Risso's dolphin

Risso's dolphin (Grampus griseus) is the only species of dolphin in the genus Grampus. It is commonly known as the Monk dolphin among Taiwanese fishermen. Some of the closest related species to these dolphins include: pilot whales (Globicephala spp.), pygmy killer whales (Feresa attenuata), melon-headed whales (Peponocephala electra), and false killer whales (Pseudorca crassidens).[4]

Risso's dolphin[1]
Size compared to an average human
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Artiodactyla
Infraorder: Cetacea
Family: Delphinidae
Genus: Grampus
Gray, 1828 [3]
Species:
G. griseus
Binomial name
Grampus griseus
(G. Cuvier, 1812)
  Distribution of Risso's dolphin

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 (also the species' genus), 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 Latin grandis piscis or French grand poisson, both meaning big fish. The specific epithet griseus refers to the mottled (almost scarred) grey colour of its body.

Description
Illustrations by Edward Drinker Cope in 1876

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.[5]

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 in toothed whales, but Risso's dolphin tend to be unusually heavily scarred.[6] Older individuals appear mostly white. Most individuals have two to seven pairs of teeth, all in the lower jaw.[5]

Length is typically 10 feet (3.0 m), although specimens may reach 13.12 feet (4.00 m).[7] Like most dolphins, males are typically slightly larger than females. This species weighs 300–500 kilograms (660–1,100 lb), making it the largest species called "dolphin".[8][9]

Range and habitat
A Risso's dolphin swims off Morro Bay

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 (a stranding was recorded in the Sea of Marmara in 2012[10]). They range as far north as the Gulf of Alaska and southern Greenland and as far south as Tierra del Fuego.[5]

Their preferred environment is just off the continental shelf on steep banks, with water depths varying from 400–1,000 m (1,300–3,300 ft) and water temperatures at least 10 °C (50 °F) and preferably 15–20 °C (59–68 °F).[5]

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
Risso's dolphin off Port San Luis, Harford Pier, at Avila Beach, California.

They feed almost exclusively on neritic and oceanic squid, mostly nocturnally. Predation does not appear significant. Mass strandings are infrequent.[5] 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.[11]

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 fishermen to feed around their boats.[12] They also travel with other cetaceans. They harass and surf the bow waves of gray whales, as well as ocean swells.[5]

Risso's dolphins have a stratified social organisation.[13] 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.[14] 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.[13] 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.[6]

Reproduction
See also: Dolphin § Reproduction and sexuality

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.[5]

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.[4] Many of these incidents have resulted in death.[14] 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.[4]

In Ireland, though not apparently in England, Risso's Dolphin was one of the royal fish which by virtue of the royal prerogative were the exclusive property of the English Crown.[15]

Conservation

The Risso's dolphin populations of the North, Baltic, and Mediterranean Seas are listed on Appendix II[16] of the Convention on the Conservation of Migratory Species of Wild Animals (CMS), since they have an unfavourable conservation status or would benefit significantly from international co-operation organised by tailored agreements.[17]

In addition, Risso's dolphin is covered by the Agreement on the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas (ASCOBANS),[18] the Agreement on the Conservation of Cetaceans in the Black Sea, Mediterranean Sea and Contiguous Atlantic Area (ACCOBAMS),[19] the Memorandum of Understanding for the Conservation of Cetaceans and Their Habitats in the Pacific Islands Region (Pacific Cetaceans MoU)[20] and the Memorandum of Understanding Concerning the Conservation of the Manatee and Small Cetaceans of Western Africa and Macaronesia (Western African Aquatic Mammals MoU).[21]

Risso's dolphins are protected in the United States under the Marine Mammal Protection Act of 1992. Currently, Japan, Indonesia, the Solomon Islands, and The Lesser Antilles hunt Risso's dolphins.[14]

Strandings

At least one case report of strandings in Japan's Goto Islands has been associated with parasitic neuropathy of the eighth cranial nerve by a trematode in the genus Nasitrema.[22] There was a recent reporting of a juvenile male Risso's dolphin that was stranded alive on the coast of Gran Canaria on 26 April 2019. This was the first documented case of capture myopathy and stress cardiomyopathy in a male juvenile Risso's dolphin that has received rehabilitation.[23]

See also

References
  1. Mead, J.G.; Brownell, R. L. Jr. (2005). "Order Cetacea". In Wilson, D.E.; Reeder, D.M (eds.). Mammal Species of the World: A Taxonomic and Geographic Reference (3rd ed.). Johns Hopkins University Press. pp. 723–743. ISBN 978-0-8018-8221-0. OCLC 62265494.
  2. Taylor, B.L.; Baird, R.; Barlow, J.; Dawson, S.M.; Ford, J.; Mead, J.G.; Notarbartolo di Sciara, G.; Wade, P. & Pitman, R.L. (2012). "Grampus griseus". IUCN Red List of Threatened Species. 2012: e.T9461A17386190. doi:10.2305/IUCN.UK.2012.RLTS.T9461A17386190.en.
  3. William Perrin (2014). Perrin WF (ed.). "Grampus Gray, 1828". World Cetacea Database. World Register of Marine Species. Retrieved 16 March 2015.
  4. Baird, Robin (2008). Encyclopedia of Marine Mammale 2nd edition. Academic Press. pp. 975–976. ISBN 9780123735539.
  5. Baird, Robin W. (2009). Perrin, William F.; Wursig, Bernd; Thewissen, J. G. M. (eds.). Encyclopedia of Marine Mammals (2nd ed.). Burlington Ma.: Academic Press. p. 975. ISBN 978-0-12-373553-9. Archived from the original on 9 November 2009.
  6. MacLeod, Colin D. (January 1998). "Intraspecific scarring in odontocete cetaceans: an indicator of male 'quality' in aggressive social interactions?". Journal of Zoology. 244 (1): 71–77. doi:10.1111/j.1469-7998.1998.tb00008.x. ISSN 0952-8369.
  7. "Grampus griseus – Risso's dolphin". Animal Diversity Web.
  8. American Cetacean Society Fact Sheet – Risso's Dolphin Archived 11 July 2007 at the Wayback Machine
  9. Risso's Dolphin. Whale Web. Retrieved 22 September 2015.
  10. First stranding record of a Risso’s Dolphin (Grampus griseus) in the Marmara Sea, Turkey (pdf). Retrieved 6 September 2017
  11. MacLeod, C.D.; Santos, M.B.; Pierce, G.J. (2014). Can habitat modelling for the octopus Eledone cirrhosa help identify key areas for Risso's dolphin in Scottish waters? (PDF) (Report). Scottish Natural Heritage Commissioned Report. 530. Scottish Natural Heritage. Retrieved 24 December 2016.
  12. Shane, Susan H. (1995). "Behavior patterns of pilot whales and Risso's dolphins off Santa Catalina Island, California" (PDF). Aquatic Mammals. 21 (3): 195–197 – via Aquatic Mammals Issue Archives.
  13. Hartman, K. L.; Visser, F.; Hendriks, A. J.E. (14 March 2008). "Social structure of Risso's dolphins (Grampus griseus) at the Azores: a stratified community based on highly associated social units". Canadian Journal of Zoology. 86 (4): 294–306. doi:10.1139/Z07-138. ISSN 0008-4301.
  14. "Risso's dolphin, Open Waters, Marine mammals, Grampus griseus at the Monterey Bay Aquarium". montereybayaquarium.org. Retrieved 27 April 2018.
  15. Ball, F, Elrington History of Dublin Volume 5 (1917) Alexander Thom and Co. Dublin< p.49
  16. "Appendix II Archived 11 June 2011 at the Wayback Machine" of the Convention on the Conservation of Migratory Species of Wild Animals (CMS). As amended by the Conference of the Parties in 1985, 1988, 1991, 1994, 1997, 1999, 2002, 2005 and 2008. Effective: 5 March 2009.
  17. Convention on Migratory Species page on the Risso's dolphin. Cms.int (25 June 1998). Retrieved 22 September 2015.
  18. Agreement on the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas. Ascobans.org. Retrieved 22 September 2015.
  19. Agreement on the Conservation of Cetaceans in the Black Sea, Mediterranean Sea and Contiguous Atlantic Area. Accobams.org. Retrieved 22 September 2015.
  20. Memorandum of Understanding for the Conservation of Cetaceans and Their Habitats in the Pacific Islands Region. Pacificcetaceans.org. Retrieved 22 September 2015.
  21. Memorandum of Understanding Concerning the Conservation of the Manatee and Small Cetaceans of Western Africa and Macaronesia. Cms.int. Retrieved 22 September 2015.
  22. Morimitsu, T; Kawano, H; Torihara, K; Kato, E; Koono, M (1992). "Histopathology of eighth cranial nerve of mass stranded dolphins at Goto Islands, Japan". Journal of Wildlife Diseases. 28 (4): 656–8. doi:10.7589/0090-3558-28.4.656. PMID 1474668.
  23. Câmara, Nakita; Sierra, Eva; Fernández, Antonio; Arbelo, Manuel; Bernaldo de Quirós, Yara; Arregui, Marina; Consoli, Francesco; Herráez, Pedro (29 January 2020). "Capture Myopathy and Stress Cardiomyopathy in a Live-Stranded Risso's Dolphin (Grampus griseus) in Rehabilitation". Animals. 10 (2): 220. doi:10.3390/ani10020220. ISSN 2076-2615. PMC 7070958. PMID 32013196.

Further reading
  • National Audubon Society Guide to Marine Mammals of the World ISBN 0-375-41141-0
  • Encyclopedia of Marine Mammals ISBN 0-12-551340-2
  • Whales, Dolphins and Porpoises, Mark Carwardine, ISBN 0-7513-2781-6
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