Asymmetry drives modularity of the skull in the common dolphin (Delphinus delphis)

Odontocetes (toothed whales) have amongst the most radically altered skull of any mammal, but no study has tested how these modifications have altered its phenotypic integration. Here, we perform the first rigorous assessment of modularity in Delphinus delphis, using a combination of cluster analysis, covariance ratio tests and maximum-likelihood methods, on 27 landmarked skulls. Cluster analysis identified ten semi-autonomous regions (modules), five consisting of bilaterally paired landmarks, while the remaining landmarks were distributed among modules representing the face, zygomatic, nasals, pterygoids and vault. The novel ten-module hypothesis, as well as a simplified version consisting of only the five major modules, was then compared with existing hypotheses formulated for terrestrial mammals based on developmental origin and/or function. We found our novel hypotheses to perform best, except when cranial asymmetry was removed. With the loss of asymmetry, a six-module hypothesis developed for terrestrial mammals was found to best fit the data. Anatomical changes in the skull related to the evolution of echolocation, as well as loss of mastication, best explain the novel modules identified in this study. Our study also provides strong evidence that the evolution of cranial asymmetry is a substantial driver of changes in modularity within whales.