G. griseus is a large dolphin, with both sexes reaching nearly 4m in length (1; 2). Their head is blunt, without a distinctive beak (1) and has a unique vertical crease, or cleft, along the anterior edge of the melon (1; 2; 3). The body is robust anteriorly and tapers down to a narrow tail stock (2). The dorsal fin is one of the tallest in proportion to body length (2) and is falcate in shape (3). The pectoral fins are long and pointed (1; 3). G. griseus has a very uniform dark grey colour that is quite often covered in white linear scars (1; 2; 3). It has been suggested that the scarring is an indicator of male ‘quality’ during male-male competition in intraspecific aggressive interactions (4). The lack of re-pigmentation on the scars is thought to be necessary in avoiding costly and dangerous fights (4). The fins are often darker than the rest of the body (1; 3; 2).
Risso’s dolphins are social odontocetes, often found in pods of 5-50 animals (5). They have also been recorded in mixed-species associations with the Striped dolphin (Stenella coeruleoalba) and the Common dolphin (Delphinus delphis) (6).
There is also evidence showing hybridisation between G. griseus and the Common Bottlenose dolphin, Tursiops truncatus (7).
Not much is known about the breeding habits of G. griseus. They reach sexual maturity when they are nearly 3m in length (5). It is not known, exactly, when the breeding and calving season is for this species. It is thought that both seasons vary highly geographically (5) and there are conflicting reports on when calving seasons are in the North Atlantic (2).
Risso’s dolphins are found in temperate and tropical waters, and prefer continental slope and steep shelf-edge habitats (7), in depths ranging from 400-1000m (2).
G. griseus is present in the Pacific, Atlantic and Indian Oceans, and the North and Mediterranean Seas in deep and continental waters (2; 3).
Risso’s dolphin is primarily a squid-eating odontocete (8; 9) and is a nocturnal predator (8).
IUCN Red List: Least Concern (10).
CMS: Appendix II (11).
Bern Convention: Appendix II (12).
Description written by Sarah Marjoram (2009)
(1) Geraci, J.R.a.V.J.L., 1993. Marine Mammals Ashore: A field guide for strandings. Texas, U.S.A.: Texas A&M Sea Grant College Program.
(2) Culik, B., 2003. Grampus griseus (G. Cuvier1812). [Online] Available at: http://www.cms.int/reports/small_cetaceans/data/g_griseus/g_griseus.htm [Accessed 17 July 2009].
(3) FAO, n.d. Species Fact Sheet: Grampus griseus. [Online] Available at: http://www.fao.org/fishery/species/2738/en [Accessed 17 July 2009].
(4) MacLeod, C.D., 1998. Intraspecific scarring in odontocete cetaceans: an indicator of male ‘quality’ in aggressive social interactions? Journal of Zoology, London, 244, pp.71-77.
(5) Fisheries, N., n.d. Risso’s dolphin: Grampus griseus. [Online] Available at: http://www.nmfs.noaa.gov/pr/species/mammals/cetaceans/rissosdolphin.htm [Accessed 20 July 2009].
(6) Frantzis, A. & Herzing, D.L., 2002. Mixed species associations of striped dolphin (Stenella coeruleoalba), short-beaked common dolphin (Delphinus delphis), and Risso’s Dolphin (Grampus griseus) in the Gulf of Corinth (Greece, Mediterranean Sea). Aquatic Mammals, 28.2, pp.188-97.
(7) Baird, R.W., 2009. Risso’s Dolphin: Grampus griseus. In Perrin, W.F..B.W.a.J.G.M.T. Encyclopedia of Marine Mammals. 2nd ed. Academic Press.
(8) Shane, S.H., 1995. Behaviour patterns of pilot whales and Risso’s dolphins of Santa Catalina Island, California. Aquatic Mammals, 21.3, pp.195-97.
(9) Blanco, C..M.A.R.a.J.A.R., 2006. Diet of Risso’s dolphin (Grampus griseus) in the western Mediterranean Sea. Scienta Marina, 70(3), pp.407-11.
(10) Taylor, B.L..R.B.J.B.S.M.D.J.F.J.G.M.G.N.d.S.P.W.&.R.L.P., 2009. Grampus griseus. [Online] IUCN Available at: www.iucnredlist.org [Accessed 14 July 2009].
(11) (CMS), C.o.t.C.o.M.S.o.W.A., Effective 5th March 2009. Appendices I and II of the Convention on the Conservation of Migratory Species of Wild Animals.
(12) Europe, C.o., 2002. Convention on the conservation of European wildlife and natural habitats: Bern Convention. [Online] Available at: http://conventions.coe.int/Treaty/FR/Treaties/Html/104-2.htm [Accessed 15 July 2009].