Under the guidance of Dr. Thad Bartlett, Lydia Overbaugh will explore the degree of behavioral flexibility in the endangered white-handed gibbon in the Huai Kha Khaeng Wildlife Sanctuary (HKK), Thailand. White-handed gibbons living at HKK inhabit an environment that is unique in many ways. First, the forest is considerably drier than other white-handed gibbon study sites, receiving less than half the amount of rainfall than elsewhere, making this forest the ecological extreme of the range of white-handed gibbons. Second, the forest is part of the largest continuous protected area in Southeast Asia, which has led to the reduction in anthropogenic stresses while maintaining a largely intact large mammal community (including elephant, tiger, 8 diurnal primates, two leopards and two canids). Finally, gibbons at HKK inhabit a mosaic landscape, including seasonally dry evergreen, hill evergreen, mixed deciduous and deciduous dipterocarp forest types. This presents a unique opportunity to examine how white-handed gibbons respond to ecological differences at the extreme of their range and in a diversity of habitats. To determine how habitat quality impacts gibbon behavior and ecology, the feeding and ranging behavior of 6 gibbon social groups will be studied over a period of 12 months to test the following predicitions: first, that gibbons in evergreen forest will (i) have smaller home ranges and travel less per day, (ii) spend less time foraging, and (iii) have a higher quality diet; and second, that these differences are a result of differences in home range quality as indicated by the density of fruiting trees.
Current hypotheses in human evolution favor an increase in the behavioral and dietary flexibility of Homo as an explanation for the success of our lineage. By examining the degree of flexibility present in a more distantly related ape species, we can better evaluate the assertion that flexibility is a uniquely human innovation. As many gibbon researchers have described gibbons as being dependent upon evergreen forest, this study will demonstrate the ways in which white-handed gibbons cope with ecologically marginal habitats. Furthermore, gibbons are primarily monogamous, but significant rates of polyandry and polygyny have been recorded for some populations and these occurrences have been linked to ecological differences such as seasonal and geographic resource availability. This project will therefore increase our understanding of the role of habitat quality in the maintenance of social monogamy in white-handed gibbons.
The proposed project will assist in graduate student training and increase the participation of women in scientific research while promoting a minority-serving institution. Additionally, the establishment of a new field site will provide opportunities for training Thai field assistants and instructing US undergraduate students. The most tangible impact of the proposed research is its applicability to primate conservation. By better understanding the degree of behavioral flexibility under distinct ecological conditions and at this species' ecological extreme, this project will inform conservation policies for white-handed gibbons. This knowledge can also provisionally be used for conservation purposes in the absence of comprehensive knowledge of other more critically endangered gibbon species.
Current hypotheses in human evolution favor an increase in the behavioral and dietary flexibility of Homo as an explanation for the success of our lineage. By examining the degree of flexibility present in a more distantly related hominoid species, we can gain a more nuanced understanding of the extent of adaptive flexibility found throughout all apes. The most distantly related apes are the gibbons, a group of 12-17 species of small-bodied apes. Gibbons are known for diets high in ripe fruit and live in territorial, socially monogamous groups inhabiting small, stable home ranges. Yet while gibbons can be described as typically monogamous, territorial ripe-fruit specialists, there is considerable variation between species in relation to diet, range use, and social structure. However, as many of these species have only been studied at one or a few locations with little ecological variation, it remains unclear whether these between-species differences are based on unique evolutionary histories or are instead the result of local ecological differences. Lar gibbons, the smallest gibbon species, inhabit a large, ecologically varied range and provide an opportunity to investigate the effects of local ecology on behavior, including diet, range use, and social structure, and reveal the extent to which behavioral flexibility exists as an ape-wide adaptive strategy. This project was conducted at the Khao Nang Rum Wildlife Research Station in the Huay Kha Khaeng Wildlife Sanctuary in western Thailand. The site is unique in many ways. First, the forest is considerably drier than other lar gibbon study sites, making this forest the ecological extreme of this species’ range. Second, the forest is part of the largest continuous protected area in Southeast Asia which has led to the reduction in anthropogenic stresses while maintaining a largely intact large mammal community (including tiger, leopard, elephant, tapir, guar, 8 diurnal primates, and two canids). Finally, gibbons here inhabit a mosaic landscape, including evergreen, mixed deciduous and deciduous dipterocarp forest types. This presents a unique opportunity to examine how lar gibbons respond to ecological differences at the extreme of their range and in a diversity of habitats. Our results demonstrate that lar gibbons are capable of living and succeeding in a variety of habitats given certain conditions. As many gibbon researchers have described gibbons as being dependent upon evergreen forest areas and restricted in their diet, the findings here will transform the way we characterize gibbons. At this site, lar gibbons living in evergreen forests show similar diet and ranging patterns as other sites with home range size varying between 15 and 21 hectares and a diet high in ripe fruit (including figs). In contrast, gibbons in mosaic forest areas have home range sizes varying from 26.4 to 50.8 and devote considerable amounts of feeding time to consuming flowers, leaves, and insects, although they clearly prefer ripe fruit when it is available. Furthermore, out of the six study groups and three additional groups encountered frequently, two groups diverted from the typical monogamous social structure seen in gibbons. Together, these findings demonstrate that lar gibbons can make important contributions to our understanding of ape evolution and specifically the factors contributing to the maintenance of social monogamy, a theoretical topic of much debate among primatologists and biological anthropologists. This project increases the participation of women in scientific research, including the co-PI, two research assistants, and two field assistants. All six assistants were selected from the local population and Thai universities and thus also increases the participation of these individuals in international scientific research while providing reliable employment in the area. The successful establishment of the field site and habituation of numerous gibbon study groups (as well as the partial habituation of several langur and macaque groups in the area) has laid the foundation for creating a field school where US undergraduate students can be trained in field methods. The most tangible impact of this project is its applicability to primate conservation. By better understanding the degree of behavioral flexibility under distinct ecological conditions and at this species’ ecological extreme, this project will inform conservation policies for lar gibbons. This knowledge can also provisionally be used for conservation purposes in the absence of comprehensive knowledge of other more critically endangered gibbon species. Preliminary results for this project were presented at the Asia-Pacific Chapter Meeting of the Association of Tropical Biology and Conservation Annual Meeting held in Banda Aceh, Indonesia in March 2013.
