Morphological integration of the kinetic skull in Natrix snakes

Abstract

© 2017 The Zoological Society of London Morphological integration, the covariation among phenotypic traits generated by common development and function, has been in the scope of evolutionary research for decades. As a morphological structure with complex development and various functions, the cranial skeleton represents a particularly interesting model for studies on morphological integration. However, most of the empirical investigations were done on akinetic and compact cranial skeletons of mammals. Here, we explore the pattern of integration in the extremely kinetic cranial skeleton of two closely related snake species, Natrix natrix and N. tessellata (Natricinae, Colubridae). In snakes, elements of jaws and palates on the left and right side are not spatially connected or firmly fused, allowing independent motion. Spatial independence of skeletal elements on the left and right side and their functional interconnections with extreme kinetic abilities, provide unique feeding performance in this group of tetrapods. By comparing patterns of symmetric and asymmetric components of variation we analysed cavariation patterns between kinetic and akinetic cranial elements. We tested whether the functionally and spatially connected bones are more integrated than disconnected ones and we examine impact of development and function on the morphological integration. We also explored whether and how allometry affects morphological integration in the snake's skull. Using micro-CT scanning 3D geometric morphometrics we showed strong covariation between the braincase and elements of the feeding apparatus, and that spatially disconnected elements are not more integrated than the connected ones. We also showed that function is the main factor that generates the pattern of morphological integration, because the signal of developmental integration is very weak and probably masked by strong functional integration of skeletal elements. Allometry has a significant impact on the morphological integration, by increasing integration of the skull, particularly integration of the lower jaw bones (compound and dentary), prefrontal, palatine and quadrate with the other skeletal elements.