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Kingdom
Animalia
Phylum
Chordata
Subphylum
Vertebrata
Class
Mammalia
Order
Cetacea
Suborder
Odontoceti
Family
Delphinidae
Species
Grampus griseus
Grampus griseus
Risso's dolphin
2010/09/05 03:07:18.371 GMT-4
By Dr. Barbara Lundrigan and Diana Tarr
Geographic Range
All temperate and tropical oceans and adjoining seas (Nowak 1984).
Habitat
Risso's dolphins are pelagic, encountered most commonly seaward of the continental slope. They are known to frequent subsurface seamounts and escarpments where they are thought to feed on vertically migrant and mesopelagic cephalopods. In Monterey Bay, California, Risso's dolphins are most concentrated over areas with steep bottom topography (Kruse 1999).
Physical Description
Mass
300 kg (average)
(660 lbs)
Adult Risso's dolphins are 4.0 to 4.3 m long. There are no teeth in the upper jaw and only two to seven (average three or four) teeth on each lower jaw (a characteristic common to squid eaters). Risso's dolphins are robust, particularly anterior to the dorsal fin. Along the body axis on the front of the head there is a slight concave groove which 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 crescentic (concavity forward) and is offset slightly left of the midline. The eye is somewhat small and elliptical and in many specimens is surrounded by a dark grey or black patch (Nishiwaki 1972, Kruse et al. 1999, MMSC 1996, 
http://www.nsrl.ttu.edu/tmot/gramgris.htm
).
Risso's dolphins are strikingly coloured. The youngest calves range in colour from iridescent gunmetal grey to fawn-brown dorsally and are creamy-white ventrally. Pale ochre-yellow highlights accentuate the muzzle. A white anchor-shape patch between the flippers resembles the chest chevron seen on pilot whales 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 colour frequently contrasts with the surrounding background (Nishiwaki 1972, Kruse et al. 1999, MMSC 1996,
http://www.nsrl.ttu.edu/tmot/gramgris.htm
).
One of the most striking characteristics of Risso's dolphins is the prominent scar pattern which criss-crosses the animals' bodies. These distinctive linear scars accumulate primarily on the animals' dorsal and lateral surfaces and have been hypothesized to result from the combined effects of lack of repigmentation of damaged tissue and a slower healing process than that observed in animals such as bottlenose dolphins. Measurements of scar widths and the distances between scars on several captive animals suggest that these marks are made by the teeth of other Risso's dolphins and squid. Less common are circular white scars located anywhere on the body, which have been attributed to bites of lampreys, cephalopods and cookie-cutter sharks, infestations of parasites and bacterial infections. Ontogenetic lightening, in conjunction with the characteristic body scars accumlated throughout life, make some older animals appear almost white (Nishiwaki 1972, Kruse et al. 1999, MMSC 1996,
http://www.nsrl.ttu.edu/tmot/gramgris.htm
).
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 December-April off South Africa. Lengths at birth range from 110 cm to 150 cm (Kruse 1999, Nowak 1999).
Key reproductive features:
gonochoric/gonochoristic/dioecious (sexes separate);
sexual
.
Behavior
Risso's dolphins are gregarious animals. Schools can comprise from one to several hundred individuals; they average about 30 animals. The largest aggregations form in response to abundant food resources. School composition appears to be extremely labile. Small subgroups of individuals within schools display the highest degree of group fidelity. Limited data suggest that cohesive subgroups may be composed of same-sex and similar-age individuals.
In the wild, Risso's dolphins engage in behaviors that are typical of delphinids. They have been observed swimming at speeds of up to 28-32 km /hr, though typically they swim 6-7 km /hr. They surface to breathe about every 7 sec during typical surfacing sequences, but may remain submerged for 30 min. They are very acrobatic- breaching, cartwheeling, lobtailing, and spyhopping (Kruse 1999, MMSC 1996).
Risso's dolphins frequently engage in percussive behaviour, such as lob-tailing and breaching. Aggressive physical contact has been documented, such as flipper slapping between individuals, stiking with flukes and dorsal fins, and body blows (Kruse 1999, MMSC 1996).
Risso's dolphins produce a wide range of sounds, including echolocation clicks, squeals, cries, and rasps. It has been suggested that there are "signature whistles" in the call repertoires of this species (Kruse 1999, MMSC 1996).
Food 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 they are kept in captivity they are fed nothing else (Nishiwaki 1972, Kruse 1999).
Economic Importance for Humans: Negative
Risso's dolphins have very little negative effect on humans. They are only rarely a by-catch in the US tuna purse seine industry, and are taken occasionally in coastal gill net and squid seining industries off the US coast. They are sometimes a nuisance to fisherman (Kruse 1999).
Economic Importance for Humans: Positive
In Japan, Risso's dolphins are taken periodically for food and fertilizer. Some are collected for live exhibition (Kruse 1999).
Conservation Status
There is limited information regarding population size of Risso's dolphin. However, there is some evidence that this dolphin may be at risk of depletion if fishery-related mortalities remain at current levels.
Pollution is the other major factor affecting the population of Risso's dolphins. Pesticides, such as DDT and dieldrin 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 (Kruse 1999).
Other Comments
The word "grampus" is Latin for "a kind of whale" and the word "griseus" is Latin for "gray". The famous Risso's dolphin "Pelorus Jack" had the habit of playing about ships and seemed to guide them into Pelorus Sound. It was observed for 24 years (around the turn of the century) escorting ships. Hybridization of Rissos' and bottlenose dolphins has been observed in the wild as well as in captivity (Kruse 1999, Nishiwaki 1972,
http://elfi.com/csiafrica.html
).
For More Information
Find
Grampus griseus
information at
Contributors
Dr. Barbara Lundrigan
(author), Michigan State University. Diana Tarr (author), Michigan State University.
References
Kruse, S., D. Caldwell, M. Caldwell. 1999. Risso's Dolphin. Pp. 183-212 in S. Ridgway, R. Harrison, eds.
Handbook of Marine Mammals, Vol 6
. London: Academic Press.
Nishiwaki, M. 1972. General Biology. Pp. 3-204 in S. Ridgway, ed.
Mammals of the Sea: Biology and Medicine
. Springfield, IL: Thomas Books.
Nowak, R. 1999.
Walker's Mammals of the World
. Baltimore and London: The John Hopkins University Press.
2010/09/05 03:07:19.304 GMT-4
To cite this page:
Lundrigan, B. and D. Tarr. 2000. "Grampus griseus" (On-line), Animal Diversity Web. Accessed September 09, 2010 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Grampus_griseus.html.
Disclaimer:
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. ADW doesn't cover all species in the world, nor does it include all the latest scientific information about organisms we describe. Though we edit our accounts for accuracy, we cannot guarantee all information in those accounts. While ADW staff and contributors provide references to books and websites that we believe are reputable, we cannot necessarily endorse the contents of references beyond our control.
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Sponsored in part by the Interagency Education Research Initiative,
the Homeland Foundation and the
University of Michigan
Museum of Zoology
.
This material is based upon work supported by the
National Science Foundation
under Grants DUE-0633095 and DRL-0628151.
The ADW Team gratefully acknowledges their support.
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Phylum Chordata 
Class Mammalia 
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