Ecological Factors Driving Cranial Morphology in Delphinidae

<p>Delphinidae have a highly modified skull, resulting from their secondary adaptation to a fully aquatic lifestyle. This is characterized by skull telescoping and the posterior migration of the nasal bones, resulting in a vertical arrangement of the nasal passages, facilitating breathing at the water surface. Across the family, skull shape is notoriously diverse, thought to be related to their ecological diversity, which makes delphinids an interesting family to study, considering the potential ecological factors driving morphological change.In this study, I used geometric morphometrics to quantify the shape changes in cranial morphology across the family Delphinidae, and test for correlations with potential ecological drivers. I tested the hypotheses that skull shape change correlates well with ecological descriptors, and that multiple ecological variables are needed to fully characterise the diverse skull morphologies present in the family. A total of 138 2D images and 34 3D models were collated from multiple sources. For each 2D image a set of 38 landmarks were digitised using ImageJ and 52 landmarks were placed on each 3D model using MorphoDig. Procrustes superimposition and Principal Component Analyses were performed in MorphoJ, thus creating a Morphospace plot of skull shape change in delphinids. Each specimen position was compared with 12 ecological variables that could be evolutionary drivers of skull shape change. These included characteristics associated with feeding ecology, acoustic ability, habitat characteristics and taxonomy. One-way ANOSIM and one-way PERMANOVA were performed to test for significance of any separations seen on the morphospace between the species assigned to each variable category. Finally, multicollinearity and multivariate correlations were carried out, to test for possible influence of multiple variables in skull shape change.My results showed that the variables best correlating with variations in skull shape are related to taxonomy, feeding ecology and melon size. Furthermore, multivariate testing showed that other feeding related variables (diet, dentition and dive depth) and whether a species occupies a coastal or a pelagic habitat, are also factors that correlate with cranial morphology. These results support previous suggestions that skull shape changes are driven by feeding differences and phylogenetic relationships, but also emphasize the importance of melon shape, which has not typically been considered as an important driver of cranial morphology. The results also support the hypothesis that more than one variable have contributed to the extensive diversity in skull shapes, implying co-dependency of several ecological factors in determining morphological change in this group.</p>