Generously Provided by the American Cetacean Society RISSO'S DOLPHINGrampus griseus
PHYSICAL DESCRIPTION: This is a
relatively robust dolphin with a rounded head, similar in shape to the more familiar pilot whale. They have an unusual external
vertical crease in the melon (the organ in the rounded head used to direct sonar clicks) that runs from
the front of the forehead to the mouth.
COLOR: Adult Risso's dolphins
are usually dark gray with extensive white scarring. The scars can be scratches, splotches, or circular marks
and, in some animals, can be so extensive that the entire body appears to be white. This scarring may come from
bites from other Risso's dolphins, squid bites, or parasites. As calves, their bodies are a uniform light gray,
which then darkens to a brownish-gray in juveniles. Risso's dolphins also have a white anchor-shaped patch,
similar to pilot whales, on their ventral/chest area
(from throat to stomach).
FINS AND FLUKE: The dorsal fin
is tall, falcate (curved) and located near the middle of the back. The flukes are broad and curved with a deep
median notch. The flippers are relatively long and thin, and pointed almost straight back. The tailstock, or
peduncle, is very slender.
Length and Weight: Length is
typically 10 feet (3 m), although animals have been recorded up to 12.5 feet (3.8 m). Like most dolphins, males
are typically slightly larger than females. Weight averages about 650 pounds (300 kg), and large individuals
may weigh up to 1100 pounds (500 kg).
Feeding: Reflecting the offshore
distribution of this species, primary prey appears to be squid, although they have also been known to feed on a
number of fish species. While the size of their squid prey is unknown, squid beaks from species that grow up to
12 feet in length have been found in the stomachs of stranded Risso's dolphins.
Mating and Breeding: While the
age at which these animals reach sexual maturity is not known, it appears to occur when the dolphins are
approximately 8.5 to 9.2 feet in length (2.6-2.8 m) in both sexes, and probably less than 13 years of age.
Little is known about calving and breeding, but evidence from the North Atlantic Ocean indicates a summer
calving season. Estimated gestation is 13-14 months. Calves are generally 4 to 5 feet long (1.2 - 1.5 m) at
birth.
Distribution and Migration:
Risso's dolphins tend to prefer offshore habitats worldwide in the warm temperate and tropical waters of both
hemispheres. They are not known to enter true polar waters, and northern-most records are Newfoundland and the
Shetlands (Atlantic), and the Gulf of Alaska (Pacific), while southern ranges are Cape Horn, the Cape of Good
Hope, Australia and New Zealand. General migrations are not known, although some individuals may shift to
cooler waters during the more productive summer season. Regional variation in movement may be related to local
environmental changes, especially warm water occurrence, and shift in prey abundance.
Natural History: Typical groups
of Risso's dolphins are 3-30 animals, although "super-pods" of up to several thousand together have been noted.
However, recent studies indicate that groups may be stable, and may consist of related individuals (especially
females), although this has yet to be confirmed. They typically travel side by side, and are fairly active.
Observations of leaping, spy hopping, fluke slapping, and flipper slapping are common. They are long-lived
animals that live an estimated twenty years minimum and possibly up to twice that long. Risso's dolphins also
form traveling pods at sea with other dolphin species, and hybrids between Risso's dolphins and bottlenose
dolphins have been recorded both in captivity and in the wild.
Status: While formerly thought
to be rare, this impression probably resulted from the typically offshore distribution. In fact, the species is
probably abundant throughout its range, although no exact numbers are known. 13,000 - 30,000 animals are
estimated to occur off of central and northern California. In a few parts of the world, Risso's dolphins are
killed for human consumption; until recently, they were commonly found in the open market in Taiwan, and they
are part of Japanese drive fisheries for dolphins. In Sri Lanka, they are one of the primary species taken in a
gill net and harpoon fishery that may have grown out of selling accidentally netted dolphins. In most areas of
the world, however, they are not currently under major threat from direct killing, but may be susceptible to
accidental fishing gear entanglement and pollution from coastal development.
Bibliography:
Acknowledgements:
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