While Risso's dolphins travel in herds averaging 15 to 46 animals in tropical and warm temperate seas, group size is about 6 or 7 in the northeastern Atlantic and in the Mediterranean Sea. Although the dolphins are fairly common, they have not been studied much. In the 1980s, however, two photo-identification surveys were begun off California and around teh Azores Islands. Besides indicating local abundance, this research promises to reveal more about the social lives of Risso's dolphins.
Risso's dolphins are sometimes taken into captivity and kept with other closely related dolphins, such as the bottlenose. A number have mated across the species barrier. At one Japanese aquarium, 13 births occurred to mixed bottlenose and Risso's dolphin parents. Most offspring died at birth or shortly afterward, but one surved for more than six years, showing characteristics of both parents.
Size: 3.3 to 3.8 m, 350 to 400 kg. Males slightly larger than females
Calves at birth: 135 to 166 cm
Teeth: 2 to 7 peglike teeh on each side of lower jaw only
Food: Squid, octopuses and fish
Habitat: Mainly deep offshore but also coastal waters
Range: Tropical to temperate world ocean
Status: Population unknown
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