The taxonomic status of the nominal dolphin species Delphinus tropicalis Van Bree, 1971

The taxonomic status of common dolphins in the Atlantic and Pacific oceans has been clarified in recent years, with the discovery that there appear to be two species, a short-beaked (Delphinus delphis) and a long-beaked (D. capensis) species. However, the taxonomy of common dolphins in the Indian Ocean and southeast Asia is still unclear. A nominal third species, Delphinus tropicalis van Bree, 1971, has been described from this area, but its validity is controversial. We reviewed records and literature on common dolphins from South Africa east to Australia and Japan, and measured 206 skulls of common dolphins from the Indo-Pacific and southern California. Other than southern Australia, we found no evidence for Delphinus delphis in the Indo-Pacific (South African specimens appear to be D. capensis). Previous reports of short-beaked common dolphins in the Indo-Pacific appear to have been cases of misidentification. The tropicalis-form has an exceptionally long and narrow rostrum with high tooth counts, but otherwise appears to resemble D. capensis, in both skeletal and external morphology. From an examination of 86 Delphinus skulls from the reported range of tropicalis (Middle East to China), we found that both tooth counts and rostral length/zygomatic width ratios were higher than for 94 D. capensis specimens from southern Japan, South Africa, and California. These measurements were greatest in the central Indian Ocean (around India). However, there was evidence of clinal variation, with both decreasing as one moves east or west from India, towards South Africa in the west or Japan in the east. We suggest that the tropicalis-form is actually a long-beaked subspecies of D. capensis, which may hybridize or intergrade with the standard capensis-form in southeast Asia and possibly along the east coast of Africa. The appropriate name is Delphinus capensis tropicalis (van Bree, 1971), and a formal description of the subspecies is provided.