dolphins whales community
RISSO'S DOLPHIN
Grampus griseus
Classification:
Risso's dolphins are in some ways an unusual species that has not been well studied, mainly due to their preference for deep, oceanic waters but research is now underway on several populations throughout their range.
Their physical appearance is unique and the numerous scars, from their major prey item, squid, and made by other Risso's dolphins, give them peculiar markings.
The amount of white scarring generally increases with age and older individuals can have a notably white head because of this.
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Appearance:
The body of an adult Risso's dolphin generally has a base colour of grey that is almost hidden by the pattern of white scars.
The Risso's dolphin is born a uniform grey with a white belly; the back changes to olive or chocolate brown as a juvenile, then becomes grey as an adult.
It has a bulbous forehead with no beak, and the mouth line slopes up to look like a smile. It has a tall falcate dorsal fin and long sickle-shaped flippers.
It has between 4-14 teeth in the lower jaw but teeth are rarely found in the upper jaw.
While the blunt head may cause it to be confused with pilot whales, and the tall dorsal fin with bottlenose dolphins or female and juvenile killer whales, other distinctive characteristics, especially the scarring, make this species relatively easy to identify in the wild.
Other names:
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Grampus
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Grey grampus
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White-headed grampus
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Gee gee's
1.
Maximum length:
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Male: 4m
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Female: 3.7m
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Calf: 1.1m
2.
Maximum weight:
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Male: 500kg
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Female: 500kg
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Calf: 20kg
3.
Diet:
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Squid
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Crustaceans
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Cuttlefish
4.
Estimated population:
Unknown
5.
Distribution map:
Behaviour:
Risso's dolphins can be highly active at the surface exhibiting a range of behaviours including breaching and spyhopping.
They do not generally bow ride, but they can be seen travelling in the wake of ships and surfing in waves.
They are generally seen in groups of between 10 and 50 animals, but larger schools, up to 4,000 individuals, have also been reported.
Risso's dolphins generally prefer deeper offshore waters where they feed almost exclusively on squid, and have been seen forming lines when hunting.
They can be found associating with other species of cetacean such as bottlenose dolphins and pilot whales.
Distribution:
The Risso's dolphin is a widely distributed species and can be found in the temperate and tropical zones of all the world's oceans.
Major threats to this species include directed hunts (for example in Japan, the Faroes, Sri Lanka, the Philippines and Taiwan), accidental entanglement in fishing gear, climate change, and noise pollution.