Body size is a major constraint in the arboreal milieu, and orthograde (upright) locomotor and postural behaviors are thought to have conveyed substantial advantages to the evolutionary development of the relatively large-bodied apes (hominoids). The results from this study will expand the current understanding of hominoid, especially gibbon, positional behavior. Additionally, as resource competitors of the critically endangered Sumatran orangutan, data on the positional behavior, forest use, and feeding/foraging strategies of endangered sympatric gibbons represents a key component to the conservation strategies at West Batang Toro, North Sumatra, Indonesia (WBT). This study also integrates with a larger project on the biodiversity of WBT, involving local and international researchers with diverse specializations, and will therefore also facilitate cross-national and inter-disciplinary ties. Lastly, the Co-PI will help educate Indonesian students and surrounding local communities about WBT and its ecological benefits.
While studies have addressed how body size and positional behavior covary among the largest representatives of this taxonomic group, far less has been reported on this relationship for the small-bodied hylobatids. This is particularly interesting as these anatomically and behaviorally specialized primates are thought to represent a lineage that has evolutionarily reduced in body size and fine-tuned the ape pattern of generalized orthogrady in order to more efficiently exploit the arboreal canopy. As a test of this hypothesis, the relationships between body size, habitat use, and positional behavior of size-variable sympatric gibbons (Hylobates agilis) and siamangs (Symphalangus syndactylus) are examined at WBT. Previously collected data by the Co-PI for Hylobates lar from Khao Yai National Park, Thailand is also used for comparison. Additionally, data from this study are compared with previously published data for other hominoids, atelids, cebids, and cercopithecoids in order to reevaluate ape locomotor and postural specializations.
Intellectual Merit: Recently, the last common ancestor of living apes has been reconstructed as a relatively large-bodied animal that utilized a positional repertoire that included a substantial amount of upright (or orthograde) locomotion and posture. For living hominoids, orthogrady is seen as a significant evolutionary development that facilitates safety in the arboreal canopy and enhances access to preferred food resources. An important caveat of this hypothesis is that the anatomically and behaviorally specialized hylobatids (especially the smaller-bodied gibbons) represent a lineage that evolutionarily reduced in body size and fine-tuned the hominoid pattern of generalized orthogrady in order to more efficiently exploit the arboreal canopy; however, there are limited tests of this caveat. Here it is argued that closely-related, size-variable, and sympatric gibbons and siamangs provide a unique situation in which to test the hypothesis that size-reduction in the small-bodied gibbons served to enhance their exploitation of the arboreal canopy. As such, a study of habitat use and positional behavior in wild Hylobates agilis (agile gibbons) and Symphalangus syndactylus (siamangs) was conducted in the West Batang Toru Forest Block, North Sumatra, Indonesia (WBTFB) between March 2010-January 2013. The relationships between body size, canopy use, and positional behavior of the aforementioned hylobatids were explored. Preliminary results suggest that body size does in fact play an important role in the differences in positional behavior and canopy use between agile gibbons and siamangs. For instance, siamangs consistently used locomotor modes (e.g., orthograde clamber/transfer) that allowed them to spread their body mass among multiple substrates, whereas agile gibbons were able to rely on more acrobatic forms of locomotion, such as ricochetal brachiation and leaping. The ability of siamangs at Batang Toru to spread their body mass among multiple substrates appears to allow them access to the same substrate size categories as the smaller-bodied gibbons. Interestingly, however, agile gibbons utilized smaller-sized substrates (<2 cm diameter) less frequently than siamangs, and this appears to be related to their ability to cross over large-sized gaps (a maximum of 15 meters was observed), which may allow them to avoid the small fragile branches that characterize the tree periphery. Conversely, larger-bodied siamangs were not observed to frequently cross gaps sizes of 3-5 m, and were never observed to cross gaps >5 m. As such, siamangs appear to be forced to cross between tree canopies via small-sized substrates. Interestingly, despite their relatively small body size and highly acrobatic nature, agile gibbons were not impervious to the constraints of body size in the arboreal setting. It could therefore be argued that body sizes above that of living hylobatids (i.e., 5-11 kg) were not necessary for the evolutionary development of orthograde locomotion and posture among hominoids. Nevertheless, as these are preliminary results, further comparative analyses (including the entire dataset and also data from other primate taxa) are a vital for a more detailed and comprehensive interpretation of the data collected during this project. Broader Impacts: Crucial to the conservation of any ecosystem is an understanding of its community ecology, and data on the positional behavior and canopy use of Endangered agile gibbons and siamangs represents a key component to the understanding of the overall primate biota of the Batang Toru Forest Complex. As such, these data can and should be used to help guide future conservation, so that more a detailed and comprehensive strategy can be employed. Additionally, this study worked in collaboration with a larger project, which focuses its efforts on the study of the overall biodiversity of the Batang Toru Forest Complex. This larger project involves local and international researchers, all of which have diverse specializations. This project therefore also helped to facilitate cross-national and inter-disciplinary ties, and the Co-PI was able to learn a considerable amount of information about the study and implementation of conservation biology and forest survey techniques. Lastly, the Co-PI participated in community outreach by teaching local Indonesian villagers/guides, local grade school students, and local university students about the techniques used within this study, in addition to more general topics such as forest ecology and conservation. It is the hope of the Co-PI that these efforts allowed participants to develop a greater understanding and appreciation of the importance of the unique forested environments that are found in close proximity to their everyday lives.
