Cranial variation in common dolphins Delphinus spp. off South Africa, with the inclusion of information from the holotype of Delphinus capensis

Abstract

The common dolphins (genus Delphinus) are widely distributed in all temperate and tropical oceans. Over this wide geographical distribution they show considerable range in morphological variation, which has led to descriptions of several species in the genus. Until recently, only two species of Delphinus were accepted, but this classification has become contentious. This study investigated the occurrence of morphologically different types of Delphinus in South African waters and assessed geographic and morphometric variations within each type. A total of 296 skulls of Delphinus spp., obtained from the Port Elizabeth Museum and the Iziko South African Museum, were photographed in their dorsal and ventral aspects for geometric morphometric analysis. Our results revealed three clusters of specimens based on analysis of the dorsal aspect of the skull, and two clusters for the ventral aspect. Significant differences in cranial size were found between clusters. Both cranial aspects showed that the main variations in skull shape occurred in the rostral region and braincase area, indicating divergent adaptations relating to these features. There was a substantial difference between the composition of the dorsal-aspect and ventralaspect clusters, suggesting the presence of only one species in South African waters. Significant differences between morphological clusters associated with the three regions within the study area (cold-temperate, warm-temperate and subtropical) are probably mediated through differences in local environmental conditions (e.g. different water temperatures and productivity).

Keywords: cranial morphology, Delphinus delphis, geographical variation, geometric morphometrics, rostral index, sexual dimorphism, taxonomy