Testing Functional Hypotheses About Variation in African Ape Scapulae Using 3D Geometric Morphometrics

Differences in scapular morphology among African apes have long been observed, but the functional significance of this variation is unclear. We identified five features of scapular morphology hypothesized to vary with arboreality and tested resulting predictions.

Fifteen 3D landmarks were collected from scapulae of adult Gorilla (n=150) and Pan (n=210) and analyzed using geometric morphometrics. Differences between taxa were visualized, and predictions were tested based on observations that Pan is more arboreal than Gorilla and G. gorilla gorilla is more arboreal than G. beringei beringei, with G. b. graueri probably intermediate to the other Gorilla subspecies. Compared to Gorilla, Pan has a cranially angled glenoid fossa and acromion process, as predicted. Contrary to predictions, Gorilla has a wider supraspinous fossa, longer acromion process, more projecting coracoid process, and more laterally placed inferior angle.

These muscle attachment sites are probably larger in Gorilla to meet the demands of stabilizing the shoulder joint when this larger animal climbs. The subspecies of Gorilla varied as predicted, with the largest attachment sites in the more arboreal G. g. gorilla and the smallest in G. b. beringei.

This study suggests that a continuum of functionally-correlated scapular shape across African apes does not exist, probably because the great size difference between the two genera results in different biomechanical demands when climbing. It may be more productive for researchers to focus on the relationship between morphology and locomotor behavior within each genus.