Grampus griseus
Risso's dolphin

All temperate and tropical oceans and adjoining seas (Nowak 1984).

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).

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).

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).

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).

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).

In Japan, Risso's dolphins are taken periodically for food and fertilizer. Some are collected for live exhibition (Kruse 1999).

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).

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).