Abstract
The characteristic long forelimbs of gibbons are a consequence of elongation of all segments (hand, forearm, and arm) but especially the forearm. While the hand is limited in size by its prehensile role, both the arm and the forearm do not seem to be directly limited in a similar manner. Why is the elongation concentrated specifically in the forearm? One hypothesis, originally applied to cursorial ungulates, relates to the need to enhance angular velocity during the swing phase at a minimum cost by distributing the heaviest forelimb loads proximally near the shoulder joint. This appears to be a plausible explanation despite complicating factors associated with competing functions of the forelimb during the support phase of arm-swinging and the fact that the animal behaviorally adjusts inertial properties of its forelimb by flexing it while reaching for the support. In order to test this hypothesis, the forelimb was modeled as a series of uniform cylinders from which the radii of gyration (k) were calculated. After converting into relative values (%k), they were compared (1) interspecifically, among two hylobatids, an orangutan, common chimpanzee, and gorilla, and (2) hypothetically, among imaginary gibbons with brachial indices extending beyond the range found in hylobatids. The results of both tests were equivocal, suggesting that forelimb mass distribution may not be the dominant factor influencing elongation of the forearms in gibbons. Even though gibbons are unique in their great dependence upon arm-swinging, efficient generation of high angular velocities during the swing phase does not appear to be the overriding function beyond others related to support, propulsion, and prehension.
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Takahashi, L.K. Forearm elongation in gibbons: Hypothesis and preliminary results. International Journal of Primatology 12, 599–614 (1991). https://doi.org/10.1007/BF02547672
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DOI: https://doi.org/10.1007/BF02547672