Demography of Agile Gibbons (Hylobates agilis) in a Lowland Tropical Rain Forest of Southern Sumatra, Indonesia: Problems in Paradise
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Gibbons are among the best-studied Asian primates, but few studies address their demography and life history strategies. We used annual censuses to study the demography of agile gibbons (Hylobates agilis) between 1998 and 2009 in rain forests of Bukit Barisan Selatan National Park, Indonesia. The population declined from 22 individuals (9 groups) to 14 individuals (5 groups) over the 12 yr of study. Infant survival to the juvenile age class was 33.3%, and 16.7% of infants survived to the subadult age class. The interbirth interval was 3.83 ± 1.15 yr and birth rate was 0.22–0.28 infants female–1 yr–1. Two groups colonized the study area but subsequently disappeared. We documented 7 immigrations, 17 disappearances, and ≥10 transients in the population. Compared to lar gibbons (Hylobates lar) and Bornean white-bearded gibbons (Hylobates albibarbis), Way Canguk’s agile gibbon population is characterized by slow reproduction, low survival, and high group turnover. We hypothesize that, although the habitat is high in fruit resources, agile gibbons may be displaced or excluded from the best fruit resources by larger and more numerous competitors, incurring costs of decreased opportunities to forage and increased travel, and leading to higher mortality for young agile gibbons. The reproductive potential of this agile gibbon population is insufficient to compensate for high mortality, and the population is unlikely to persist without immigration from outside the area. Given the agile gibbons’ endangered status and limited capacity to respond demographically to change, it is likely that intensive management interventions will be required to conserve this species.
KeywordsAgile gibbon Demography of agile gibbons Resource competition Sumatra, Indonesia
This research is a collaborative effort by the Wildlife Conservation Society and the Indonesian Ministry of Forestry, Directorate General for Nature Conservation (PHKA). Our research was funded by the Wildlife Conservation Society, the U.S. Fish and Wildlife Service Great Apes Conservation Fund (grant no. 98210-1-G084), the Disney Conservation Fund, and E. McBean. We thank A. Dwiyahreni, M. Iqbal, A. Nurcahyo, M. Nusawalo, M. Prasetyaningrum, M. Rusmanto, Sunarto, D. Suyadi, N. Winarni, Aris, Bawk, Tegu, and Wariono for assistance with data collection over the years. A. Elder provided valuable insights into the interspecific relationships of Way Canguk primates, L. Morino provided details of clouded leopard predation on a juvenile siamang at Way Canguk, and R Palombit and C. van Schaik provided information on the relationships of lar gibbons and siamangs in northern Sumatra. Our manuscript was greatly improved by the comments of J. Setchell and 2 anonymous reviewers.
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