Patterns of sexual dimorphism in Pan and Gorilla limb bones

While it is well-established that Pan and Gorilla differ in degree of size-based sexual dimorphism, less is known about their patterns of shape-based sexual dimorphism in the postcranium or about geographic variation in these patterns. Using a geographically diverse sample, differences between the genera in patterns of sexual dimorphism in the limb bones were investigated.

Forty-three linear measurements of ten limb bones were collected from Gorilla (n=266) and Pan (n=274) adults, size-corrected using geometric means, and analyzed using t-tests and principal components analyses (PCAs). Male and female Gorilla are best separated on PC2 and PC3, with high loadings for size-corrected lengths and midshaft widths and low loadings for size-corrected measurements of the proximal and distal ends. Pan sexes are not as strongly distinguished but are best separated on PC1 and PC2, on which size-corrected lengths, hand and foot bone widths, and elbow and knee measurements load most heavily. T-tests and PCAs of size-corrected measurements show greater shape differences between sexes in Gorilla than in Pan .

Examination of PCA plots by species, subspecies, and population, however, show that patterns of sexual dimorphism differ at each of these levels and do not simply reflect the genus-level pattern. For example, in Gorilla , the female distribution pattern varies among populations. In Pan , P. troglodytes troglodytes and P. t. verus display some separation of sexes, while little sexual dimorphism is apparent in the distributions of P. t. schweinfurthii and P. paniscus . Patterns of shape-based sexual dimorphism in limb bones exhibit geographic variation within each genus.