Factors affecting reproduction in rehabilitant female orangutans: young age at first birth and short inter-birth interval

Abstract

This study investigated the reproductive parameters of free-ranging rehabilitant female orangutans. We aimed to assess the factors that influence these parameters and provide information that could assist with the management of orangutan reintroduction programs. We analyzed the birth records of free-ranging female rehabilitants at Bukit Lawang, Bukit Tigapuluh, Sepilok, Camp Leakey, Kaja Island, Sungai Wain, and Meratus and compared them with reproductive parameters reported in wild and zoo populations. Females' ages at first birth were 10.6-14.7 years, significantly earlier than those of wild and zoo orangutans. Computed inter-birth intervals (IBIs) calculated by the Kaplan-Meier method were 65.1-90.1 months; the values for Camp Leakey and Bukit Lawang rehabilitants were significantly shorter than those reported for wild Sumatran orangutans. Infant mortality rates were 18-61%; the values for Bukit Lawang and Sepilok were significantly higher than those reported for wild Sumatran and zoo orangutans. In rehabilitants, young ages at first birth and shorter IBIs may result from the high energy intake enabled by provisioning, although the possibility exists that they reflect underestimations of age on arrival at rehabilitation centers. The observed high infant mortality rate may reflect poor mothering skills due to human rearing and/or increased disease transmission. This study demonstrates that accelerated reproductive rates (younger age at first birth and shorter IBI) are common in provisioned rehabilitant females on both Sumatra and Borneo.

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

“…Chi-squared analyses show that the sex ratio at birth of first-generation Camp Leakey mothers does not differ significantly from that of secondgeneration Camp Leakey mothers (v 2 = 2.002, df = 1, p [ 0.05). Comparing the entire Camp Leakey sample to published data for Sepilok (Kuze et al 2012), Sepilok had a significantly lower ratio of males to females (Sepilok sex ratio at birth = 0.07, n = 27; Pearson's chi-squared test: v 2 = 23.49, df = 1, p \ 0.001). When compared to data from captive (zoo) Pongo (Singleton and van Schaik 2002), the difference was barely significant, with Camp Leakey again having a higher ratio of males to females than in the zoo (zoo sex ratio at birth = 0.51, n = 755; Pearson's chisquared test: v 2 = 3.854, df = 1, p = 0.05).…”

“…However, infant mortality at Camp Leakey was significantly lower than that among the provisioned rehabilitant population at Sepilok (58 %, Kuze et al 2012; Pearson's chi-squared test, v 2 = 10.62, df = 1, p \ 0.001), where over half of all infants did not survive past their third year. At Tanjung Puting National Park, over a 40-year period, only two cases of infant mortality among the wild orangutan population were noted: a young male who disappeared during late infancy, and a stillborn female infant (Galdikas, unpublished data).…”

“…Emerging evidence suggests that this same pattern can be seen when comparing the reproductive parameters of wild orangutan populations with those of provisioned freeranging ex-captive populations (Kuze et al 2012). Wild orangutans are individuals who developed from birth to independence with their biological mothers and who have never received food provisioning or care from humans.…”

“…Data were recorded daily by Galdikas and local field assistants, and are based on direct continuous long-term observation, not hearsay or incomplete records (e.g., Yeager 1997; Kuze et al 2012). Four primary reproductive parameters are included in this analysis.…”

“…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.…”