Scapula form and biomechanics in gorillas

Gorillas are generating renewed interest as mounting evidence from field and molecular studies strongly suggests the western lowland (Gorilla gorilla gorilla) and eastern mountain (Gorilla gorilla beringei) gorillas are considerably more distinct than has previously been accepted. Schultz (1927, 1930, 1934) was one of the earliest investigators to document morphological differences between the two groups, noting differences in pedal, limb and scapular morphology. These differences led Schultz to conclude that while lowland gorillas retained some features suited to an arboreal habitat, the mountain gorilla had evolved into a specialized terrestrial quadruped. In particular, he noted that mountain gorillas exhibited lower values for the scapular index, higher values for ratios of infraspinous fossa vs. scapula length and spine length vs. scapula length and variability in the extent of curvature of the vertebral border. However, Schultz' observations were based upon small sample sizes of mostly adult specimens. This study extends Schultz' preliminary work by assessing, with appreciably larger sample sizes, patterns of relative growth of the scapula in these two subspecies of Gorilla. Scapula measurements were obtained for ontogenetic series of G.g. gorilla (n = 366) and G. g. beringei (n = 43). Statistical analyses reveal mountain gorillas exhibit significantly (P < 0.05) greater spine lengths and scapula breadths and smaller scapula lengths than lowland gorillas of comparable superior border lengths. However, at comparable body weights, mountain gorillas exhibit significantly shorter spines and superior borders than lowland gorillas. These differences in scapula proportions are evaluated in the context of biomechanical predictions regarding scapula form and locomotion.