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The Risso’s dolphin is perhaps one of the most recognizable specie of dolphin. Found in almost all the world’s oceans (and some seas, such as the Mediterranean) in temperate and tropical zones, this species ranges all the way from its northern range of the Gulf of Alaska and Greenland to the southernmost tip of South America and New Zealand, typically sticking to deeper waters along continental shelves.
With a fairly robust body, bulbous head and large curved dorsal fin leading to a narrow tail and caudal peduncle, the Risso’s dolphin is difficult to misidentify should you encounter one. As one of the largest dolphins growing up to 13 feet and 1,100 lbs, this species lacks a defined beak that is typically seen in dolphins and sports only 2-7 pairs of teeth in the lower jaw only – a low number of teeth when compared to other toothed cetaceans. However, perhaps the most distinguishable feature of this dolphin is its coloration. Born a dark black or grey, Risso’s dolphin calves lighten to a whiter coloration as they age, though this is mainly due to the vast amount of scars their bodies accumulate. Since the scars this species acquires from tooth rakes from other dolphins, their squid prey fighting back or parasitic fishes (cookie cutter sharks, lamprey, etc.) do not heal as well as other cetaceans, the scars tend to be permanent and therefore whiten the dolphin as it ages, making this species incredibly distinguishable from others.
Diet & Behavior
Risso’s dolphins eat a few different species, including some fish and crustaceans, but most of their diet is cephalopods (e.g. octopus and squid). Squid is their prey of choice and usually is the dominant species in their diet. This explains why Risso’s are mainly nocturnal foragers. These squid come up closer to the surface at night to feed, usually in large numbers, making it a perfect time and place for a Risso’s to stop by for a snack. Teeth aren’t needed for chewing, especially on cephalopods, so like many other squid eating species they only have a few pairs of teeth (on their bottom jaw only- more on these later!) that help to catch their prey, but then it is swallowed whole.
While Risso’s (and their prey) favor deeper water, it is not uncommon for them to be sighted in shallower coastal areas, usually following their preferred prey. These dolphins can dive up to 1000 feet and hold their breath for up to 30 minutes, but as their prey comes to the surface at night, they needn’t do that often and usually do shallower, shorter dives. Before a dive they may take up to 10-12 breaths at 15-20 second intervals before diving and will often lift their tails up out of the water (think of a humpback whale diving), which is called fluking.
The don’t just use their fluke for diving, but also in social interactions – along with their other body parts. They can be very active at the surface, leaping, slapping flippers/flukes and spyhopping. Often they are aggressive towards each other, using their teeth in aggressive interactions (often involved in mating) and causing rake marks. These produce scars, and while rake marks are common on many toothed whale species, Risso’s are unique in the longevity of the scars which can last for years (and causing older individuals to be almost all white). It is thought that there is a loss of pigment during the healing process, and that this may have evolved to establish a social hierarchy. Perhaps the more scarred the individual, the higher the social status. Fortunately for researchers, the longevity of scars allows them to be used for photo-ID purposes of tracking individuals over time to learn more about their social and population structure (the Jonian Dolphin Conservation (JDC) research group uses an automated ID system, check it out!)
This work has helped us know more about their social structure. Group size usually ranges from 10 to 30-50 individuals, though they can be solitary or in superpods up to 1000s of individuals. The Risso’s ‘stratified’ social structure is different than the fission/fusion structure of most dolphin species. They have these larger groups, but within that there are smaller, more stable subgroups. Individuals are loyal to these subgroups (they have high fidelity to their groups). Adult males have long-term bonds, and mothers with calves will group together. Juveniles on the other hand, have much less fidelity and will leave and join different groups quite often.
Not much is known about their reproduction, but animals are sexually mature around 10 years of age (when they are 8.5-9ish feet long). Gestation is 13-14 months every 2.5 years (on average). Breeding and calving may occur year round, but the peak calving period varies geographically (e.g. off Japan it is summer to fall, but in California it is fall to winter). An interesting reproductive fact is that hybrids between Risso’s and bottlenose dolphins exist in captivity and in the wild!
These hybrids may be a result to their tendency to hang out with other species. They are known to harass and surf the bow waves of Gray whales. In the Gulf of Corinth in the Mediterranean Risso’s have been seen associating with striped dolphins and common dolphins, and Risso’s were only seen there in mixed species groups. It is commonly noted that Risso’s form travelling pods at sea with other dolphin species. They seem to be the social butterflies of the cetacean world!
Risso’s dolphins are considered fairly common throughout their range, and are listed as a “Species of Least Concern” by the IUCN. They are, however, protected by the Marine Mammal Protection Act (MMPA) and CITES Appendix II throughout their range.
- Climate Change: Like many species, Risso’s are threatened by climate change primarily due to the impacts changing ocean temperatures and climates may have on their prey species.
- Entanglement: Unfortunately, entanglement in fishing gear occurs quite frequently, which makes sense as these dolphins often hang around beside fishing vessels looking for scraps or foraging on the same prey species. They are most susceptible to longlines, trawls, and gillnets.
- Hunting: Legal hunting of Risso’s dolphins for meat occurs in the following countries: Japan, Indonesia, the Caribbean, Sri Lanka, and the Solomon Islands. Animals captured but not killed for meat are sometimes sold to aquariums.
- Ocean Noise: Sonar, especially loud sonar such as that employed during naval trainings, can startle Risso’s and cause them to flee. This can result in mass stranding events.
- Contaminants: Like many other marine mammals, Risso’s are exposed to many contaminants such as PCBs, which are stored in their blubber. As a long-lived species, they are likely to accumulate contaminants in larger amounts (both from individual exposure and through bioaccumulation through their food), and are more at risk of metabolizing them in times of nutritional stress.
- Risso’s dolphins were named after French-Italian naturalist Antoine Risso, who lived from 1777-1846 (he also gave his name to several fish species!).
- Their Latin name, Grampus griseus, is both literal and accurate: Grampus is a combination of the word for large and ‘piscis’ (meaning “fish”), while griseus means “grey-mottled or grizzled”.
- Risso’s dolphins are the largest of the delphinids that are still called a dolphin (killer whales are actually the largest dolphin species). They are also often considered part of the Blackfish grouping, which includes killer whales, false killer whales, pygmy killer whales, pilot whales, and melon-headed whales.
- Interestingly, although Risso’s are very distinctive their large dorsal fin can actually cause confusion with observers thinking they are watching killer whales.
- They have unique frequencies of call for each individual that enables them to be recognized by conspecifics.
- One of the most famous Risso’s dolphins was named “Pelorus Jack”. Pelorus Jack became famous for guiding boats across Cook Strait in New Zealand and was responsible for the first government protection for a cetacean after someone fired a rifle at the dolphin in 1904. They even named a Scottish ceilidh dance after it!