Risso's dolphin

Identification

The Risso's dolphin (Grampus griseus) is a large dolphin with a robust, stocky body that becomes more slender behind the dorsal fin. Its head is bulbous, with no beak, a particularly large melon, and a mouth that slants upwards towards the eyes. A peculiar, deep V-shaped crease extends from the blowhole to the tip of the rostrum.

At birth, the Risso is a uniform light grey colour, but this darkens to a chocolate brown / black as the animal ages. Large animals are typically silver-grey and the body is covered with scratches and scars inflicted from conspecifics or the sharp beaks of squid during feeding. Old, mature individuals may be almost entirely white in colour. The pectoral flippers are long, narrow and curved. The dorsal fin is tall and falcate, and set at the mid-point of the body. The tail flukes are dark and broad, with a distinct median notch, pointed tips, and an overall concave outline.

Risso's dolphins are often seen travelling and surfacing slowly, although they may be highly active often breaching clear of the water and slapping their heads, tails or sides on the surface. They sometimes surface with only their tail or head showing above the water. When travelling (at a speed of about 5 knots) they generally show the top of the head and dorsal fin as they blow. When diving, they may raise their tail flukes out of the water and descend vertically. Risso's may remain motionless on the surface of the water for several seconds after surfacing. They also spy hop (pic, left), revealing the whole head and body down to the flipper for 3 seconds or more, as if surveying their surroundings.

Distribution & Habitat

Risso's dolphins are generally considered to be a widespread, warm water and pelagic species, favouring waters along the continental slopes. They have been recorded in waters with a wide range of surface temperatures - from 4.5 degrees to 28 degrees centigrade. Surveys in the eastern tropical Pacific, however, often found Risso's dolphins in relatively near-shore areas and the species is most often sighted in inshore waters around the British Isles, in contrast to some other areas within its range. Definite, well-defined migrations have not been documented for the Risso, although their numbers may fluctuate locally from season to season, or over the longer term.

Natural History & Ecology

Risso's dolphins reach a maximum length of 4 m and a weight of 500 kg. There is some evidence, from strandings records, that males are slightly longer than females. Males attain sexual maturity at a length of just under 300 cm, females slightly less, and at least 3 or 4 years of age.

Knowledge of the reproduction and breeding of the Risso's dolphins is still limited. Births may occur in winter, but calves may be born all year round. Off Britain, breeding is probably mainly between, although not restricted to, April and July. Gestation has been estimated to be 13 to 14 months. Hybridisation with bottlenose dolphins both in the wild and in captivity has been reported.

The diet of Risso's dolphin consists of mainly of cephalopods (especially squid), crustaceans and occasionally small fish. Vegetative matter has been found in the stomachs of stranded Risso's dolphins, as with other cetaceans. It may be that weed is swallowed incidentally; alternatively it has been suggested that cetaceans, like some canids, seek herbal remedies when they are ill.

Risso's dolphins often swim in ‘echelon' formation, lined up abreast at evenly spaced intervals in co-operative behaviour to improve the effectiveness of foraging and hunting. They are generally considered to forage in deep water, although in some places Risso's can be found feeding in shallow inshore waters.

Social Behaviour

The Risso's dolphin tends to form small to medium-sized pods of 2 to 45 animals, but may be encountered singly or in large groups of 200 to 1000 individuals. The species has a cohesive social structure in which individuals stay together for extended periods of time. Groups composed of mature males, females, juveniles and calves are observed. However, the details of sex and age composition of group are not well known. In larger schools (more than 60 animals), it is thought that segregation by age and sex occurs, since distinctive groups of calfless adult animals, juvenile groups, and females with calves have been observed.

The social behaviour of Risso's has been described as ‘rough and obviously physical', and behaviours include slapping, splashing and sometimes striking one another. The high degree of scarring on the skin of these dolphins is thought to be attributable to this intraspecific behaviour. The species is also known to be highly gregarious, associating with other small cetaceans in mixed species groups.