Grey dolphin, Grey grampus, Monk dolphin
Risso's dolphin (Grampus griseus) is one of the most mysterious and little known-species of dolphin with a rather unusual appearance. It is named after Antoine Risso, whose study of the animal formed the basis of the recognized description by Georges Cuvier in 1812. Some of the closest related species to Risso's dolphins include: Pilot whales, Pygmy killer whales, Melon-headed whales, and False killer whales.
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NocturnalNocturnality is an animal behavior characterized by being active during the night and sleeping during the day. The common adjective is "nocturnal",...
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DiurnalDiurnal animals are active during the daytime, with a period of sleeping or other inactivity at night. The timing of activity by an animal depends ...
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CarnivoreA carnivore meaning 'meat eater' is an organism that derives its energy and nutrient requirements from a diet consisting mainly or exclusively of a...
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MolluscivoreA molluscivore is a carnivorous animal that specializes in feeding on molluscs such as gastropods, bivalves, brachiopods, and cephalopods. Known mo...
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AquaticAn aquatic animal is an animal, either vertebrate or invertebrate, which lives in water for most or all of its life. It may breathe air or extract ...
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PrecocialPrecocial species are those in which the young are relatively mature and mobile from the moment of birth or hatching. Precocial species are normall...
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CosmopolitanAnimals with cosmopolitan distribution are those whose range extends across all or most of the world in appropriate habitats. Another aspect of cos...
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NatatorialNatatorial animals are those adapted for swimming. Some fish use their pectoral fins as the primary means of locomotion, sometimes termed labriform...
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ViviparousAmong animals, viviparity is the development of the embryo inside the body of the parent. The term 'viviparity' and its adjective form 'viviparous'...
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MigratingAnimal migration is the relatively long-distance movement of individual animals, usually on a seasonal basis. It is the most common form of migrati...
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starts withRisso's dolphin has a relatively large anterior body and dorsal fin, while the posterior tapers to a relatively narrow tail. The bulbous shape of the head has a vertical crease in front. Infants are dorsally grey to brown and ventrally cream-colored, with a white anchor-shaped area between the pectorals and around the mouth. In older calves, the nonwhite areas darken to nearly black and then lighten (except for the always dark dorsal fin). Linear scars mostly from social interaction eventually cover the bulk of the body; scarring is a common feature of male to male competition in toothed whales, but Risso's dolphin tend to be unusually heavily scarred. The pronounced appearance of these scars results from the lack of repigmentation, which may be advantageous as a display that reduces further challenges from other males. Older individuals appear mostly white. Most individuals have two to seven pairs of teeth, all in the lower jaw.
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.
Social animals are those animals that interact highly with other animals, usually of their own species (conspecifics), to the point of having a rec...