This animals is one of the most mysterious and little known-species of dolphin with a rather unusual appearance. The snout of Risso’s dolphin is blunt. The animal has a bulbous head, emerging almost vertically from the upper jaw. Another identifying feature of Risso’s dolphin is long, white bands of scar tissue, descending either side of the dolphin's body. These streaks are believed to be a result of squid bites or bites from conspecifics, caused during playing or fighting. The skin color of newborn calves is grey, becoming chocolate brown as they grow up. Adult individuals exhibit slaty or black upper-side, slightly colored with blue of purple. The under-side is whiter, while the flippers and tail are darker.
The area of this dolphin's distribution extends from Newfoundland, the Gulf of Alaska and the North Sea southwards to South America, Southern Africa, reaching Australia and New Zealand. The animal is also known to occur in partially enclosed water bodies such as Red Sea, Gulf of Mexico and Mediterranean Sea. Risso's dolphin displays migratory behavior, travelling north by winter, occasionally reaching as far north as the Gulf of Alaska and the Shetland Islands. Preferred habitat of this dolphin is deep, temperate, tropical and shelf-edge offshore waters. The animal most frequently occurs at depths of 400 - 1000 meters. However, those in northern Europe and around oceanic islands, tend to live in shallower, coastal areas.
These highly social animals usually form groups, varying from 10 to 50 individuals with an average of 30. However, Risso's dolphins are also known to gather in very large groups of up to 4,000 dolphins, when there is enough food. Their groups often include individuals of the same age and sex. The species have been observed mixing with other cetaceans to form foraging groups. Risso's dolphins are night feeders, since cephalopods, which are the main prey species of these animals, come to the water's surface during the nighttime hours. Males of this species can often be seen bothering False killer whales, Bottlenose dolphins and other animals. The animals are also known for their aggressive physical contacts, including flipper slapping between conspecifics, striking with flukes and dorsal fins as well as body blows. Risso's dolphins interact with each other, using different forms of communication such as breaching, chasing, biting, leaping out of the water and lob-tailing.
Risso’s dolphins are carnivores (molluscivores). They primarily feed upon the greater Argonaut or the paper nautilus, supplementing its diet with a wide variety of fish, krill, crustaceans as well as cephalopods.
Little is known about the mating behavior and system of this animal. However, reproduction is affected the by social structure of this species. Thus, females of Risso's dolphin are known to form single-sex groups, which attract mating males. Considering the global distribution of Risso's dolphins, these animals are likely to mate and give birth at any time of year, though populations off South Africa most frequently breed in December-April. Gestaiton period lasts for 13 - 14 months, yielding a single baby, which is born precocial. Soon after the birth, the calf is ready to swim. Females and their young usually gather into separate groups, where mother dolphins help each other raise calves. Within these pods, it is common to care for another female's calf, while its mother is foraging for food. Young dolphins live in these groups until they become sexually mature. Calves are weaned within 12 - 18 months after birth, gaining their sexual maturity at a length of 8.5-9 feet or 2.6-2.8 meters.
Presently, one of the potential concerns to the global population of Risso's dolphin is climate change, since rising ocean level and increasing temperature of the water's surface may lead to decline as well as changes in distribution of its prey species. The animal also suffers from overfishing; particularly in Japan, where Risso's dolphin is fished for its meat. In some parts of its range, this dolphin is often incidentally caught in fishing gears. Other notable threats to the population of this species include anthropogenic sounds, usually produced by navy sonar or seismic exploration: these sounds, which resonate through the ocean, are extremely dangerous for dolphins, causing gas-bubble disease, which, in turn, leads to high number of mortality among this species.
According to IUCN, the Risso’s dolphin is locally common and widespread throughout its range but no overall population estimate is available. However, there are estimates of this species in specific areas: the California – Oregon - Washington subpopulation is now estimated at only 16,066 dolphins; Hawaiian waters hold 2,351 dolphins; off Sri Lanka - 5,500-13,000 dolphins; off the eastern Sulu Sea (Dolar) - 1,514 dolphins; off the eastern United States - 20,479 dolphins; in the northern Gulf of Mexico - 2,169 dolphins; in three areas of concentrated occurrence off Japan - 83,300 dolphins; in the eastern tropical Pacific - 175,000 dolphins. Currently, Risso’s dolphins are classified as Least Concern (LC) on the IUCN Red List.
Due to their diet, Risso's dolphins likely have a significant influence on the abundance fish and krill.