How are they recognised?
Risso’s Dolphins have a bulb-shaped head with a distinct vertical crease along the upper surface of the melon (a mass of adipose tissue found in the forehead of all toothed cetaceans and used in communication and echolocation). Unlike many other dolphins, Grampus griseus has no beak.
The dorsal fin is tall and its flippers are long and pointed. The upper part of the body is robust, and tapers to a relatively narrow tail. Adults range from 2.6 to 4 m in length and weigh about 400 kg. The Risso’s Dolphin is the fifth largest of the ocean dolphins. Males and females are similar in appearance.
Colour patterns change dramatically with age. The youngest calves have a grey to brown back and a creamy-white abdomen, darkening to nearly black, with the abdominal patches remaining white. As they mature, they lighten, and only the dorsal fin remains dark. This is as a result of the majority of the dorsal and side surfaces becoming covered with linear scars. Older animals can appear almost completely white.
These distinctive scars are caused by other Risso’s Dolphins, predators, prey, and parasites. They are believed to remain due to the lack of re-pigmentation in damaged tissue and a slower healing process compared to other dolphins.
Where are they found?
Grampus griseus has an extensive distribution, inhabiting temperate, subtropical and tropical waters in the oceans of both hemispheres.
The seasonal movements of Risso’s Dolphins are unclear. They seem to be present year-round throughout most of their geographic range. Residents of the northern-most parts appear to migrate seasonally. For example, populations which summer off the coast of Northern Scotland migrate during the winter to the Mediterranean Sea, while summer populations off the coast of California migrate to overwinter in Mexican waters.
What is their habitat?
Risso’s Dolphins inhabit oceanic and continental slope waters, ranging from 400 to 1,000 m deep. They are often seen seaward of continental shelves and slopes, or close to seamounts or submarine canyons. The preference for this habitat may be explained by the high marine productivity of these areas, as it enhances feeding opportunities.
Grampus griseus is most commonly found in waters between 15 and 20o, and rarely in waters below 10o.
How do they mate?
There is little information about the reproduction of Grampus griseus. It is likely that breeding and calving occur year-round due to its wide distribution. However, it peaks seasonally depending on the hemisphere. For example, during winter it peaks in Eastern Pacific, and in summer in the North Atlantic and Western Pacific.
Females are usually considered mature by 8 to 10 years. Yet size, rather than age, is often a better indicator of sexual maturity in marine cetaceans. Most males reach sexual maturity at a length of 2.6 to 2.8 m.
After a gestation period that lasts 13 to 14 months, a single calf is born. Newborn calves range from 1.1 to 1.7 m in length and weigh an average of 20 kg. They begin swimming immediately after birth. Weaning is complete by 12 to 18 months following parturition.
Female Risso’s Dolphins are the primary caregivers for calves. Paternal care, which is rare in other cetaceans, has not been documented in this species. Female dolphins and their calves form nursery pods. Often, while a calf’s mother is foraging for food, another female provides care. The young do not leave the group until a few years before sexual maturity.
How do they behave?
Risso’s Dolphins are very social. They typically travel in pods of 10 to 50 (usually around 30). However, large gatherings of up to 4,000 dolphins have been reported, very probably in response to abundant food resources.
Cohesive subgroups are formed based on age and sex, with strongest associations occurring between adult males and between adult females.
Grampus griseus often mixes with groups of other cetaceans, such as Bottlenose Dolphins and Pacific White-Sided Dolphins.
Female reproductive success is positively influenced by the social support and foraging benefits of large pods. For example, while a female is searching for food, she is able to leave her calf in the care of other females in the group. As a result, female pods tend to be significantly larger than those of males.
At the water’s surface, Grampus griseus can display an impressive variety of acrobatics.
What do they eat?
Risso’s Dolphins consume large amounts of fish, krill, crustaceans, and primarily cephalopods (squid, cuttlefish, octopus, and nautilus). For this reason, they are believed to feed at night, when prey travels from the depths to the water’s surface.
Do they have predators?
There is no data available regarding predators specific to Risso’s Dolphin.
What is their lifespan?
Average lifespan in the wild: 30-35 years.
Are they endangered?
The Risso’s Dolphin is widely distributed worldwide and relatively abundant. However, little is known about this species. Details of distribution are generally lacking and estimates of abundance are available for only a few regions.
Grampus griseus is threatened by overfishing in some parts of its range. In Sri Lanka about 1,300 individuals are taken annually in fisheries, providing meat for human consumption and fish bait. In Japan Risso’s Dolphins are also hunted for meat and fertilizer. About 250-500 are killed in a drive fishery that takes place once a year. Direct killing additionally occurs in the Caribbean, Indonesia, and Philippines.
This species is also taken occasionally as by-catches in other fisheries. It has been reported in the North Atlantic, the Mediterranean Sea, the Southern Caribbean, the Azores, Peru, the Solomon Islands, and the US Coast. Moreover, small numbers have also been captured for live exhibition in aquariums and oceanariums.
Because Risso’s Dolphins are deep-divers that feed on squid and rely on echolocation to hunt, they are likely to be vulnerable to sound pollution caused by navy sonars and seismic exploration.
Increasing levels of plastics and other refuse at sea may pose a threat for populations of small cetaceans. Predicted impacts of global climate change on the marine environment may also affect this species, although possible consequences are currently unclear.
Due to the high levels of pollutants accumulated in small cetaceans’ tissues, the consumption of their meat could be harmful not only for children and pregnant women, but also for the general population.
Evaluated by the IUCN Red List of Threatened Species as Least Concern (2012).
Did you know?
- Grampus griseus has no teeth in its upper jaw, but has 2 to 7 pairs of sharp peg-like teeth in the lower jaw.
- About 77% of time is spent travelling, 13% in social activity, 5% in feeding, and 4% resting.
- Hybrid offspring between the Bottlenose Dolphin and the Risso’s Dolphin has been known to occur.
- Risso’s Dolphins are one of many hosts for sea lampreys (jawless fish that resemble eels).