Risso's dolphins (Scientific name: Grampus griseus) are sometimes called gray dolphins. They are found in the temperate and tropical zones of all the world’s oceans. These cetaceans generally prefers deeper offshore waters, especially near the continental shelf edge and slope, where they can dive to at least 300 meters (1,000 feet) and hold their breath for 30 minutes. They are also very active on the ocean surface. [Source: NOAA]
Risso's dolphins are typically found in groups of between 10 and 30 animals, though they have been reported as solitary individuals, in pairs, or in loose aggregations in the hundreds or thousands. Occasionally, this species associates with other dolphins and whales. They are sometimes considered part of a subfamily referred to as “blackfish,” which also includes false killer whales, pygmy killer whales, melon-headed whales, long-finned pilot whales, and short-finned pilot whales. Risso’s dolphins have an estimated lifespan of at least 35 years.
Risso's dolphins to prey on a mix of near surface, open ocean and bottom-dwelling marine organisms — mainly fish (such as, anchovies), krill, and cephalopods (such as, squid, octopus, and cuttlefish) and crustaceans. One of their most important prey item is the greater argonaut, which is also known as the paper nautilus. Although they are capable of making very deep dives, usually they make shorter dives of just a few minutes. Risso’s dolphins feed mainly at night, when their prey is closer to the surface. Most of their diet consists of squid, and they have been known to move into continental shelf waters when following their preferred prey. They also feed along the continental shelf in waters between 600 and 800 meters deep. [Source: Kelsey Hans, Animal Diversity Web (ADW) /=]
In the scientific name the word "grampus" is Latin for "a kind of whale" and the word "griseus" is Latin for "gray". Pelorus Jack, a famous Risso's dolphin, had the habit of playing about ships and seemed to guide them into Pelorus Sound in New Zealand. He was observed for 24 years around the turn of the century) escorting ships. [Source: Kelsey Hans, Animal Diversity Web (ADW) /=]
Websites and Resources: Britain-based Whale and Dolphin Conservation Society uk.whales.org ; Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Fishbase fishbase.se; Encyclopedia of Life eol.org; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems ; Monterey Bay Aquarium montereybayaquarium.org ; MarineBio marinebio.org/oceans/creatures
Risso's Dolphin Habitat and Where They Are Found
Risso's dolphins have a cosmopolitan distribution, meaning they can be found worldwide. They are generally found in temperate, subtropical, and tropical oceans and seas from latitudes 64° north to 46° south. Their preferred habitats appear to be mid-temperate waters of the continental shelf and slope between 30° and 45° latitude. They prefer deeper waters (1,000 meters, 3,300 feet) with steep bottom topography, but they are known to inhabit shallower coastal areas. [Source: NOAA]
In the Northern Hemisphere, their range includes the Gulf of Alaska, Gulf of Mexico, Newfoundland, Azores, Norway, Japan, Russia, and Red Sea. They are known to inhabit the Mediterranean Sea but are rare in the Black Sea. They do not appear to inhabit the Persian Gulf and some other very shallow, enclosed bodies of water. In the Southern Hemisphere, their range includes Argentina, Australia, Chile, South Africa, and New Zealand.
Risso's dolphins are typically found in waters with depths of 400 to 1,200 meters (1312 to feet), or even more. T hey are often seen near the edges of continental shelves, or near bathymetric features such as seamounts and submarine canyons. They are most commonly found in waters ranging in temperature from 15º to 20º C (59º to 68º F), but will inhabit waters cold as 10ºC (50º F). [Source: Kelsey Hans, Animal Diversity Web (ADW) /=]
Risso’s dolphins are present year round throughout most of their geographic range. Little is known of their migration patterns or movements but they may be affected by movements of spawning squid and oceanographic conditions. Information on the migrations of some populations and groups is known. Residents of the northern-most parts of their range migrate seasonally between summering and wintering grounds For example, populations off the coast of northern Scotland during the summer, migrate to the Mediterranean during the winter, and populations off the coast of California during the summer, migrate to Mexican waters during winter.
Risso's Dolphin Physical Characteristics
Risso's dolphins are medium-sized cetaceans. They range in length from 2.6 to 5 meters (8.5 to 16.4 feet), with their average length being 2.8 meters (9.2 feet). They range in weight from 300 to 500 kilograms (660 to 1100 pounds). Sexual Dimorphism (differences between males and females) is not so apparent. Both sexes are roughly equal in size and look similar. These dolphins are endothermic (use their metabolism to generate heat and regulate body temperature independent of the temperatures around them) and homoiothermic (warm-blooded, having a constant body temperature, usually higher than the temperature of their surroundings). [Source: Kelsey Hans, Animal Diversity Web (ADW) /=]
Risso's dolphins have a robust body with a narrow tailstock. Risso's dolphins have blunt, squarish, bulbous head with a vertical crease unique to the species and lack the beak associated with with many dolphin species. They have a tall, curved, sickle-shaped dorsal fin located mid-way down their back. Risso’s dolphins have two to seven pairs of peg-like teeth in the front of their lower jaw to capture prey and usually none in their upper jaw. This low number of teeth is unusual when compared with other cetaceans. [Source: NOAA **]
Risso's dolphin flippers are long, pointed, and recurved. They have two to seven pairs of sharp peg-like teeth in their lower jaw, which are specialized for capturing prey, fighting predators, and competing with conspecific for mates and resources. Evolutionary retention of these teeth may be partly due to their significance in male-male interactions. /=\
Risso's dolphins displays highly variable coloration. Calves have a dark cape and saddle, with little or no scarring on their body. As Risso's dolphins age, their coloration lightens from black, dark gray, or brown to pale gray or almost white. Adult bodies are usually heavily scarred, with scratches from teeth raking between dolphins, as well as circular markings from prey (such as, squid), cookie-cutter sharks, and lampreys. These distinctive 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 slow healing process. Some mature adults swimming just under the water's surface appear white because of the extensive scarring. Tooth rakes and scars from other Risso’s dolphin tend to be long and parallel and may act as an indicator of male fitness during aggressive social interactions. Risso's dolphins may be confused with bottlenose dolphins, false killer whales, and killer whales due to the shape and size of their dorsal fin. However, their blunt heads and extensive scarring stes them apart. ** /=\
Risso's Dolphin Behavior
Risso's dolphins are diurnal (active during the daytime), nocturnal (active at night), motile (move around as opposed to being stationary), and social (associates with others of its species; forms social groups). In terms of home range, There is no information regarding home-range size in Risso’s dolphins. [Source: Kelsey Hans, Animal Diversity Web (ADW) /=]
Risso's dolphins are typically found in groups that average between 10 and 30 animals, but they have been reported as solitary individuals, in pairs, or in loose aggregations of hundreds and thousands. Occasionally, this species associates with other dolphins and whales, such as bottlenose dolphins, gray whales, northern right whale dolphins, and Pacific white-sided dolphins. [Source: NOAA]
When at the surface, Risso's dolphins have a small inconspicuous blow if backlit (which is more distinct after long dives), and their head partially emerges at a 45° angle. Before diving, they usually take 10 to 12 breaths at 15- to 20-second intervals and will often display their tails (known as flukes). Risso’s dolphins are very active on the surface, often leaping out of the water, slapping their flippers or tails on the water surface, and raising their heads vertically out of the water. They occasionally porpoise — or move in and out of the water in a series of high-speed leaps — most often when being pursued or hunted by predators. [Source: NOAA]
According to Animal Diversity Web: Unlike most cetaceans, which tend to have either a fusion-fission or matrilineally based social system, Risso's dolphins display a stratified social system. Clusters are formed based on age and sex class, with strongest associations occurring between adult females and adult males. Female reproductive success is positively influenced by the increased social support and foraging benefits of larger pods. For example, while a given female is searching for food, she is able leave her calf in the care of other females in the group. As a result, female pods tend to be much larger than those of males.
Male formations experience a trade off in relation to size, as foraging benefits and habitat defense increase with pod size, but reproductive benefits decrease due to increased competition for mates. Risso's dolphins are very social, and as many as 4,000 individuals have been documented in a single pod. Although estimates for the average pod size have varied over the years, recent studies suggest a mean pod size of three to 12 individuals. /=\
Risso’s dolphins spend 77 percent of their time traveling, 13 percent engaged in social activity, five percent feeding, and about 3.7 percent resting. They feed at night, as that is when their primary prey, cephalopods, travel to the ocean surface. Risso's dolphins use a variety of behaviors to communicate with members of their own species, such as chasing, biting, aerial acrobatics, lob-tailing, and breaching. Males are often found harassing other species, such as false killer whales and bottlenose dolphins. Aggressive physical contact has been documented, such as flipper slapping between individuals, stiking with flukes and dorsal fins, and body blows Risso’s dolphins have been recorded associating and forming groups with other cetaceans, including bottlenose dolphins and Pacific white-sided dolphins. Hybrid offspring between bottlenose dolphins and Risso’s dolphins have been known to occur, both in captivity and in the wild.
Risso's Dolphin Perception and Communication
Risso’s dolphins communicate with vision and sound and sense using vision, touch, sound, echolocation (emitting sound waves and sensing their reflections to determine the location of objects) and chemicals usually detected by smell. [Source: Kelsey Hans, Animal Diversity Web (ADW) /=]
Risso's dolphins make a number of different vocalizations, including barks, buzzes, grunts, chirps, whistles, and simultaneous whistle and pulse sounds. Whistle and burst-pulse vocalizations thought to be unique to this species have not been recorded in other dolphin and whales.
Risso’s dolphins use echolocation to locate, identify, and determine the distance of various objects in their environment. The clicks made by dolphin are used in echolocation. The clicks of Risso's dolphins have a peak frequency of 65 kHz, 3-dB bandwidths of 72 kHz, and durations of 40 to 100 Ms, all of which are consistent with other delphinids. Risso’s dolphins are also able to emit sonar clicks in the water while the majority of their forehead is above water, a characteristic unique to this species. In addition to broadband clicks,
Risso's Dolphin Mating and Reproduction
Risso’s dolphins are viviparous, meaning they give birth to live young that developed in the body of the mother, and engage in year-round breeding, with seasonal peaks depending on hemisphere. The number of offspring is one. The gestation period ranges from 13 to 14 months. Females reach sexual maturity at eight to 10 years.
There is little information available regarding reproductive behavior in Risso’s dolphins though other Cetaceans tend to be either polygynous and polyandrous.. Individuals become sexually mature when they reach a length of about 2.6 to 2.8 meters (8.5 to 9 feet).
The peak of the breeding and calving season may vary geographically especially in the North Pacific, with most animal births occurring from summer to fall in Japanese waters and from fall to winter in California waters. Breeding and calving mainly occur during summer in the north Atlantic Ocean and and winter in the eastern Pacific.
Risso's Dolphin Parenting and Offspring
Young are precocial. This means they are relatively well-developed when born. They begin swimming immediately after birth. Parental care is provided by females. During the pre-birth and pre-weaning stages provisioning and protecting is done by females. The post-independence period is characterized by the association of offspring with their mothers. The weaning age ranges from 12 to 18 months.
Newborns range from 1.1 to 1.5 meters (3.6 to 5 feet) in length and average 20 kilograms (44 pounds) at birth. The youngest calves range in colour from iridescent gunmetal grey to fawn-brown on the back and are creamy-white on the bottom. 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.
Mother-calf pods form, and young usually do not leave the group until a few years before sexual maturity. Alloparental care (care to young provided by individuals other than parents) has been recorded amongst females. Often, while a calve's mother is foraging for food, another female provides care.
Risso's Dolphin, Humans, Threats and Conservation
Risso’s dolphins are not endangered or threatened. They are abundant and have a broad geographic range. The International Union for Conservation of Nature (IUCN) Red List classifies them as a species of “Least Concern”. The Convention on International Trade in Endangered Species (CITES) places them in Appendix II, which lists species not necessarily threatened with extinction now but may become so unless trade is closely controlled: Like all marine mammals, they are protected under the Marine Mammal Protection Act (MMPA). The Specially Protected Areas and Wildlife (SPAW) Annex II lists one population segmen — the Wider Caribbean.
One of the main threats to Risso’s dolphins is becoming entangled or captured in commercial fishing gear such as gillnets, longlines, and trawls. These dolphins are sometimes a by-catch in the U.S. tuna purse seine industry, and are taken occasionally in coastal gill net and squid seining industries off the U.S. coast. They are sometimes a considered a nuisance to fisherman. Risso’s dolphins are directly hunted for meat and oil in Indonesia, Japan, the Caribbean (the Lesser Antilles), Sri Lanka, and the Solomon Islands. [Source: NOAA]
Underwater noise threatens dolphin populations, interrupting their normal behavior and driving them away from areas important to their survival. Increasing evidence suggests that exposure to intense underwater sound in some settings may cause some dolphins to strand and ultimately die.
Contaminants enter ocean waters from many sources, including oil and gas development, wastewater discharges, urban runoff, and other industrial processes. Once in the environment, these substances move up the food chain and accumulate in predators at the top, such as Risso’s dolphins. Because of their long lifespan and blubber stores, Risso’s dolphins accumulate contaminants including trace metals and organochlorines like PCBs and DDTs in their bodies, threatening their immune and reproductive systems. Coastal populations are generally are more susceptible. [Source: NOAA]
Image Sources: Wikimedia Commons, NOAA
Text Sources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.
Last Updated June 2023