Risso's Dolphin
(Grampus griseus)

Risso's doplhin.
Three Risso's dolphins in Monterey Bay, CA. Copyright Tom Kieckhefer
Risso's dolphin.
Risso's dolphin with distinctive markings. Photo courtesy of Tom Jefferson.

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Group of 40-50 Risso's dolphins in the western North Atlantic.
Sound courtesy of William A. Watkins, Woods Hole Oceanographic Institution.
Description
The Risso's dolphin is the fifth largest delphinid. Adults reach approximately 4 m (13 ft) in length. Risso's dolphins are distributed world wide and are mostly seen in temperate and tropical waters. These dolphins are offshore animals that have a preference for shelf-edge habitats, in depths between 400-1000 m. Like pilot whales, Risso's dolphins feed exclusively on squid. They travel singly or in groups of hundreds of individuals.

Risso's dolphins have an extremely tall dorsal fin and blunt head with square melon and no rostrum. When calves are born, they are light in color. As calves age, they become almost black but then pale with age. However, their dorsal fin stays dark, and along with the extensive scarring on their body, they are easy to identify. The vocalizations of the Risso's dolphin have only briefly been described. They are known to echolocate with short duration (40 to 70 microsecond), broadband signals with peak frequencies between 50 and 110 kHz.
 
Additional Resources

  • Scripps Institution of Oceanography, "Voices in the Sea." (Link)