Risso's dolphin (Grampus griseus)
Baird, R.W. 2009. Risso's dolphin (Grampus griseus). Pages 975-976 in Encyclopedia of Marine Mammals. Edited by W.F. Perrin, B. Wursig and J.G.M. Thewissen. Academic Press, San Diego, CA.
Risso’s dolphin ( Grampus griseus ) is the fifth largest member of the family Delphinidae, with adults of both sexes reaching up to about 4m in length (Fig. 1). The common name comes from the person (M. Risso) who described the type specimen to G. Cuvier in 1812. Risso’s dolphins are unusual looking for a variety of reasons. Their anterior body is extremely robust, tapering to a relatively narrow tail stock, and they have one of the tallest dorsal fins in proportion to body length of any cetacean (Fig. 2). The bulbous head has a distinct vertical crease or cleft along the anterior surface of the melon. Color patterns change dramatically with age. Infants are gray to brown dorsally and creamy-white ventrally, with a white anchor-shaped patch between the pectoral flippers and white around the mouth. Calves then darken to nearly black, while retaining the ventral white patch. As they mature they lighten (except for the dorsal fin, which remains dark in adults in most populations), and the majority of the dorsal and the lateral surfaces of the body become covered with distinctive linear scars, most of which are presumably caused by intraspecific interactions. Older animals can appear almost completely white on the dorsal surface or when swimming just beneath the surface. No evidence of sexual dimorphism has been reported. From a distance Risso’s dolphins are most frequently confused with killer whales (Orcinus orca) due to the large size of their dorsal fin.