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Risso's dolphins are
robust blunt-headed animals, lacking distinct beaks. The flippers are
long, pointed, and recurved the dorsal fin is tall and falcate.
Risso's dolphins have mouthlines that slope upward. Newborns are 1.2-1.5 m
long and adults range up to 3.8 m long. This a large delphinid, with
weights of up to 400 kg recorded, although the maximum may be near 500 kg.
One of the most
distinctive features of the Risso's dolphin is a vertical crease on the
front of the melon. At sea, the best identification characteristic is
coloration and scarring. Adults range from dark gray to nearly white, but
are typically covered with white scratches, spots, and blotches. The chest
has a whitish anchor-shaped patch and the appendages tend to be darker
than the rest of the body. Young animals range from light gray to dark
brownish-gray and are relatively unmarked. The teeth are also unique
there are 2-7 pairs in the front of the lower jaw, usually none in the
upper jaw. Some or all of the teeth may be worn-down in adults, or missing
altogether.
Range
Pantropical/Temperate
This is a widely-distributed species, inhabiting deep waters of the outer
continental shelf and slope from the tropics through the temperate regions
in both hemispheres. They may be associated with strong bathymetric
features or oceanic fronts. The species range is roughly 60°N to 60°S, in
waters where sea surface temperature is greater than 10°C.
Atlantic Ocean
In the western Atlantic, the Risso's dolphin ranges from Newfoundland
along the east coast of Canada and the U.S. in the Gulf of Mexico
and on rare occasions south along the coasts of Central and South America
to Cabo de Hornes in Chile. In the western Atlantic, Risso's dolphins can
be found from the British Isles and North Sea south along the coast
of Europe in the Mediterranean near the Canary Islands
and in the waters along Cape Province in South Africa.
Pacific Ocean
In the western Pacific, Risso's dolphins are found in the Sea of Okhotsk,
near the Kuril Islands, Kamchatka Peninsula, and Japan south to
Geographe Bay and Sydney in Australia and the North Island of New
Zealand. In the eastern Pacific, they are found from the Gulf of Alaska
in the waters off British Columbia and the western United States off
Baja California and in the Gulf of California in the eastern
tropical Pacific in Hawaiian waters in rare instances and
south along the coast of South America all the way to Chile.
Indian Ocean
The Risso's dolphin's range includes the Indian Ocean from Africa to
Indonesia, with sightings concentrated near the Natal coast of Africa, the
Bay of Bengal including the Gulf of Aden and the Gulf of Oman and
the waters near Sri Lanka.
Habit
Tropical/Temperate - Risso's dolphins are pelagic, often associated with
sea mounts, distinct bottom topography, or upwelling currents. These large
dolphins are often seen surfacing slowly, although they can be energetic,
sometimes breaching or porpoising, and occasionally bowriding. Herds tend
to be small to moderate in size (< 100), but groups of up to 4,000 have
been reported. Risso's dolphins commonly associate with other species of
cetaceans. Hybrids between this species and bottlenose dolphins have been
recorded, both in captivity and possibly in the wild. In at least the
North Atlantic, there appears to be a summer calving peak.
Diet dominated by
Squid &other invertebrates - Risso's dolphins feed on crustaceans and
cephalopods, but seem to prefer squid. Squid bites may be the cause of
some of the scars found on the bodies of these animals. They appear to
feed mainly at night. |
