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Squirrel Monkeys And Its Impact On The Body Size

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Additionally, the attainable range of shoulder mobility relative to over-all body size has an impact upon the fore-limb reaching that occurs over the course of a diagonal sequence gait. Squirrel monkeys, which display a more upright limb posture relative to other arboreal quadrupeds, do not have the ability to achieve extensive shoulder mobility (Schmidt 2005; Whitehead and Larson 1994). Compared to Cercopithecids, including rhesus macaques, however, squirrel monkeys possess an increasingly mobile proximal humerus joint, for they must create torque and movement around limited support axes (Nakatsukasa 1994; Schmidt 2005:366). The articular proportions between species (see Appendix IV), however, do not initially appear to reflect that difference in shoulder mobility, for the rhesus macaque specimens possess a greater mass / humeral head surface area compared to the squirrel monkey (see Figure 4). This initial variation reflects Cheng and Scott’s (2000) investigation, which highlighted a larger proximal humerus in terrestrial macaques necessary for the ability to reach forward, to the extent that Cercopthecids are able, and subsequently move the body across a flat surface (Cheng and Scott 2000). There remains, however, the fact that the larger mass / humeral head ratios of the rhesus macaques does not correspond to similarly greater stature proportions (see Figure 6, Table III). In order to explain that difference, it is useful to examine the stature / humeral head

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