Risso's dolphin is the fifth largest of the delphinids. Its anterior body is extremely robust, tapering to a relatively narrow tail stock and its dorsal fin is one of the tallest in proportion to body length of any cetacean, exceeded only by that of the adult male killer whale (Orcinus orca). The bulbous head has a distinct vertical crease or cleft along the anterior surface of the melon. Color patterns change dramatically with age. Infants are dorsally grey to brown, then darken to nearly black and lighten while maturing (the dorsal fin remaining dark). In ageing animals, the majority of the dorsal and lateral surfaces become covered with distinctive linear scars. Older animals can appear completely white on the dorsal surface.

This species is subject to by-catch, and is included in the UK Biodiversity Action Plan list of species of conservation concern (Biodiversity Steering Group, 1995). All species of cetaceans are given protection under the Wildlife and Countryside Act 1981 and the Wildlife (Northern Ireland) Order 1985. All cetacean species are listed on Annex A of EU Council Regulation 338/97 and therefore treated by the EU as if they are on CITES Appendix I therefore prohibiting their commercial trade. Risso's dolphin is also listed on Annex IV of the EC Habitats Directive, which prohibits all killing, capture, disturbance, sale, exchange or keeping of the animials. The directive requires that all accidental killing and capture be monitored by all member states. Also this species is listed under Appendix II of the Bonn Convention (North and Baltic Sea populations), and Appendix II of the Bern Convention. The Bonn Convention is the parent Convention to the 'Agreement on the Conservation of Small Cetaceans in the Baltic and the North Seas' (ASCOBANS) formulated in 1992. The UK is one of seven European countries which have signed the agreement which includes the protection of specific areas, monitoring, research, information exchange, pollution control and heightened public awareness. Measures included are specifically aimed at protecting dolphins and porpoises in the North Baltic Seas (Anon, 1999e).

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

Mediterranean and Red Sea.

Tropical and temperate waters worldwide.

occurs (regularly, as a native taxon) in multiple nations

Canada

Origin: Native

Regularity: Regularly occurring

Currently: Present

Confidence: Confident

Type of Residency: Transient

United States

Origin: Native

Regularity: Regularly occurring

Currently: Present

Confidence: Confident

Type of Residency: Year-round

Global Range: Not uncommon worldwide in tropical, subtropical, and warm temperate oceans. In the North Atlantic: Newfoundland to Lesser Antilles in the west, Sweden to the Mediterranean in the east. South to Argentina and South Africa in the south Atlantic. In the Pacific Ocean: Alaska and the Kuiles south to central Chile, Australia, and New Zealand. Occurs also in the Indian Ocean and Red Sea. May wander into cooler waters in summer. See Leatherwood et al. (1980) for information on distribution in the eastern Pacific Ocean.

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

Head-body length: 2.6 – 4 m; Weight: 300 – 500 kg.

Length: 400 cm

• Marine

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

Risso's dolphins are pelagic, mostly occurring seaward of the continental slope.

Habitat Type: Marine

Comments: Pelagic waters; generally in water >180 m deep (>1000 m deep according to IUCN 1991). In the northern Gulf of Mexico, occurs mainly in steep sections of the upper continental slope, where water depth is 350-975 m and gradient is greater than 24 m per 1.1 km (Baumgartner 1997).

Non-Migrant: No. All populations of this species make significant seasonal migrations.

Locally Migrant: No. No populations of this species make local extended movements (generally less than 200 km) at particular times of the year (e.g., to breeding or wintering grounds, to hibernation sites).

Locally Migrant: No. No populations of this species make annual migrations of over 200 km.

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 )

Comments: Diet mainly squid, occasionally small fishes.

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:

• sea lamprey (Petromyzon marinus)

There is no information available regarding predators specific to Risso's dolphin.

Usually occurs in groups; off California, groups averaged usually between 30-50, sometimes over 200; in the eastern tropical Pacific, groups averaged 15-26; in the northeastern Atlantic and Mediterranean, average group size was 6-7, with maximum of 20-30 (Leatherwood et al. 1980, IUCN 1991). Commonly associates with pilot whales.

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 ; acoustic ; echolocation

Risso's dolphins feed primarily on squid and are also known to feed on a variety of fish. Risso's dolphins are a gregarious species that form groups of between 3-30 individuals on average; however they have been known to form massive schools of thousands of dolphins. They are an agile species that perform leaps and fluke and flipper slapping.

Risso's dolphins are thought to reach sexual maturity when they reach between 2.6-2.8 m in length in both sexes. Little is known about the reproductive habits of this species. One young born at any time of year after gestation of 13-14 months. The lifespan is estimated at 20-40 years.

Comments: Active day/night.

On average Risso's dolphins live at least 30 years.

Average lifespan

Status: wild: 30 years.

Average lifespan

Status: wild: 20.0 years.

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.

Average birth mass: 20 kg.

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); 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

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

Accidental?

Not Assessed (not resident).

Very rare.

Canada

Rounded National Status Rank: NNA - Not Applicable

United States

Rounded National Status Rank: NU - Unrankable

Rounded Global Status Rank: G5 - Secure

Comments: Hunted only incidentally in small cetacean fisheries throughout the world (Leatherwood and Reeves 1983). The only known substantial threat is local--incidental take that occurs in association with the Sri Lankan gillnet fishery in the Indian Ocean (IUCN 1991).

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.

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

Comments: Sometimes maintained in marine aquaria. Meat is used locally for human consumption (IUCN 1991).