Habitat and Distribution
Risso's dolphins seem to prefer offshore temperate waters. However, they have been found in regions where temperatures range from 4.5 to 28 degrees celsius. They are generally found along continental slopes, but have also been seen near-shore where the continental shelf is narrow. Risso's dolphins travel as far north as Newfoundland and the Shetlands in the North Atlantic, and the Gulf of Alaska in the North Pacific. They have been seen as far south as Cape Horn, the Cape of Good Hope, Australia and New Zealand.
The migration of these dolphins is not known, although this species has been seen moving into cooler waters during the more productive summer months, possibly in search of food. Risso's dolphins may also migrate regionally when environmental conditions change, in search of warmer waters or more prey.
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Physical Appearance
Risso's dolphins have a circular shaped body with no beak and a blunt head. They have up to seven pairs of teeth in the front of their lower jaw and none in the upper jaw.
Color: Risso's dolphins vary in color from blueish gray, to grayish brown, or pale white, depending on the age of the dolphin. Risso's dolphins usually get lighter in color as they age. They have many white scratches and scars on their bodies. These scars may be caused by bites from other Risso's, squid bites, or parasites. The scars look like white lines or circular marks covering the body. Risso's dolphins also have a white anchor shaped patch on their underside, ranging from their throat to their stomach.
Length: Male Risso's dolphins are generally larger than the females. The average length is 10 feet (3 m), but they can grow to be 12.5 feet (3.8 m). Risso's average about 650 pounds in weight, and larger individuals may weigh up to 1100 pounds.
Fins: A tall curved dorsal fin is located in the middle of the dolphin's back. Both the flippers and the dorsal fin come to a distinct pointed tip. The flippers are long, thin, and pointed straight back.
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Behavior
Risso's dolphins travel in groups of several hundred to thousands, but smaller groups of 3-30 members are more common. These dolphins typically surface and swim slowly, but they can be very active at times. Common behavior includes leaping and jumping out of the water along with flipper and tail slapping. Risso's dolphins sometimes aggregate with bottlenose, or other dolphin species. They produce a wide variety of sounds through echolocation, including clicks, squeals, and cries, all used for communication.
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Breeding and Reproduction
Little is known about the reproductive habits or life history of Risso's dolphins. Males reach sexual maturity when they reach 300 cm and females reach sexual maturity at a length slightly less than 300 cm. Both males and females reach this length when they are about 3-4 years of age. It is thought calves are born in the winter months, but births may occur year round. A female is pregnant from 13-14 months and babies range from 110 cm to 150 cm at birth.
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Diet
Risso's dolphins generally eat fish, octopus and squid. When hunting and foraging, Risso's dolphins swim aside each other in a co-operative behavior to help with prey capture. These dolphins usually hunt in deep water, however in some areas they have been seen feeding closer to shore.
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Fun Facts
Risso's dolphins can live up to 30 years in the wild.
A famous Risso's dolphin named "Pelorus Jack" was known for escorting ships into the Pelorus Sound. The crew of these ships witnessed this behavior for 24 years beginning in 1912.
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