Distal Humerus Morphology and Ecological Variation Among Gorilla Subspecies

Several features of the distal humerus have been considered to reflect varying locomotor adaptations among apes. Ecological diversity among Gorilla subspecies offers the opportunity to test the functional significance of these features. Gorilla gorilla gorilla lives in lowland forests, traveling long distances terrestrially to find fruit trees and climbing trees to obtain fruit, while Gorilla beringei beringei lives in montane forests lacking fruit trees and relies on terrestrial herbaceous vegetation, resulting in less climbing and shorter day ranges. Gorilla beringei graueri occupies habitats over a range of altitudes. Six linear distal humerus measurements were collected from G. g. gorilla (n=146), G. b. beringei (n=26), and G. b. graueri (n=22), and variables were size-adjusted using the geometric mean. Based on Bonferroni-corrected t-tests, male and female G. g. gorilla had significantly larger capitular widths than G. b. beringei, as predicted, but significantly smaller trochlear widths and olecranon fossa depths contrary to predictions. In separate analyses of males and females, principal components analyses separated these two taxa on PC1, and anteroposterior trochlear diameter had the highest loading, although the larger diameters (shallower trochlear notches) observed in G. g. gorilla were not predicted. Despite its range of habitats, G. b. graueri does not exhibit intermediate morphology. In males, G. b. graueri clusters with G. b. beringei on PC1, while in females, G. b. graueri clusters with G. g. gorilla on PC1. Results suggest the distal humerus may not reflect locomotor variation within Gorilla in the same ways it reflects such variation among ape genera.