When on the surface, these dolphins are known for their extremely active behavior, which includes breaching, flipper-slapping, lobtailing and spyhopping.Meanwhile, each dolphin has its own unique frequency, which helps identify individuals. They use a wide variety of whistles and clicks to communicate with each other.The upper jaw of this animal has no teeth.In 1882, this man was the first person to describe this dolphin to Georges Cuvier. The word 'Grampus' is the combination of two Latin words: 'granis' - meaning large or great and 'piscis', meaning fish. The scientific name of this species is 'Grampus griseus', where the word 'griseus' means grizzled or mottled with gray.Ecological nicheĭue to their diet, Risso's dolphins likely have a significant influence on the abundance fish and krill. Currently, Risso’s dolphins are classified as Least Concern (LC) on the IUCN Red List. 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. Population numberĪccording to IUCN, the Risso’s dolphin is locally common and widespread throughout its range but no overall population estimate is available. 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. In some parts of its range, this dolphin is often incidentally caught in fishing gears. The animal also suffers from overfishing particularly in Japan, where Risso's dolphin is fished for its meat. 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. 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.
Young dolphins live in these groups until they become sexually mature. Within these pods, it is common to care for another female's calf, while its mother is foraging for food. Females and their young usually gather into separate groups, where mother dolphins help each other raise calves. Soon after the birth, the calf is ready to swim. Gestaiton period lasts for 13 - 14 months, yielding a single baby, which is born precocial. 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. Thus, females of Risso's dolphin are known to form single-sex groups, which attract mating males. However, reproduction is affected the by social structure of this species. Little is known about the mating behavior and system of this animal. 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. 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.
Males of this species can often be seen bothering False killer whales, Bottlenose dolphins and other animals. 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. The species have been observed mixing with other cetaceans to form foraging groups. Their groups often include individuals of the same age and sex. However, Risso's dolphins are also known to gather in very large groups of up to 4,000 dolphins, when there is enough food. These highly social animals usually form groups, varying from 10 to 50 individuals with an average of 30.
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