distribution. The species can be found in temperate, subtropical and tropical waters of oceans worldwide. ("NOAA fisheries", 2010; Amano and Miyazaki, 2004; Nuno, 2008; Pawloski, et al., 2003; Stewart, et al., 2002)has an extensive
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. ("NOAA fisheries", 2010; Leatherwood, et al., 1980; Nuno, 2008; Pawloski, et al., 2003; Taylor, et al., 2010)
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. (Culik, 2010)
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. ("NOAA fisheries", 2010; Jefferson, et al., 1993; Pawloski, et al., 2003; Stewart, et al., 2002)
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, ("NOAA fisheries", 2010; Jefferson, et al., 1993; Stewart, et al., 2002)
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. ("NOAA fisheries", 2010; Hartman, et al., 2008; Jefferson, et al., 1993; MacLeod, 1998)
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. ("NOAA fisheries", 2010; Jefferson, et al., 1993; MacLeod, 1998; Stewart, et al., 2002)
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. ("NOAA fisheries", 2010)
There is no information regard the mating system of Cetaceans tend to be either polygynous and polyandrous. (Stewart, et al., 2002). However, other
There is little information available regarding reproductive behavior in 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. ("NOAA fisheries", 2010; Nuno, 2008). 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
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. (Hartman, et al., 2008)
On average Risso's dolphins live at least 30 years. ("NOAA fisheries", 2010)
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 3 to 12 individuals. (Hartman, et al., 2008; Kruse, et al., 1999; Nuno, 2008; Stewart, et al., 2002)
Risso’s dolphins spend 77% of their time traveling, 13% engaged in social activity, 5% feeding, and about 3.7% 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 conspecifics, 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 (Kruse 1999, MMSC 1996). ("NOAA fisheries", 2010; Hartman, et al., 2008; Kruse, et al., 1999; Nuno, 2008; Stewart, et al., 2002)
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. ("NOAA fisheries", 2010; Kruse, et al., 1999)
There is no information regarding home-range size in Risso’s dolphins.
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. (Corkeron and Van Parijs, 2001; Pawloski, et al., 2003)
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. ("NOAA fisheries", 2010; Jefferson, et al., 1993; Kruse, et al., 1999; Raga, et al., 2006; Stewart, et al., 2002)
There is no information available regarding predators specific to Risso's dolphin. (Stewart, et al., 2002)
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. ("NOAA fisheries", 2010; Stewart, et al., 2002)
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. ("NOAA fisheries", 2010; Kruse, et al., 1999)
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. (Stewart, et al., 2002; Storelli and Macrotrigiano, 2000)
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. (Stewart, et al., 2002)
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. (Kruse, et al., 1999)
Kelsey Hans (author), University of Michigan-Ann Arbor, Phil Myers (editor), University of Michigan-Ann Arbor, John Berini (editor), Animal Diversity Web Staff.
the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.
body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.
uses sound to communicate
Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
an animal that mainly eats meat
uses smells or other chemicals to communicate
the nearshore aquatic habitats near a coast, or shoreline.
having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.
The process by which an animal locates itself with respect to other animals and objects by emitting sound waves and sensing the pattern of the reflected sound waves.
animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.
parental care is carried out by females
union of egg and spermatozoan
A substance that provides both nutrients and energy to a living thing.
eats mollusks, members of Phylum Mollusca
having the capacity to move from one place to another.
specialized for swimming
active during the night
An aquatic biome consisting of the open ocean, far from land, does not include sea bottom (benthic zone).
mainly lives in oceans, seas, or other bodies of salt water.
reproduction that includes combining the genetic contribution of two individuals, a male and a female
associates with others of its species; forms social groups.
uses touch to communicate
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
uses sight to communicate
reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.
breeding takes place throughout the year
young are relatively well-developed when born
2010. "NOAA fisheries" (On-line). Office of Protected Resources. Accessed March 12, 2011 at http://www.nmfs.noaa.gov/pr/species/mammals/cetaceans/rissosdolphin.htm.
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