Skull morphometric variability related to offshore and inshore ecotypes of the common bottlenose dolphin (Tursiops truncatus) from northwestern Mexico

The distribution of common bottlenose dolphins Tursiops truncatus is associated with environmental factors, which influence morphological adaptations. In the present study, we examined the cranial characteristics of this species from the Pacific Northwest of Mexico to detect the main variations using geometric morphometric analysis. From stable isotope concentrations (δ¹³C), we identified 24 specimens of the inshore coastal ecotype and 18 of the offshore ecotype. Maturity was determined using the degree of fusion of the maxillae and premaxillae. Allometry was detected in the skull lateral region, indicating that the relationship between shape and size is maintained in adults. After eliminating the allometric effect by discarding the smallest individuals, morphometric comparisons indicated differences between the ecotypes that were associated mainly with the intersection between the frontal bone and zygomatic process, the anteriormost point in the curvature of the zygomatic process, and the meeting of the suture between squamosal and exoccipital bones, with the supramastoid crest to anteriormost in the curvature of the temporal crest of the intersection between the parietal and interparietal. No significant differences were found in the ventral view. The cranial differences between both Tursiops truncatus ecotypes could be mainly associated with the different types of feeding.