1Human Origins Program, Department of Anthropology, National Museum of Natural History, Smithsonian Institution, 2School of Biosciences, University of Birmingham, 3Department of Anthropology, Rutgers, The State University of New Jersey, 4Center for the Advanced Study of Hominid Paleobiology, The George Washington University
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The relatively short first ray compared with digits 2-5 of orangutan hands and feet has led to the general assumption that the orangutan pollex and hallux are not extensively recruited during positional behaviors. Earlier studies of captive orangutans appeared to support the assumption that the first rays were rarely used in favor of grasps that involved digits 2-5, perhaps reflecting locomotor adaptations for grasping thin vine-like substrates encountered in their natural habitats. However, the natural habitats of orangutans are now known to include a wide variety of differently sized substrates and a reexamination of how orangutans use their manual and pedal digits in the wild is clearly warranted. We examined 326 minutes of digital video of wild orangutans from the Tuanan Research Station, located in Central Kalimantan, Indonesia, and recorded in seconds all instances and durations of digit use. Of the 6781 seconds where the hallux was visible, 43.6% involved actions that included recruitment of the hallux whereas only 19.5% involved toe-only actions (p<.001). Of the 5848 seconds where the pollex was visible, 53.2% involved actions that included recruitment of the pollex whereas only 11.4% involved finger-only actions (p<.001). Other interesting findings include slightly more frequent use of the pollex compared with the hallux (p=0.05), toes are used roughly twice as often compared with fingers (p<.001), and the left and right hands are utilized equally (p=0.49). In total, these data demonstrate that wild orangutans recruit their first rays far more often than has been assumed in studies of functional morphology.