Allometric analysis of the skull in Pan and Gorilla by geometric morphometrics.

Classical metric analyses have shown that much of the morphological variation between Pan and Gorilla is due to size-related shape differences. Geometric morphometrics, which operates a removal of size (as a consequence of GLS superimposition of landmark points), can shed further light on the issue. With these methods and with particular reference to facial and basal regions of the cranium, we examine here a sample including skulls at different ontogenetic stages of Gorilla gorilla and Pan troglodytes. Landmark co-ordinates were acquired in two dimensions from lateral view. Multivariate analysis of transformed data shows that the first principal component (representing 61.2% of total variance) does not separate the two genera, while sets the whole sample in an ontogenetic scaling perspective which involves mostly muzzle growth and airorhynchy The second principal component (16.8% of total variance) is related to an increasing midface flattening along the two ontogenetic series, with Gorilla trajectory displaced toward a larger level of midface projection. The cranial shape variation observed along the first principal component corresponds to the pattern observed when the shape vector is regressed on centroid size, where shape results clearly size-related (r = 0.92).