Reproductive parameters over a 37-year period of free-ranging female Borneo orangutans at Sepilok Orangutan Rehabilitation Centre |
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Authors: | Noko Kuze Symphorosa Sipangkui Titol Peter Malim Henry Bernard Laurentius N Ambu Shiro Kohshima |
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Institution: | (1) Department of Zoology, Graduate School of Science, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan;(2) Sabah Wildlife Department, 5th Floor Block B, Wisma Muis, 88100 Kota Kinabalu, Sabah, Malaysia;(3) Institute for Tropical Biology and Conservation, Universiti Malaysia Sabah, Locked Bag 2073, 88999 Kota Kinabalu, Sabah, Malaysia;(4) Graduate School of Bioscience and Biotechnology, Tokyo Institute of Technology, 2-12-1-W3-43 Ookayama Meguro-Ku, Tokyo 152-8551, Japan |
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Abstract: | We analysed the reproductive parameters of free-ranging female orangutans at Sepilok Orangutan Rehabilitation Centre (SORC)
on Borneo Island, Sabah, Malaysia. Fourteen adult females produced 28 offspring in total between 1967 and 2004. The average
censored interbirth interval (IBI) (i.e. offspring was still alive when mother produced a next offspring) was 6 years. This
was shorter than censored IBIs reported in the wild but similar to IBIs reported for those in captivity. The nonparametric
survival analysis (Kaplan-Meier method) revealed a significantly shorter IBI at SORC compared with wild orangutans in Tanjung
Putting. The infant (0–3 years) mortality rate at SORC of 57% was much higher than rates reported both in the wild and captivity.
The birth sex-ratio was significantly biassed toward females: 24 of the 27 sex-identified infants were females. The average
age at first reproduction was 11.6 years, which is younger than the age in the wild and in captivity. The high infant mortality
rate might be caused by human rearing and increased transmission of disease due to frequent proximal encounters with conspecifics
around the feeding platforms (FPs). This young age of first reproduction could be because of the uncertainty regarding estimated
ages of the female orangutans at SORC. It may also be affected by association with other conspecifics around FPs, which increased
the number of encounters of the females with males compared with the number of encounters that would take place in the wild.
Provision of FPs, which improves the nutritional condition of the females, caused the shorter IBI. The female-biassed birth
sex-ratio can be explained by the Trivers and Willard hypothesis. The female-biassed sex ratio could be caused by the mothers
being in poor health, parasite prevalence and/or high social stress (but not food scarcity) due to the frequent encounters
with conspecifics around FPs. |
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Keywords: | Interbirth interval Infant mortality Biassed birth sex ratio Age at first reproduction Provision |
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