Espolarte - Marine mammals - Risso's dolphin

Risso's dolphin

Grampus griseus

Length:3 m, max. 4 m; 1,5 m at birth
Weight:300 kg, max. 600 kg
Social structure:lives in groups of 330 animals
Distribution:tropical and warm temperate zones of all oceans

Taxonomy

Order: Cetacea
Suborder: Odontoceti
Family: Delphinidae
Genus: Grampus
Species: Grampus griseus

Risso's dolphin is an oceanic dolphin, a member of the Delphinidae family. Some authors, such as Lyall Watson, used to classify it as a member of the Globicephalidae family; this family should also include pilot whales, killer whales and some other species.

This dolphin species was named after a French naturalist called Risso, who had found a beached specimen in 1811 near Nice, Provence. He described and sketched the animal carefully, and sent the description and the drawings to Cuvier, most renowned French biologist of the period. Another dolphin has stranded near Brest; the skull and the hide of this was also sent to Cuvier. The scientist has described the dolphin found near Nice as Delphinus aries (ram dolphin) and the other one as Delphinus griseus (grey dolphin). In 1828, Gray classified this dolphin, together with the killer whale and several other species, into the genus Grampus created by him.

The name Grampus probably derives from the French words grand poisson (great fish); fishermen and whalers used to name killer whales, and sometimes other medium-sized whales so. The species name griseus refers to the greyish colour of the animal.

Names

Description

This is a fairly large dolphin, with an average length of 3 m and an average weight of 300 kg. Largest animals may reach a length of 4 m and may weigh 600 kg. Newborn Risso's dolphins are about 1,5 m long.

Risso's dolphins are robust in front of their dorsal fin, and have a narrow tailstock. They have no beak. There is a peculiar vertical crease on the forehead. The flippers are long and pointed, the dorsal fin is also long (50 cm) and leans backwards.

The colouration changes with age. Newborn calves are uniform dark grey, but later change to chocolate brown. Older animals gradually fade, their throat and belly becomes pale grey or almost white, while fins, flippers and flukes remain dark. In oldest animals, the tip of the flippers can also become white. Each dolphin changes its colour in its own pace: some older animals retain their dark colour, while others become almost entirely white. The skin of Risso's dolphins is full of scars, caused by the teeth of their companions.

Risso's dolphins have an unique arrangement of teeth: they have no teeth in their upper jaw, but do have 27 (usually 3 or 4) large, oval-shaped teeth on each side of the lower jaw. There are 68 or 69 vertebrae.


Photo: Isabel Guzmán

Range

This dolphin inhabits warm temperate and tropical zones of all oceans. It prefers the continental slope zones, where the sea is 200 to 2000 m deep.

Identification

The most conspicuous feature of the Risso's dolphin is its scarred body, by good light conditions easily noticeable from a distance. In some cases, this species may be confused with bottlenose dolphins (Tursiops truncatus) or false killer whales (Pseudorca crassidens), which differ from the Risso's dolphin in the following characteristics:

A dead animal found on the beach may easily be recognized from its scars, long dorsal fin and creased forehead. If only a skeleton is found, this species can still be identified from its typical teeth.

Natural history, behaviour

Risso's dolphin feeds primarily on squid. Usually lives in groups of 5 to 30 animals, but much larger schools have also been observed. Calves are born during winter.

In some cases, they rise gently to the surface to breathe, showing only their fins, but sometimes they also porpoise. They spend 3 to 4 minutes near the surface, breathing every 15 to 20 seconds. This period is followed by a dive, usually lasting only one or two minutes, but dives lasting half an hour also occur. Risso's dolphins sometimes spyhop, rising their head vertically above the surface, exposing also the flippers. When spyhopping, both eyes look in the same direction, making the dolphin capable of some degree of stereoscopic vision.

They are sometimes friendly, sometimes wary with humans. They may also bowride in front of large ships, or breach in the wake of the ship. They often can be seen together with other species of dolphin, such as spinner dolphins (Stenella longirostris), northern rightwhale dolphins (Lissodelphis borealis) and longfin pilot whales (Globicephala melaena). In captivity, Risso's dolphins and bottlenose dolphins have cross-breeded.

The most famous representative of the species was a certain Pelorus Jack, an unusually bright-coloured male from New Zealand. He was first observed in 1888, near the harbour of French Pass; he was named after a near sound. Pelorus Jack has never been seen together with other dolphins, but he regularly appeared near ships, swimming in front of the watercraft and seemingly acting as a pilot, guiding the ship along the safest route. Though these waters are full of dangerous rocks, no accident has ever happened to ships guided by Pelorus Jack. In 1904, a drunken passenger of the steamer Penguin shot at the dolphin and wounded him; this cruel human was all but lynched by his fellow passengers. Pelorus Jack recovered from his injury, but he never helped the Penguin again, and that ship eventually wrecked. The famous dolphin was last seen in 1912. He probably died of old age, but some locals supposed that he had been harpooned and killed by whalers.

Conservation status

Risso's dolphin is a fairly abundant species. According to recent research, there are about 60.000 animals along United States coasts, 85.000 in the West Pacific and 175.000 in the East Pacific. There is no reliable estimate of the global population. The species is classified as data deficient.

This dolphin is hunted for meat in several regions. Substantial numbers are killed by Japanese whalers and fishermen of Srí Lanka; a few is taken off Indonesia and the Caribic island Saint Vincent.