Risso’s dolphins use echolocation to locate, identify, and determine the distance of various objects in their environment. One of the most well-known sounds of delphinids are clicks. 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 dolphins make a number of different vocalizations, including barks, buzzes, grunts, chirps, whistles, and simultaneous whistle and pulse sounds. Whistle and burst-pulse vocalizations have not been reported in other cetaceans and are thought to be unique to this species.
Communication Channels: visual ; acoustic
Perception Channels: visual ; tactile ; acoustic ; echolocation ; chemical
There is no information available regarding predators specific to Risso's dolphin.
Risso's dolphins have blunt, squarish heads and lack the beak typical of other delphinids. The dorsal fin is tall and falcate, and their flippers are long, pointed, and recurved. The anterior part of the body is very robust, tapering to a narrow tailstock. Adults range from 2.6 to 4 m in length, with an average body mass around 400 kg. The sexes are similar in size. Newborns range from 1.1 to 1.5 m in length and average 20 kg at birth. Along the body axis on the melon (i.e., beak, eyes, blowhole) there is a slight concave groove which is a unique characteristic of this species. Sexual dimorphism has not been reported in this species.
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. (Nishiwaki 1972, Kruse et al. 1999, MMSC 1996,
This species displays highly variable coloration. 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. In older animals, lip colour frequently contrasts with the surrounding background. Coloration fades with age, and some adults appear almost completely white due to the linear scarring that accumulates on individuals over time. 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 slower healing process than that observed in animals such as bottlenose dolphins. Scarification can be caused by other Risso's dolphins, predators (e.g., cookie cutter sharks), prey, or by parasites like sea lamprey. Intraspecific, tooth rake, scars tend to be long and parallel and may act as an indicator of male fitness during aggressive social interactions.
Risso's dolphins lack teeth in their upper jaws, but have 2 to 7 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 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 make them unmistakable.
Range mass: 300 to 500 kg.
Range length: 2.6 to 5 m.
Average length: 2.8 m.
Other Physical Features: endothermic ; homoiothermic; bilateral symmetry
Sexual Dimorphism: sexes alike
On average Risso's dolphins live at least 30 years.
Average lifespan
Status: wild: 30 years.
Average lifespan
Status: wild: 20.0 years.
Risso's dolphins are pelagic, but prefer habitat on steep slopes, ranging from 400 to 1,200 meters. They 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 59 to 68 degrees F, but will inhabit waters cold as 50 degrees F.
Risso’s dolphins are present year round throughout most of their geographic range. 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.
Range depth: 400 to 1,200 m.
Habitat Regions: temperate ; tropical ; saltwater or marine
Aquatic Biomes: pelagic ; benthic ; coastal
Grampus griseus has an extensive distribution. The species can be found in temperate, subtropical and tropical waters of oceans worldwide.
Biogeographic Regions: indian ocean; atlantic ocean ; pacific ocean ; mediterranean sea
Other Geographic Terms: cosmopolitan
Risso's dolphins are known to prey on a mix of neritic, oceanic, and occasionally bottom dwelling organisms. Their diet consists of fish, krill, crustaceans, and cephalopods. Their most important prey item is the greater argonaut, which is also known as the paper nautilus. They often follow prey into shallow waters along the continental shelf, and prefer to feed between 600 and 800 m below the surface of the sea.
Animal Foods: fish; mollusks; aquatic crustaceans
Primary Diet: carnivore (Molluscivore )
Risso's dolphins consume large amounts of fish, krill, crustaceans, and cephalopods and likely have a significant influence on the abundance of these animals. Risso's dolphins are one of many hosts for sea lamprey, which is common in shoreline habitat throughout north Atlantic.
Commensal/Parasitic Species:
In Indonesia and the Caribbean, Risso’s dolphins are hunted for their meat and oil. In Japan, they are taken periodically for food and fertilizer. Small numbers are sometimes collected for live exhibitions.
Positive Impacts: food ; body parts are source of valuable material
Risso’s 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 high trophic level consumers. As a result, their tissues accumulate pollutants that are prevalent throughout their geographic range, a process known as bioaccumulation, and consuming the meat of this species could be harmful.
Risso's dolphins are abundant and have a broad geographic range. As a result, they are classified as a species of "least concern" on the IUCN's Red List of Threatened Species. However, because little is known of current population trends, it is difficult to estimate potential conservation needs. Potential threats include direct killings for meat and oil in the Indian Ocean, and by-catch in the north Atlantic, the Mediterranean Sea, the southern Caribbean, the Azores, Peru, and the Solomon Islands. Because this species relies on echolocation to hunt, it is also thought that anthropogenic sounds may influence local populations. Recent climate change may also influence their range and abundance, however, potential effects are currently unclear.
US Federal List: no special status
IUCN Red List of Threatened Species: least concern
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. It was observed for 24 years (around the turn of the century) escorting ships.
There is no information regard the mating system of Grampus griseus. However, other Cetaceans tend to be either polygynous and polyandrous.
There is little information available regarding reproductive behavior in Grampus griseus. Most females are sexually mature by 8 to 10 years old, however, size is often a better indicator of sexual maturity than age in marine cetaceans. Most males reach sexually maturity at a length of 2.6 to 2.8 m. Gestation lasts 13 to 14 months, and average mass of newborns calves is 20 kg. Weaning is complete by 12 to 18 months after parturition. Breeding and calving occur year-round, but peak during summer and winter in the north Atlantic and eastern Pacific, respectively.
Breeding season: Grampus griseus breeds year round, but peaks seasonally depending on hemisphere.
Range gestation period: 13 to 14 months.
Range weaning age: 12 to 18 months.
Range age at sexual or reproductive maturity (female): 8 to 10 years.
Key Reproductive Features: year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization ; viviparous
Average number of offspring: 1.
Female Risso's dolphins are the primary care givers to calves, and paternal care, which is rare in other cetaceans, has not been documented in this species. Newborns are precocial and begin swimming immediately after birth. Mother-calf pods form, and young usually do not leave the group until a few years before sexual maturity. Alloparental care has been recorded amongst females. Often, while a calve's mother is foraging for food, another female provides care.
Parental Investment: precocial ; female parental care ; pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female); post-independence association with parents
Mediterranean and Red Sea.
Accidental?
Risso's dolphin (Grampus griseus) is a dolphin, the only species of the genus Grampus. Some of the closest related species to these dolphins include: pilot whales (Globicephala spp.), pygmy killer whales (Feresa attenuata), melon-headed whales (Peponocephala electra), and false killer whales (Pseudorca crassidens).[5]
Risso's dolphin is named after Antoine Risso, whose study of the animal formed the basis of the recognized description by Georges Cuvier in 1812.[6] The holotype referred to specimen at the Muséum National d'Histoire Naturelle, an exhibit using preserved skin and skull obtained at Brest, France.[7]
The type and sole species of the genus Grampus refers to Delphinus griseus Cuvier 1812. A proposition to name this genus Grampidelphis in 1933, when the taxonomic status of 'blackfish' was uncertain, and conserving the extensive use of "Grampus" for the 'killer' Orcinus orca", also suggested renaming this species (Grampidelphis exilis Iredale, T. & Troughton, E. le G. 1933).[8] These were recognised as synonyms after publication of the Catalog of Whales (Hershkovitz, 1966).[7]
Another common name for the Risso's dolphin is grampus (also the species' genus), although this common name was more often used for the orca. The etymology of the word "grampus" is unclear. It may be an agglomeration of the Latin grandis piscis or French grand poisson, both meaning big fish. The specific epithet griseus refers to the mottled (almost scarred) grey colour of its body.
Risso's dolphin has a relatively large anterior body and dorsal fin, while the posterior tapers to a relatively narrow tail. The bulbous shape of the head has a vertical crease in front.[9]
Infants are dorsally grey to brown and ventrally cream-colored, with a white anchor-shaped area between the pectorals and around the mouth. In older calves, the nonwhite areas darken to nearly black, and then lighten (except for the always dark dorsal fin). Linear scars mostly from social interaction eventually cover the bulk of the body; scarring is a common feature of male to male competition in toothed whales, but Risso's dolphin tend to be unusually heavily scarred. The pronounced appearance of these scars results from the lack of repigmentation, which may be advantageous as a display that reduces further challenges from other males.[10] Older individuals appear mostly white. Most individuals have two to seven pairs of teeth, all in the lower jaw.[9]
Length is typically 10 feet (3.0 m), although specimens may reach 13 feet (4.0 m).[11] Like most dolphins, males are typically slightly larger than females. This species weighs 300–500 kilograms (660–1,100 lb), making it the largest species called "dolphin".[12][13]
Risso's dolphins are found worldwide in temperate and tropical waters, in the Indian, Pacific and Atlantic Oceans, also the Persian Gulf, Mediterranean and Red Seas, but not the Black Sea (a stranding was recorded in the Sea of Marmara in 2012[14]). They range as far north as the Gulf of Alaska and southern Greenland and as far south as Tierra del Fuego.[9]
Their preferred environment is just off the continental shelf on steep banks, with water depths varying from 400–1,000 m (1,300–3,300 ft) and water temperatures at least 10 °C (50 °F) and preferably 15–20 °C (59–68 °F).[9] They are recorded diving to depths of 600 metres in pursuit of prey.[15]
The population around the continental shelf of the United States is estimated to be in excess of 60,000. In the Pacific, a census recorded 175,000 individuals in eastern tropical waters and 85,000 in the west. No global estimate exists.
Since 2017, Risso´s dolphins have began to appear off the Norwegian coast, at Bleik´s Canyon, Andenes. The repeated regular sightings imply an expansion of their natural range. Possible explanations are changing climate and currents as well as northwards expansion of prey species or competition with other species in their natural range, e.g. pilot whales.
They feed almost exclusively on neritic and oceanic squid, mostly nocturnally. Predation does not appear significant. Mass strandings are infrequent.[9] Analysis carried out on the stomach contents of stranded specimens in Scotland showed that the most important species preyed on in Scottish waters is the curled octopus (Eledone cirrhosa).[16]
A population is found off Santa Catalina Island where they are sympatric with short-finned pilot whales (Globicephala macrorhynchus) and both species feed on the squid population. Although these species have not been seen to interact with each other, they take advantage of the commercial squid fishing that takes place at night. They have been seen by fishermen to feed around their boats.[17] They also travel with other cetaceans. They surf the bow waves of gray whales, as well as ocean swells.[9]
Risso's dolphins have a stratified social organisation.[18] These dolphins typically travel in groups of between 10 and 51, but can sometimes form "super-pods" reaching up to a few thousand individuals. Smaller, stable subgroups exist within larger groups. These groups tend to be similar in age or sex.[19] Risso's experience fidelity towards their groups. Long-term bonds are seen to correlate with adult males. Younger individuals experience less fidelity and can leave and join groups. Mothers show a high fidelity towards a group of mother and calves,[18] but it is unclear whether or not these females stay together after their calves leave or remain in their natal pods.
Like many dolphin species, they use echolocation to target cephalapods and fish that are feeding below. Tagging of a population in the Azores revealed that Grampus griseus plan whether to make a shallow or deep dive, with different strategies that create profitable foraging for the considerable expenditure in time and energy. Risso's can achieve depths over 600 metres by exhausting their lungs and using several spins to rapidly descend, almost vertically, and increase the time spent foraging. This allows the species to exploit a deep and dispersed layer of prey such as squid, those taking refuge during daylight when they become more vulnerable to predation.[15]
Risso's dolphins do not require cutting teeth to process their cephalopod prey, which has allowed the species to evolve teeth as display weapons in mating conflicts.[10]
Gestation requires an estimated 13–14 months, at intervals of 2.4 years. Calving reaches seasonal peaks in the winter in the eastern Pacific and in the summer and fall in the western Pacific. Females mature sexually at ages 8–10, and males at age 10–12. The oldest specimen reached 39.6 years.[9]
Risso's dolphins have successfully been taken into captivity in Japan and the United States, although not with the regularity of bottlenose dolphins or orcas. Hybrid Risso's-bottlenose dolphins have been bred in captivity.
Like other dolphins and marine animals, there have been documentations of these dolphins getting caught in seine-nets and gillnets across the globe.[5] Many of these incidents have resulted in death.[19] Small whaling operations have also been cause of some of these deaths. Pollution has also affected many individuals who have ingested plastic. Samples from these animals shows contamination within their tissue.[5]
In Ireland, though not apparently in England, Risso's Dolphin was one of the royal fish which by virtue of the royal prerogative were the exclusive property of the English Crown.[20]
A famed individual named Pelorus Jack was widely reported between 1888 and 1912, travelling with ships navigating the Cook Strait in New Zealand. A law protecting the animal was passed after a public outcry, renewed twice more, but suggested be invalid by its reference to Fisheries acts that did not concern marine mammals.[21]
The Risso's dolphin populations of the North, Baltic, and Mediterranean Seas are listed on Appendix II[22] of the Convention on the Conservation of Migratory Species of Wild Animals (CMS), since they have an unfavourable conservation status or would benefit significantly from international co-operation organised by tailored agreements.[23]
In addition, Risso's dolphin is covered by the Agreement on the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas (ASCOBANS),[24] the Agreement on the Conservation of Cetaceans in the Black Sea, Mediterranean Sea and Contiguous Atlantic Area (ACCOBAMS),[25] the Memorandum of Understanding for the Conservation of Cetaceans and Their Habitats in the Pacific Islands Region (Pacific Cetaceans MoU)[26] and the Memorandum of Understanding Concerning the Conservation of the Manatee and Small Cetaceans of Western Africa and Macaronesia (Western African Aquatic Mammals MoU).[27]
Risso's dolphins are protected in the United States under the Marine Mammal Protection Act of 1992. Currently, Japan, Indonesia, the Solomon Islands, and The Lesser Antilles hunt Risso's dolphins.[19]
At least one case report of strandings in Japan's Goto Islands has been associated with parasitic neuropathy of the eighth cranial nerve by a trematode in the genus Nasitrema.[28] There was a recent reporting of a juvenile male Risso's dolphin that was stranded alive on the coast of Gran Canaria on 26 April 2019. This was the first documented case of capture myopathy and stress cardiomyopathy in a male juvenile Risso's dolphin that has received rehabilitation.[29]
Risso's dolphin (Grampus griseus) is a dolphin, the only species of the genus Grampus. Some of the closest related species to these dolphins include: pilot whales (Globicephala spp.), pygmy killer whales (Feresa attenuata), melon-headed whales (Peponocephala electra), and false killer whales (Pseudorca crassidens).
