Geographical variation in head shape of a Neotropical group of toads: the role of physical environment and relatedness

Abstract

In this study we review the morphological variation within the Rhinella crucifer species group using geometric morphometrics. We sampled 270 specimens from 78 localities comprising all genetic units delimited for the group. We placed 12 landmarks and 89 semi-landmarks defining morphological structures of the anterior region of the body (head and parotoid glands) on standardized photographs of dorsal aspects of specimens. We checked for the existence of size-free morphological variation using exploratory multivariate analyses and tested for differences among categories (genetic units) using canonical variate analyses. We investigated the effects of relatedness by conducting canonical analyses hierarchically, and tested for phylogenetic signal using reconstruction of morphologies on a tree derived from mitochondrial data. We then corrected for relatedness using phylogenetic principal component analysis, and tested for the influence of the physical environment (temperature, humidity and altitude) with a partial Mantel test of matrix correlation. Our results revealed that there is size-free shape variation in the group. Shape changes are related to specific structures in the head, with landmarks and semi-landmarks highlighting changes in a complementary way. We were able to statistically detect the effect of phylogenetic distance with landmarks when considering the closest genetic units as a single category. A significant proportion of the variation in head shape can be explained by environmental variables, suggesting that conditions of the physical environment should also be considered as a source of morphological variation.