Phylogenetic comparative analysis of suspensory adaptations in primates

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.