Variation in skull size and shape of two snake species (Natrix natrix and Natrix tessellata)

We examined morphological differences in cranium size and shape between closely related snake species, Natrix natrix and Natrix tessellata (Natricinae, Colubroidea), as well as variation within species. These two snake species have similar ecology and habitat preferences but differ in feeding strategies. Our hypothesis was that divergence in size and shape of cranial elements between species depends on their functional role and anatomical relationships. To analyse complex, kinetic crania, we applied computed microtomography and 3D geometric morphometrics. We analysed size and shape of six cranial elements separately. We selected two “non-trophic” structures (akinetic braincase and mobile nasals) and four movable “trophic” skeletal elements (maxillae, quadrates, pterygoids and compound bones) which are involved in prey capture and swallowing. Our results showed that N. natrix and N. tessellata significantly differ in size and shape of all analysed cranial elements. In both species, cranium is significantly larger in females than in males. To account for possible differences in shape due to differences in size, we estimated allometric and non-allometric component of shape variation. For all elements, except nasals, allometry accounted for a significant proportion of the variance in shape. The analysis of non-allometric component of shape variation revealed significant dimorphism in shape of the braincase and maxilla between N. tessellata females and males, and marginally significant sexual dimorphism in shape of maxilla in N. natrix. These results indicated that sexual dimorphism in skull shape is species specific and not entirely caused by selection for larger size in females.