The Risso’s Dolphin (Grampus griseus), sometimes called the gray dolphin, is the only species of dolphin in the genus Grampus. Risso’s dolphins are cetaceans found worldwide in temperate and tropical waters, usually in deep waters rather than close to land. As well as the tropical parts of the Indian, Pacific and Atlantic Oceans, Rissos are also found in the Mediterranean and Red Seas, though are absent from the Black Sea. Their preferred environment is just off the continental shelf on steep banks, with water depths varying from 400 – 1000 metres and water temperature at least 10° Centigrade and preferably in excess of 15° Centigrade.
Another common name for Risso’s dolphin is the ‘Grampus’, although historically, it was the common name used to describe the orca (killer whale). The specific name griseus refers to the mottled (almost scarred) grey color of the dolphins body. These dolphins belong to the family Delphinidae, the dolphin family, which includes members such as common dolphins, pacific white-sided dolphins, and bottlenose dolphins. They also belong to the order Artiodactyla.
Risso’s dolphins are closely related to pilot whales, pygmy killer whales, melon-headed whales, and false killer whales. The population of Risso’s dolphins around the continental shelf of the United States has been recorded to be in excess of 60,000. In the Pacific a census recorded 175,000 individuals in eastern tropical waters and 85,000 in the west. No global estimate of population exists. Risso’s dolphins in the United States are not endangered or threatened. Like all marine mammals, they are protected under the Marine Mammal Protection Act.
Risso’s dolphin is named after Antoine Risso, whose description formed the basis of the first public description of the animal, by Georges Cuvier, in 1812.
Risso’s Dolphin Characteristics
Risso’s dolphins are medium sized, with a typical length of 10 feet (3 metres), although some have been recorded to measure up to 12.5 feet (3.8 metres). Like most dolphins, males are typically slightly larger than females. Their weight averages about 650 pounds (300 kilograms) and large individuals may weigh up to 1100 pounds (500 kilograms).
Risso’s dolphins are dark gray all over when they are born. As they mature, their coloring becomes more chocolate brown or pale gray with paler undersides. Mature adults swimming just under the water’s surface usually appear white. Their flippers and tail remain darker.
Risso’s dolphins have broad flukes that have pointed tips. They have a very tall dorsal fin which can measure around 50 centimetres in length. The tip of this dorsal fin may be pointed or curved. They have a single blowhole.
Instead of the usual dolphin beak, Risso’s have a blunt snout and a rounded, bulbous head that slopes steeply towards their mouths. Their mouths curve upwards, giving the appearance of a ‘smile’ which is a common feature on all dolphins.
Risso’s dolphins can quite easily be identified, particularly when they mature. This is because they become scarred and battered which is caused by other Risso’s dolphins teeth. Risso’s have teeth only in the front of their lower jaw which are used when playing or fighting.
Risso’s Dolphin Lifespan
Risso’s dolphins are believed to have a life span of at least 20 years, but have been known to live up to 40 years old.
Risso’s Dolphin Diet
Risso’s dolphins prey on neritic, oceanic, and occasionally bottom dwelling organisms. Their diet consists of fish, krill, crustaceans, and cephalopods, such as anchovies, squid, octopus, and cuttlefish.
They eat mostly at night, when their prey is closer to the surface. Their preferred depth to eat at is between 600 and 800 m below the surface of the sea. They have been known to move into continental shelf waters when following squid.
Risso’s Dolphin Behaviour
Risso’s dolphins are typically found in groups of between 10 and 30 animals, but they have been reported as solitary individuals, in pairs, or in loose aggregations of hundreds and thousands. They are also often seen in the company of other whales and dolphins, such as bottlenose dolphins, gray whales, northern right whale dolphins, and Pacific white-sided dolphins.
Their groups are formed based on age and sex class, with strongest associations occurring between adult females and adult males. When hunting for food, these groups spread out in a long line. They are not known for undertaking large migrations, but they do travel within their local distribution area in search of areas where prey is abundant.
Risso’s dolphins spend 77% of their time traveling, 13% engaged in social activity, 5% feeding, and about 3.7% resting. They are often seen breaching, clearing the water and slapping their heads, tails or sides on the surface. They also engage in chasing, spy hopping (lifting their head above the surface of the water to have a look around), biting, aerial acrobatics and lob-tailing. Juvenile dolphins are particularly energetic.
These animals have been known to be aggressive and flipper slapping between individuals, striking with flukes and dorsal fins, and body blows have been witnessed.
Some groups are shy, however, some allow humans to approach close to them. Risso’s dolphins do not often ‘bow ride’ in front of boats like some other dolphins, but may swim beside or in the bubbly wake that a boat leaves.
Risso’s dolphins can dive to at least 1,000 feet and hold their breath for 30 minutes, but they usually make shorter dives of just a few minutes. When at the surface, they have a small inconspicuous blow if backlit (which is more distinct after long dives), and their head partially emerges at a 45° angle. Before diving, they usually take 10 to 12 breaths at 15- to 20-second intervals and will often display their tails.
Risso’s Dolphin Communication
Risso’s Dolphins are highly social and there often seems to be much communication going on between animals in a group with much dramatic splashing at the surface.
They also use echolocation to locate, identify, and determine the distance of various objects in their environment. One of the most well-known sounds of delphinids are clicks. Risso’s dolphins are also able to emit sonar clicks in the water while the majority of their forehead is above water, a characteristic unique to this species. They also make a number of different vocalizations, including barks, buzzes, grunts, chirps, whistles, and simultaneous whistle and pulse sounds.
Risso’s Dolphin Reproduction
Little is known about mating between this dolphin species. It is thought they are polygynous and polyandrous. Breeding and calving may occur year-round, but the season may vary geographically (especially in the North Pacific), with most animal births occurring from summer to fall in Japanese waters and from fall to winter in California waters.
Individuals reach sexual maturity when they reach a length of about 8.5 to 9 feet, which is usually when females are between 8 and 10 years of age and when men are between 10 and 12 years of age. Risso’s dolphins have a gestation period of 13 to 14 months.
Females give birth to a single calf with calves usually being between 3.5 to 5.5 feet in length and weigh about 45 pounds. They can nurse for up to 3 years before being weaned.
Risso’s Dolphin Location and Habitat
Risso’s dolphins have an extensive distribution and can be found in temperate, subtropical and tropical waters of oceans worldwide. Their preferred habitats appear to be mid-temperate waters of the continental shelf and slope between 30° and 45° latitude. In the Northern Hemisphere, their range includes the Gulf of Alaska, Gulf of Mexico, Newfoundland, Azores, Norway, Japan, Russia, the Mediterranean Sea and the Red Sea. In the Southern Hemisphere, their range includes Argentina, Australia, Chile, South Africa, and New Zealand.
Risso’s dolphins are present year round throughout most of their geographic range. Not much is known about migratory patterns within this species, but it is thought that residents of the northern-most parts of their range migrate seasonally between summering and wintering grounds.
Their range depth is between 400 and 1,200 m.
Risso’s Dolphin Predators
Risso’s dolphins are probably preyed upon by killer whales, sharks and possibly false killer whales (although no incidents of attacks on Risso’s dolphins have actually been observed).
Risso’s Dolphin Conservation
Risso’s dolphins are abundant and have a broad geographic range. As a result, they are classified as a species of “Least Concern” on the IUCN’s Red List of Threatened Species. However, unfortunately there is not a lot of information about the population status of these animals, so it is difficult to know what their conservation needs are. Risso’s dolphins are protected in the United States under the Marine Mammal Protection Act of 1992. In the Mediterranean Sea, the animals are protected by the regional ACCOBAMS and in the North and Baltic Sea by the ASCOBANS Agreements.
Bycatch is among the worst threats to these animals, meaning that they are caught up in fishing nets and then drown. In some regions, they are intentionally killed for their meat or oil, or killed by fishermen who consider them to be competition. Currently, Japan, Indonesia, the Solomon Islands, and The Lesser Antilles hunt Risso’s dolphins.
As a deep-diving species, they are also threatened by noise pollution, caused mainly by ship traffic and military sonar. This species is also heavily burdened by toxins such as mercury and chlorides, particularly in the Mediterranean Sea, but also in other regions.
