adaptive significance remains unclear. We determined paternity of orang-utans at Camp Leakey in Tanjung Puting National Park, within the home range of one long-term dominant male, Kusasi, before, during and after his period of dominance, in comparison with subordinate male conspecifics. We found that Kusasi fathered substantially more offspring conceived during his dominant period than any other male and that socially subordinate, unflanged males only fathered offspring during periods of rank instability. We conclude that orang-utan male bimaturism is consistent with an evolutionarily stable reproductive strategy and that reproduction within the range of a dominant, flanged male is highly skewed in his favour, while unflanged males may largely wait for reproductive opportunities.
KeywordsMale bimaturism Sexual selection Secondary sexual characteristics Paternity Reproductive success
We thank the Indonesian State Ministry of Research and Technology (RISTEK), the Indonesian State Ministry of Forestry (PHKA), the Indonesian Institute of Sciences (LIPI), the Indonesian Natural Resource Conservation Agency (BKSDA) and the Tanjung Puting National Park Office (BTNTP) for granting their permission to undertake this research and to our formal counterpart, S. H. Limin of CIMTROP at Universitas Palangka Raya, for his generous support of this work. We are indebted to A. E. Leiman OBE, S. Brend, P. A. Racey, S. B. Piertney and L. Howe who were instrumental to facilitating this research; to D. Rachmawan, J. R. Houslay, L. E. Koslowski, E. Tan and the Indonesian assistants who assisted with sample collection; to G. L. Shapiro, A. E. Russon and R. Garriga for their data on Kusasi’s history and to R. J. Turton, M. Arandjelovic and V. Städele for assistance with analyses. We thank the Los Angeles Zoo and Botanical Gardens, Como Park Zoo and Conservatory and Henry Vilas Zoo (all USA) for providing biomaterials from their orang-utans for use in developing the multiplex procedure, following approval by recommendation from the Orangutan Species Survival Plan (SSP). We are grateful for the comments of M. A. van Noordwijk and two anonymous reviewers, which greatly improved this manuscript.
This work was supported by the University of Aberdeen (UoA) Expedition Fund; UoA Small Grants Fund; UoA Alumni Annual Fund; the John Reid Trust; the Royal Geographical Society with Institute of British Geographers; the Royal Scottish Geographical Society; the Gilchrist Educational Trust; the Orangutan Foundation UK; the North of England Zoological Society at Chester Zoo; Darwin College, University of Cambridge; the Primate Society of Great Britain Charles A Lockwood Memorial Grant; the ARCUS Foundation; the Leverhulme Centre for Human Evolutionary Studies, University of Cambridge; the Miss Millie Foundation; Henry Vilas Zoo; The Orang-utan Conservation Genetics Trust and the Max Planck Society. GLB was supported by the Orang Utan Republik Foundation through the 2012 LP Jenkins Memorial Fellowship and by the German Academic Exchange Service (DAAD). The long-term research conducted at Camp Leakey, which facilitated this genetic study, was primarily funded by the Orangutan Foundation International.
Compliance with ethical standards
All applicable international, national and/or institutional guidelines for the care and use of animals were followed. Biological samples were exported from the Republic of Indonesia and from the United States of America to the European Union, via the United Kingdom, with permits 14459/IV/SATS-LN/2008, 00459/IV/SATS-LN/2011, 11US49805A/9 and 477248/01-27 under the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). All experiments described herein comply with the current laws of the countries in which they were performed. The authors declare that they have no conflict of interest.
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