(Cuvier, 1812) - Risso's dolphin
Risso's dolphins are robust blunt-headed animals without distinct beaks. The flippers are long, pointed, and recurved; the dorsal fin is tall and falcate. Risso's dolphins have mouthlines that slope upward. One of the most distinctive features is a vertical crease on the front of the melon. However, at sea, the best identification character is the coloration and scarring. Adults range from dark grey to nearly white, but are typically covered with white scratches, spots, and blotches. The chest has a whitish anchor-shaped patch, and the appendages tend to be darker than the rest of the body. Young animals range from light grey to dark brownish grey and are relatively unmarked.
The teeth are also unique; there are 2 to 7 pairs in the front of the lower jaw and usually none in the upper jaw. Some or all of the teeth may be worn-down in, or missing from, adults.
Can be confused with
Risso's dolphins are generally easy to identify when seen at close range; however, from a distance they may be confused with other large delphinids with a tall dorsal fin (such as bottlenose dolphins, false killer whales, and killer whales). When visible, the light, extensively scarred bodies and squarish heads of Risso's dolphins make them unmistakable.
Newborns are 1.2 to 1.5 m long and adults range up to at least 3.8 m long. Weights of up to 400 kg have been recorded, and the maximum may be near 500 kg.
This is a widely distributed species, inhabiting deep oceanic and continental slope waters from the tropics through the temperate regions in both hemispheres. They are found from Newfoundland, Norway, the Kamchatka Peninsula, and Gulf of Alaska in the north to the tips of South America and South Africa, southern Australia, and southern New Zealand in the south.
Biology and Behaviour
These large dolphins are often seen surfacing slowly, although they can be energetic, sometimes breaching or porpoising, and occasionally bowriding. Herds tend to be small to moderate in size, but groups of up to 4000 have been reported. Risso's dolphins commonly associate with other species of cetaceans. Hybrids between this species and the bottlenose dolphin have been recorded, both in captivity and in the wild.
In the North Atlantic, there appears to be a summer calving peak.
Risso's dolphins feed on crustaceans and cephalopods, but seem to prefer squid. Squid bites may be the cause of some of the scars found on the bodies of these animals.
Risso's dolphins have been taken in small numbers, (both incidentally and intentionally) in drive, gillnet, seine, and harpoon fisheries throughout the species' range. In Sri Lanka, they are apparently the second most commonly taken cetacean in fisheries, providing fish and meat for human consumption and fish bait; stocks there may be adversely affected.