Species

Risso's dolphin

May 2, 2011, 12:04 pm
Content Cover Image

Risso's Dolphin. Source: Robert Pitman/NOAA

Grampus griseus (common name: Risso's dolphin),a marine mammal, is a member of the family of oceanic dolphin, part of the order of cetaceans. The species is so named for its coloration and size.  Griseus means "grizzled," or, "mottled with gray."  Also, the word Grampus, may be a combination of the Latin words, granis and piscis.  The word granis means large, and the word piscis means fish.  The dolphins are also named after the zoologist Risso, who in 1812 was the first person to describe the species.

This dolphin is well known for its striking colors and heavy scarring along its body.  Also, Risso's dolphin is quite gregarious, living in groups between three to 50 animals, and also has been known to interact with other whales and dolphins. They, like many other dolphins, are thought to communicate via echolocation.  This dolphin has been seen virtually all over the world's oceans, in warm and tropical waters. 

  caption Risso's Dolphin stamp, collection of Georges DeClercq

caption Size comparison of an average human and a Risso's dolphin (Grampus griseus). Source: Chris huh/Wikipedia

 Conservation Status caption Size comparison of Risso's dolphin to an average-sized human adult. Source: Chris Huh/Wikipedia

Scientific Classification

Kingdom: Animalia (Animals)
Phylum:--- Chordata
Class:------ Mammalia (Mammals)
Order:-------- Cetacea (Cetaceans)
Family:-------- Delphinidae (Porpoise)
Genus:----------Grampus
Species:----------- Grampus griseus (G. Cuvier, 1812)

Common names

Grampus
Grey dolphin
Grijze dolphin
Mottled grampus
Risso's dolphin
Risso’s grampus
White-headed grampus

Physical description

Risso's dolphin is a large, stocky species with a blunt head, with a total body length ranging from 4.0 to 4.3 meters long.  They are easily recognized as they are heavily scarred and become whiter with age as the number of scars increases.  The scars are thought to be caused by the teeth of other Risso's dolphins, from playing or fighting, however, it is also thought that some of the scars are the result of squid bites, cephalopods, lampreys, cookie-cutter sharks, and infections. 

Risso's dolphins are strikingly colored.  The youngest calves range in color from iridescent gunmetal grey to fawn-brown dorsally, and are creamy-white ventrally.  Pale reddish-yellow highlights accentuate the muzzle.  A white anchor-shaped patch between the flippers resembles the V-shaped patch on the chest of the pilot whale, but is typically brighter and more extensive.  Calves become silver-grey, then darken to nearly black, retaining the ventral patches of white.  As animals age further, their heads, abdomens, and flanks lighten.  In older animals, lip color frequently contrasts with the surrounding background.  Also, these dolphins have no teeth on the upper jaw, and have very few teeth on the lower jaw. 

Along the body axis on the front of the head, there is a slight concave groove that is a unique characteristic of this species.  It is reported that the cleft becomes more prominent with age, and that the skin within it pulses during vocalization.  The relatively large blowhole is crescent shaped (concavity forward) and is offset slightly left of the midline.  The eye is somewhat small and elliptical.

Reproduction

Little is known of the life history or the reproduction of Risso's dolphins.  Births are thought to occur during the warmer months, which are from December to April in South Africa.  Lengths at birth range from 110 cm to 150 centimeters.

Lifespan

It is thought that these dolphins may live as long as 24 years.  A well-known Risso’s dolphin called, "Pelorus Jack" was sighted in a New Zealand harbor for over 20 years.

Behavior

Like most dolphins, this species is a highly social animal, typically occurring in groups ranging between 3 to 50 individuals, and may mix with different species of cetaceans (bottlenose dolphin, gray whale, northern right-whale dolphin, and pacific white-sided dolphin). When groups are hunting, they spread out into a long line.  This species tends to ride alongside or in the wake of boats, and young individuals often breach (clear the water), slap their flippers on the surface of the water or 'spyhop' (lift their heads clear of the water).  A number of sounds are produced, including characteristic 'signature whistles',  and many of these vocalizations are important in detecting prey using echolocation.

Distribution

This dolphin is widely distributed, inhabiting tropical and warm temeratures in both hemispheres.  Some of the places Risso's dolphin inhabits include Madagascar, Kenya, Mozambique, Tanzania, Somalia

caption Risso's Dolphin distribution. Source: MarineBio.org  

Habitat

It is thought that this dolphin shows a preference for deep, warm temperate and tropical waters in offshore areas.  They prefer deep off shore waters, but can be seen close to shore around oceanic islands.  In Britain and Ireland, most records are within 11 km of the coast.  In Monterey Bay, California, Risso's dolphins are most concentrated over areas with steep bottom topography.

Feeding habits

Risso's dolphins are known to prey on a mix of neritic, oceanic, and occasionally bottom dwelling cephalopods. The most important food item is small squid, and when kept in captivity they are fed nothing else.  However, this dolphin species has also been observed to feed upon fish and crustaceans. 

Conservation status

A UK Biodiversity Action Plan priority species, Risso's dolphin is protected in UK waters by the Wildlife and Countryside Act 1981 and the Wildlife (Northern Ireland) Orders, 1985; it is illegal to intentionally kill, injure, or harass any cetacean (whale or dolphin) species in UK waters.  The Agreement on the Conservation of Small Cetaceans in the Baltic and North Seas (ASCOBANS) has been signed by seven European Countries, including the UK.  Provision is made under this agreement to set up protected areas, promote research and monitoring, pollution control and increase public awareness.

This species is listed on the IUCN Red List as Least Concern. 

Threats

Pollution is a major factor affecting the population of Risso's dolphins.  Pesticides, such as DDT, were found in high concentrations in a specimen taken off the Pacific Coast of Japan. There is also evidence that the eating of trash such as plastic bags, soda cans and pieces of rope may cause fatalities.  Increasing levels of plastics and other "throw-aways" at sea may pose a threat to wild populations.  In addition, this dolphin is vulnerable to hunting, environmental change, and entanglement in fishing nets, which results in drowning.

Also, in Japan, Risso's dolphins are taken periodically for food and fertilizer. Some are collected for live exhibition.

References and Further reading

  • Mammals of Texas: Online Edition, Risso's Dolphin
  • NOAA Office of Protected Resources, Risso's Dolphin (Grampus griseus)
  • IUCN Red List
  • Encylcopedia of Life, Grampus griseus (G. Cuvier, 1812) Risso's dolphin
  • Cawardine, M., Hoyt, E., Fordyce, R.E. and and Gill, P. (1998) Whales and Dolphins, the Ultimate Guide to Marine Mammals. Harper Collins Publishers, London.
  • Council of Europe: Bern Convention (June, 2002)
  • UKBAP (June, 2002)
  • WDCS (June, 2002)
  • Cetacea.org. (June, 2002)
  • Macdonald, D.W. (2001) The New Encyclopedia of Mammals. Oxford University Press, Oxford.
  • Animal Diversity Web (June, 2002)
  • Bruyns, W.F.J.M., (1971). Field guide of whales and dolphins. Amsterdam: Publishing Company Tors.
  • Howson, C.M. & Picton, B.E. (ed.), (1997). The species directory of the marine fauna and flora of the British Isles and surrounding seas. Belfast: Ulster Museum. [Ulster Museum publication, no. 276.]
  • Jefferson, T.A., Leatherwood, S. & Webber, M.A., (1994). FAO species identification guide. Marine mammals of the world. Rome: United Nations Environment Programme, Food and Agriculture Organization of the United Nations.
  • Kinze, C. C., (2002). Photographic Guide to the Marine Mammals of the North Atlantic. Oxford: Oxford University Press.
  • NBN (National Biodiversity Network), (2002). National Biodiversity Network gateway. , 2008-10-31
  • OBIS, (2008). Ocean Biogeographic Information System. http://www.iobis.org, 2008-10-31
  • Reid. J.B., Evans. P.G.H., Northridge. S.P. (ed.), (2003). Atlas of Cetacean Distribution in North-west European Waters. Peterborough: Joint Nature Conservation Committee.
  • Banks, R. C., R. W. McDiarmid, A. L. Gardner, and W. C. Starnes. 2003. Checklist of Vertebrates of the United States, the U.S. Territories, and Canada
  • Borges, P.A.V., Costa, A., Cunha, R., Gabriel, R., Gonçalves, V., Martins, A.F., Melo, I., Parente, M., Raposeiro, P., Rodrigues, P., Santos, R.S., Silva, L., Vieira, P. & Vieira, V. (Eds.) (2010). A list of the terrestrial and marine biota from the Azores. Princípia, Oeiras, 432 pp.
  • Foote (2008) Mortality rate acceleration and post-reproductive lifespan in matrilineal whale species. Biol Lett, 4:189-191.
  • Gordon, D. (Ed.) (2009). New Zealand Inventory of Biodiversity. Volume One: Kingdom Animalia. 584 pp
  • IUCN (2008) Cetacean update of the 2008 IUCN Red List of Threatened Species.
  • Jan Haelters
  • Koninklijk Belgisch Instituut voor Natuurwetenschappen: Beheerseenheid Mathematisch Model Noordzee en Schelde-estuarium: Oostende
  • Mead, James G., and Robert L. Brownell, Jr. / Wilson, Don E., and DeeAnn M. Reeder, eds. 2005. Order Cetacea. Mammal Species of the World: A Taxonomic and Geographic Reference, 3rd ed., vol. 1. 723-743
  • Müller, Y. (2004). Faune et flore du littoral du Nord, du Pas-de-Calais et de la Belgique: inventaire. [Coastal fauna and flora of the Nord, Pas-de-Calais and Belgium: inventory]. Commission Régionale de Biologie Région Nord Pas-de-Calais: France. 307 pp.
  • North-West Atlantic Ocean species (NWARMS)
  • Perrin, W. (2011). Globicephala melas (Traill, 1809). In: Perrin, W.F. World Cetacea Database. Accessed through: Perrin, W.F. World Cetacea Database 2011-02-05
  • Ramos, M. (ed.). 2010. IBERFAUNA. The Iberian Fauna Databank
  • Rice, Dale W. 1998. Marine Mammals of the World: Systematics and Distribution. Special Publications of the Society for Marine Mammals, no. 4. ix + 231
  • Richard Weigl (2005) Longevity of Mammals in Captivity; from the Living Collections of the World. Kleine Senckenberg-Reihe 48: Stuttgart.
  • Ronald Nowak (1999) Walker's Mammals of the World. Johns Hopkins University Press: Baltimore.
  • Sergeant, D.E. The Biology of the Pilot or Pothead Whale in Newfoundland Waters. Fisheries Research Board of Canada under the Control of the Honorable the Minister of Fisheries Ottawa.
  • Tinker, S.W. Whales of the World. Bess Press, Inc.
  • Traill, 1809. Nicholson's J. Nat. Philos. Chem. Arts, 22:81.
  • UNESCO-IOC Register of Marine Organisms
  • Wilson, Don E., and DeeAnn M. Reeder, eds. 1993. Mammal Species of the World: A Taxonomic and Geographic Reference, 2nd ed., 3rd printing. xviii + 1207
  • Wilson, Don E., and F. Russell Cole. 2000. Common Names of Mammals of the World. xiv + 204
  • Wilson, Don E., and Sue Ruff, eds. 1999. The Smithsonian Book of North American Mammals. xxv + 750
  • van der Land, J. (2001). Tetrapoda, in: Costello, M.J. et al. (Ed.) (2001). European register of marine species: a check-list of the marine species in Europe and a bibliography of guides to their identification. Collection Patrimoines Naturels, 50: pp. 375-376

 

Glossary

Citation

Life, E. (2011). Risso's dolphin. Retrieved from http://www.eoearth.org/view/article/155758

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