Shape Variation in the Dermatocranium of the Greater Short-Horned Lizard Phrynosoma hernandesi (Reptilia: Squamata: Phrynosomatidae)

Abstract

Dermatocranial shape and horn morphology display great disparity among the species of Phrynosoma. Ontogenetic change in dermatocranial shape in a series of 79 specimens of the short-horned Phrynosoma hernandesi (54F: 25M) was examined using geometric morphometric techniques. A multivariate ANCOVA of Procrustes residuals with sex as a factor and ln(centroid size) as the covariate indicated sexual shape dimorphism. Separate multivariate regressions of Procrustes residuals on ln(centroid size) for each sex indicated that allometry accounts for ~52–54% of the total sample shape variance. Comparisons of ontogenetic shape change between sexes indicate that sexual shape dimorphism is minimal and of uncertain biological significance. Groupings of multivariate regression coefficients by magnitude and sign suggest that allometric integration of the dermatocranium is not uniform over the dermatocranium. Principal component analysis of the landmark configurations corrected for sex and allometry yields a first principal component which describes shape variance concentrated in the posterolateral and posterior regions of the dermatocranium, and again is indicative of non-uniform shape variation over the dermatocranium. Our findings for P. hernandesi indicate that the adult shape of the dermatocranium may contribute to a passive defence against predation. We hypothesize that the complexity in dermatocranial shape demonstrated here for P. hernandesi indicates parcellation of shape variance, which may contribute to explanations of the pronounced dermatocranial disparity exhibited by the species of Phrynosoma.