Cetacean Society International

Working for whales, dolphins and porpoises worldwide

CSI Photo Gallery

Risso's Dolphin

(Grampus griseus)

Risso's dolphins, like this older pair near Massachusetts, USA, are often called "grampus". Coloring is variable, but generally they are gray at birth, turn brown as immatures, and fade to light gray as adults. Their dorsal fins usually remain dark, clear of the many scars most individuals acquire as they age. Those distinctive scars may come from their squid prey, or perhaps tooth rakes made by other Risso's, but other dolphin species have been seen to rake Risso's dolphins as well. While Risso's dolphins may scar each other for social reasons, for example to establish dominance, why do other species also do it? Sometimes groups forage spread out in a line, searching for prey near the surface. When one dolphin in the line finds food will all the dolphins have a share, or do the dominant ones eat first?
These adult Risso's dolphins, swimming near the Outer Hebrides, Scotland, here show the species' characteristic blunt head, scarring, and the white forehead of older dolphins. Groups of 12 to 40, with occasional aggregations from 100 to more than 1000, are found in all marine waters, except the Black Sea, with surface temperatures of 50-82°F (10-28°C). Their favorite prey seem to be squid, and favorable hunting may keep them seasonally or locally over steep and fairly deep shelf waters. Growing to up to 12 feet 6 inches (3.66 m) long, the Risso's dolphin has a lifespan of more than 30 years. (Photo courtesy Colin D. MacLeod, UK.)