Risso's dolphin is the only species of dolphin in the genusGrampus. It is commonly known as the Monk dolphin among Taiwanese fishermen. Some of the closest related species to these dolphins include: pilot whales, pygmy killer whales, melon-headed whales, and false killer whales.
Taxonomy
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. Another common name for the Risso's dolphin is grampus, although this common name was more often used for the orca. The etymology of the word "grampus" is unclear. It may be an agglomeration of the Latingrandis piscis or French grand poisson, both meaning big fish. The specific epithetgriseus refers to the mottled grey colour of its body.
Description
Risso's dolphin has a relatively large anterior body and dorsal fin, while the posterior tapers to a relatively narrow tail. The bulbous 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. Linear scars mostly from social interaction eventually cover the bulk of the body; scarring is a common feature in toothed whales, but Risso's dolphin tend to be unusually heavily scarred. Older individuals appear mostly white. Most individuals have two to seven pairs of teeth, all in the lower jaw. Length is typically, although specimens may reach. Like most dolphins, males are typically slightly larger than females. This species weighs, making it the largest species called "dolphin".
Range and habitat
They are found worldwide in temperate and tropical waters, usually in deeper waters rather, but close to land. As well as the tropical parts of the Indian, Pacific and Atlantic Oceans, they are also found in the Persian Gulf and Mediterranean and Red Seas, but not the Black Sea. They range as far north as the Gulf of Alaska and southern Greenland and as far south as Tierra del Fuego. Their preferred environment is just off the continental shelf on steep banks, with water depths varying from and water temperatures at least and preferably. The population around the continental shelf of the United States is estimated 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 exists.
Ecology
They feed almost exclusively on neritic and oceanic squid, mostly nocturnally. Predation does not appear significant. Mass strandings are infrequent. Analysis carried out on the stomach contents of stranded specimens in Scotland showed that the most important species preyed on in Scottish waters is the curled octopus. A population is found off Santa Catalina Island where they coexist with pilot whales to feed on the squid population. Although these species have not been seen to interact with each other, they take advantage of the commercial squid fishing that takes place at night. They have been seen by fisherman to feed around their boats. They also travel with other cetaceans. They harass and surf the bow waves of gray whales, as well as ocean swells. Risso's dolphins have a stratified social organisation. These dolphins typically travel in groups of between 10 and 51, but can sometimes form "super-pods" reaching up to a few thousand individuals. Smaller, stable subgroups exist within larger groups. These groups tend to be similar in age or sex. Risso's experience fidelity towards their groups. Long-term bonds are seen to correlate with adult males. Younger individuals experience less fidelity and can leave and join groups. Mothers show a high fidelity towards a group of mother and calves. But, it is unclear whether or not these females stay together after their calves leave or remain in their natal pods.
Behavior
Social behavior
Risso’s dolphins do not require cutting teeth to process their cephalopod prey, which has allowed the species to evolve teeth as display weapons in mating conflicts.
Reproduction
Gestation requires an estimated 13–14 months, at intervals of 2.4 years. Calving reaches seasonal peaks in the winter in the eastern Pacific and in the summer and fall in the western Pacific. Females mature sexually at ages 8–10, and males at age 10–12. The oldest specimen reached 39.6 years. Risso's dolphins have successfully been taken into captivity in Japan and the United States, although not with the regularity of bottlenose dolphins or orcas. Hybrid Risso's-bottlenose dolphins have been bred in captivity.
Human interactions
Like other dolphins and marine animals, there have been documentations of these dolphins getting caught in seine-nets and gillnets across the globe. Many of these incidents have resulted in death. Small whaling operations have also been cause of some of these deaths. Pollution has also affected many individuals who have ingested plastic. Samples from these animals shows contamination within their tissue.
At least one case report of strandings in Japan's Goto Islands has been associated with parasiticneuropathy of the eighth cranial nerve by a trematode in the genus Nasitrema. There was a recent reporting of a juvenile male Risso's dolphin that was stranded alive on the coast of Gran Canaria on April 26,2019. This was the first documented case of capture myopathy and stress cardiomyopathy in a male juvenile Risso's dolphin that has received rehabilitation.
