Spear Jeffrey K
Department of Organismal Biology and Anatomy, University of Chicago, 1027 E 57th Street, Chicago, 60637, USA; Center for the Study of Human Origins and Department of Anthropology, New York University, 25 Waverly Place, New York, 10003, USA; New York Consortium in Evolutionary Primatology, New York, USA.
J Hum Evol. 2025 Jan;198:103616. doi: 10.1016/j.jhevol.2024.103616. Epub 2024 Nov 27.
The evolution of suspensory locomotion in primates has been of great interest to biological anthropologists since the early 20th century due to the contentious hypothesis that suspension in hominoids may have been a preadaptation for bipedalism. Studies of fossil hominoids regularly look for traits (or lack thereof) indicative of suspension, but many fossils exhibit potentially confusing mosaics of traits, and there is ongoing debate regarding whether certain traits are truly associated with suspension or whether they might more accurately represent allometric trends, developmental byproducts, or adaptation to cautious climbing. Here, I test the association between 27 morphological traits and forelimb suspension in extant primates using phylogenetically informed comparative methods, a broad comparative sample (nearly 1500 individuals representing 74 genera), and a systematic review of behavioral literature. I find that clavicle length, olecranon length, mediolateral scapula breadth (but not craniocaudal height), and glenoid and scapula spine angle are all strongly associated with suspension. The association is strongest for clavicle and olecranon lengths when the 'suspensory' category is highly exclusive, whereas it is strongest for scapula breadth, glenoid angle, and spine angle when the category is highly inclusive (i.e., also including taxa that use only limited amounts of suspension). Humeral head height above the greater tuberosity appears to be associated with nonquadrupedal locomotion generally rather than suspension specifically. Insertions for the biceps and deltoid muscles are significantly more distal in suspensory taxa only when size-standardized by a body size proxy, not when standardized by the length of the load arm. Overall, a majority of hypothesized traits are not actually associated with suspension in a phylogenetic comparative context. Morphological adaptations that do characterize suspension are expressed in a mosaic fashion that depends on the degree of suspension practiced, other behaviors used, and evolutionary history. Most of these traits may be related to an enhanced range of motion at the shoulder.
自20世纪初以来，灵长类动物悬荡式运动的进化一直是生物人类学家非常感兴趣的研究领域，这是因为存在一个有争议的假说，即类人猿的悬荡行为可能是两足行走的一种预适应。对化石类人猿的研究经常会寻找表明悬荡行为的特征（或缺乏这些特征），但许多化石呈现出可能令人困惑的特征组合，并且关于某些特征是否真的与悬荡行为相关，或者它们是否更准确地代表了异速生长趋势、发育副产品或对谨慎攀爬的适应，目前仍存在争议。在这里，我使用系统发育比较方法、广泛的比较样本（代表74个属的近1500个个体）以及对行为文献的系统综述，来测试27种形态特征与现存灵长类动物前肢悬荡行为之间的关联。我发现锁骨长度、尺骨鹰嘴长度、肩胛骨的中外侧宽度（但不是前后高度）以及关节盂与肩胛冈的角度都与悬荡行为密切相关。当“悬荡”类别高度排他时，锁骨和尺骨鹰嘴长度的关联最为强烈；而当该类别高度包容时（即也包括仅使用有限悬荡行为的类群），肩胛骨宽度、关节盂角度和肩胛冈角度的关联最为强烈。肱骨头高于大结节的高度似乎通常与非四足运动相关，而不是与特定的悬荡行为相关。仅当通过体型代理进行大小标准化时，悬荡类群中二头肌和三角肌的插入点才明显更靠远端，而通过负载臂长度进行标准化时则不然。总体而言，在系统发育比较的背景下，大多数假设特征实际上与悬荡行为并无关联。确实表征悬荡行为的形态适应以镶嵌的方式表现，这取决于所采用的悬荡程度、其他使用的行为以及进化历史。这些特征中的大多数可能与肩部更大的活动范围有关。
