Interspecific size- and sex-related variation in the cranium of European brown frogs (Genus Rana)

The vertebrate skull is considered as phylogenetically conserved, but shows extensive diversification in many clades due to various environmental, climate and habitat influences. We explored the cranial size and shape differences of three closely related brown frog species, Rana temporaria, R. dalmatina and R. graeca to emphases the interspecific variation and intraspecific sexual differences, the allometric patterns in cranial shape changes and the relationship between cranial shape, phylogeny, and ecological similarity. Brown frogs significantly differ in cranial size and shape. Interspecific cranial shape changes are under strong influence of size variation. When size is considered, our results show that R. dalmatina is the most divergent species, while differences between R. dalmatina and R. temporaria cranial shapes are diminished when size is removed and peculiar features of R. graeca cranium arise (the most robust and rounded cranium). In both cases, with size and without size, major shape changes are related to the position of quadratum and width of the cranium at the level of anterior margin of eye. When comparing non-allometric shape with ecological similarity and phylogenetic relatedness we found that ecologically similar but phylogenetically more distant species shared the cranial morphology. Intraspecific patterns of cranial phenotypic variation in brown frogs showed significant cranium sexual size and shape dimorphism and absence of allometric scaling between sexes. More detailed studies on the relation of morphology and ecology in brown frogs are necessary to explain the mechanism behind cranial size and shape variation