Reproductive parameters over a 37-year period of free-ranging female Borneo orangutans at Sepilok Orangutan Rehabilitation Centre

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.

Reproductive parameters of female orangutans (Pongo pygmaeus wurmbii) 1971–2011, a 40-year study at Tanjung Puting National Park, Central Kalimantan, Indonesia

“…The rate of infant mortality at Camp Leakey was significantly lower than that at Sepilok (Kuze et al 2008). Kuze et al (2008) speculate that their extremely high (58 %) observed rate of infant mortality had two possible causes: (1) human rearing causing poor infant care skills, and (2) increased sociality due to provisioning leading to increased aggression and disease transmission.…”

“…The first-generation mothers' mean age at first reproduction (mean Gen1 = 14.2, n Gen1 = 11, SD Gen1 = 1.17, range Gen1 = 12-16) was significantly older than that of the secondgeneration mothers (mean Gen2 = 12.1, n Gen2 = 8, SD Gen2 = 0.99, range Gen2 = 11-13) (Mann-Whitney U test; n Gen1 = 11, n Gen2 = 8, U = 6.5, p \ 0.001). Ages at first reproduction of the first-and second-generation Camp Leakey mothers were each compared to two other orangutan populations for which published data sets are available: Sepilok Orangutan Rehabilitation Center (''Sepilok,' ' Kuze et al 2008) and wild orangutans at Tanjung Puting National Park (''Wild TPNP,'' Galdikas, personal observation as published in Tilson et al 1993). The distribution of age at first reproduction varied significantly across populations (CL Gen 1-Sepilok-Wild TPNP, independent-samples Kruskal-Wallis test, p \ 0.005; CL Gen 2-Sepilok-Wild TPNP, independent-samples KruskalWallis test, p \ 0.05).…”

“…Kuze et al (2008) speculate that this can also be explained by the Trivers and Willard (1973) hypothesis: mothers at Sepilok suffer from low health because of the stress caused by a high degree of gregariousness around feeding platforms and a high level of parasite exposure, and thus they produce a disproportionately high number of daughters to sons. Furthermore, captive Pongo also exhibited a significantly different sex ratio at birth from the Camp Leakey orangutans.…”

“…Kuze et al (2008) speculate that their extremely high (58 %) observed rate of infant mortality had two possible causes: (1) human rearing causing poor infant care skills, and (2) increased sociality due to provisioning leading to increased aggression and disease transmission. These factors may also be contributing to the infant mortality rate at Camp Leakey, although it is not as extreme as at Sepilok.…”

“…Kuze et al (2008Kuze et al ( , 2012 showed that these orangutans demonstrated a low mean age at first reproduction, a low ratio of males to females at birth, and a high rate of infant mortality relative to wild orangutan populations, such as those found at Gunung Palung and Tanjung Puting (Galdikas and Wood 1990;Knott 2001;Knott et al 2009). Interestingly, however, Kuze et al (2008Kuze et al ( , 2012 found that the IBI of rehabilitated orangutans at Sepilok (6.2 years) was comparable to that of wild populations of the same subspecies (P. p. morio) at Kutai National Park (6.1 years, as reported in Wich et al 2004;. Many of the differences in observed life history parameters between this population of provisioned P. p. morio and other wild-unprovisioned and thus likely nutritionally stressed-populations of Bornean orangutans can be explained by the ecological energetics hypothesis.…”