Risso's Dolphin

Introduction to Risso's Dolphin

Grampus(Grampus griseus) , which is also known as the Risso’s dolphin, is considered to be the only species of dolphins in the genus of Grampus. It is also commonly known as the Monk dolphin among the many Taiwanese fishermen. The term grampus has been originally derived from the Latin terms for great(Grandis) and fish(Pisces) and these terms have been applied to many large fishes and it has also been used to address many cetaceans which include large killer whale that is also the members of the family Delphinidae.


Pilot whales (Globicephala spp.), pygmy killer whales (Feresa attenuata), melon-headed whales (Peponocephala Electra), and false killer whales (Peponocephala Electra) are some of the nearest relatives of these dolphins (Pseudorca crassidens).


Grampus, also known as Risso's dolphin, is a typical offshore resident of tropical and temperate ocean waters and a member of the dolphin family (Delphinidae). The grampus has a blunt head and a distinct longitudinal forehead crease and measures about 4 meters (13 feet) in length. It is unusual to dolphins in that it normally has no upper teeth and a lower jaw of zero to seven teeth. The heads and trunks of older males are extensively scarred, presumably from encounters with other grampuses.


In this article we are going to discuss the Risso’s dolphin which is also known as Grampus, its geographic range, habitat, Physical description, Reproduction, lifespan, food habits, and also all the related frequently asked questions will also be answered.


Taxonomy of the Risso’s Dolphin

Risso's dolphin was named after Antoine Risso, whose account was used as the basis for Georges Cuvier's first public description of the species in 1812. Grampus is one of the common names of the Risso's dolphin. The grampus was the common name that was also used to describe the orca. The origins of the term "grampus" are unknown. It may be a combination of the Latin grandis piscis and the French grand poisson, all of which signify large fish. The epithet griseus refers to the mottled (almost scarred) grey color of the animal's fur.


Range and Habitat 

Range 

  • The Risso’s dolphins can be found in deep oceanic and continental slope waters in both hemispheres, from the tropics to the temperate zones. They can be found in Newfoundland, Norway, the Kamchatka Peninsula, and the Gulf of Alaska in the north to the southernmost points of South America and Africa, as well as southern Australia and New Zealand.

  • The Risso's dolphins are found worldwide in temperate and tropical waters. They can be found in the Indian ocean. The Pacific Ocean and the Atlantic oceans but they can also be found in the Mediterranean, Red Sea, and the Persian Gulf except for the Black sea. They can be found all the way from the Gulf of Alaska to southern Greenland, and all the way down to Tierra del Fuego.

Habitat 

  • The Risso’s dolphins are very large dolphins that are often found surfacing slowly although they are also found to be very energetic in nature sometimes breaching or porpoising, and occasionally bow-riding. Herds are mostly small to modest in number, although groups of up to 4,000 have been seen. Other populations of cetaceans also interact with Risso's dolphins. Both in captivity and in the wild, hybrids between this animal and the bottlenose dolphin have been observed.

  • There seems to be a summer calving peak in the North Atlantic. Crustaceans and cephalopods are eaten by Risso's dolphins, but they tend to prefer squid. Squid bites may be to blame for some of the marks on these animals' bodies.

  • The Risso's dolphins usually prefer the environment which is just off the continental shelf on the steep banks, with the water depth that varies between 1300 to 3300 feet and the water temperature which should be at least around 10-degree celsius and the preferable temperature should be between 15–20 degree celsius.

  • Around the continental shelf, it is estimated that the population of the  Risso's dolphins is around sixty thousand. In the Pacific ocean, the population of the  Risso's dolphins is said to be two lakhs but there are not accurate global numbers of these species. 

Physical Description of the Risso’s Dolphin

  • The fifth largest dolphin in the Delphinidae family is the Risso's dolphin. The common name was given to the species by Antoine Risso, a French-Italian naturalist who first described it in 1812.

  • Risso’s dolphins are very unusual looking as their anterior body is considered to be extremely robust in nature. They lack a distinct beak and are blunt-headed. Their very tall dorsal fin, as well as their distinctive coloration and scarring, make them readily identifiable at sea. Adults are greyish in colour and usually have white scratches, spots, and blotches on them. They will grow to be up to 4 metres long.

  • Risso’s dolphins scars are presumed to be caused by the intra-species interactions. They are usually gregarious, travelling in groups of 10 to 50 people, with the biggest groups seen including over 4,000 creatures.They are found to travel in groups with other cetacean species such as the grey whales. 

  • The Risso’s dolphins are known to be very shy and they avoid boats and human contact.Based on few behavioural findings, they seem to eat mostly squid and hunt at night.

  • Risso's dolphins have a squarish, blunt head and no beak like most delphinids. Their flippers are long, pointed, and recurved, and their dorsal fin is tall and falcate. The body's front half is very powerful, tapering to a thin tailstock.An adult Risso's dolphins  ranges from 3 to 4 meters in length and has an average body mass of around 400 kilograms. The sexes are found to be similar in size.The newborn Risso's dolphins range  from 1.1 to 2 meters in length and have an average body mass of 20 kilograms at birth. A faint concave groove runs along the melon's body axis, which is the beak, eyes, and blowhole  that is considered to be a   special feature of this species. There hasn't been any evidence of sexual dimorphism in this genus.

  • When it comes to the young calves of the Risso's dolphins they don't have a particular color as the youngest calves are iridescent gunmetal grey to fawn-brown on the dorsal side and creamy-white on the ventral side. The muzzle is highlighted with pale ochre-yellow highlights. The white anchor-shaped patch between the flippers looks similar to the chest chevron seen on pilot whales, except it's usually lighter and larger. Calves turn silver-grey, then darken to nearly black, with white spots on the ventral side. Animals' heads, abdomens, and flanks lighten as they get older.

  • The Risso's dolphins as they age are the colour of their body. The colour of the lips always varies with the surroundings. Because of the linear scarring that accumulates on people over time, some adults become almost entirely white as they mature. The scars appear mainly on the animals' dorsal and lateral surfaces, and are thought to be the result of a combination of factors including a lack of repigmentation of exposed tissue and a longer healing period than seen in bottlenose dolphins.

  • The Risso’s dolphins are known to lack teeths in their upper jaws but they do have a 8 pairs of sharp peg like teeths in their lower jawas which are basically present to capture and eat their prey.The teeth are also used to fight the predators and also to compete with other animals for the resources. Because of the appearance and scale of their dorsal fin, Risso's dolphins can be mistaken for bottlenose dolphins, false killer whales, and killer whales. Their blunt heads and significant scarring, though, distinguish them.

  • The image given below describes the Risso's dolphins

[Image will be Uploaded Soon]


Reproduction in the Risso's Dolphins

  • In the Risso's dolphins, the gestation period is between 13 to 14 months at an interval of 2.4 years.Calving peaks seasonally in the eastern Pacific during the winter, and in the western Pacific during the summer and fall. Most females are sexually mature by the age of eight to ten, but in marine cetaceans, size is always a greater measure of sexual maturity than age. At a length of 2.6 to 2.8 m, most males attain sexual maturity. The estimated gestation period is 13 to 14 months, and the average weight of newborn calves is 20 kilogrammes. Weaning is completed between 12 and 18 months after birth. Breeding and calving take place all year, but in the north Atlantic and eastern Pacific, breeding and calving are at their best in the summer and winter, respectively.

  • A woman Risso's dolphins are the calves' main caregivers, and parental care, which is uncommon in other cetaceans, has not been recorded in this group. Newborns are precocial, and they begin swimming as soon as they are born. Mother-calf pods develop, and young do not normally leave the group until they are sexually mature. Females have been shown to provide alloparental treatment. When a calf's mother is out foraging for fruit, another female takes care of the calve.

Behaviour of the Risso's Dolphins

  • Risso's dolphins are known to have a hierarchical social structure. Clusters are established based on age and sex class, with adult females and adult males having the greatest connections. The expanded social reinforcement and foraging advantages of larger pods have a favourable impact on female reproductive success. For example, a female will leave her calf in the care of other females in the group when she goes food hunting. As a result, female pods are significantly larger than male pods.

  • In male Risso's dolphins there is a formation where there is a trade-off between foraging and habitat protection as pod size increases, but reproductive advantages decline due to intensified competition for mates. Risso's dolphins are highly social creatures, with pods containing up to four thousand people. While figures for the average pod size have fluctuated over time, recent research indicates a three to twelve person average pod size.

  • Risso's dolphins spend 77% of their time travelling, 13% socialising, 5% feeding, and 3.7 percent resting. They eat at night because that's when their main prey, cephalopods, come to the surface of the ocean. To interact with their peers, Risso's dolphins engage in a number of activities, including chasing, chewing, aerial acrobatics, lob-tailing, and breaching. Males of other animals, such as false killer whales and bottlenose dolphins, are also seen harassing them.

  • The Risso's dolphins are also found to be very aggressive as the use of aggressive physical action, such as flipper slapping, sticking with flukes and dorsal fins, and body blows, has been recorded.Other cetaceans, such as bottlenose dolphins and Pacific white-sided dolphins, have been seen associating and joining communities with Risso's dolphins. Both in captivity and in the wild, hybrid offspring of bottlenose dolphins and Risso's dolphins have been observed.

Conservation of the Risso's Dolphins

The Risso's dolphins are abundantly found everywhere in the world and they do have a geographic range. As they are found everywhere they are classified as the species of ‘least concern’ on the IUCN's Red List of Threatened Species.However, estimating future conservation requirements is complicated since little is understood about global population patterns of the Risso's dolphins. They are directly killed for meat and oil in the Indian Ocean, as well as by-catch in the north Atlantic, Mediterranean Sea, southern Caribbean, Azores, Peru, and Solomon Islands, are both potential threats for these species. Since this creature hunts by echolocation, it's possible that anthropogenic sounds have an effect on local communities. Recent climate change may have an impact on their distribution and abundance, although the implications are currently unknown.

FAQs (Frequently Asked Questions)

Q1: What are the Food Habits of the Risso's Dolphins?

Ans: Risso's dolphins are known to eat a variety of species, including neritic, oceanic, and bottom-dwelling organisms. Fish, krill, crustaceans, and cephalopods make up the majority of their diet. The greater argonaut, also known as the paper nautilus, is one of their most important prey items. They often pursue prey into deeper waters around the continental shelf, preferring to feed 600 to 800 metres underneath the sea's surface.

Q2: What is the Role of Risso's Dolphins in the Ecosystem?

Ans: Risso's dolphins are known to eat a lot of tuna, krill, crustaceans, and cephalopods, and they probably have a big impact on the abundance of these species. Sea lamprey, which is abundant in shoreline habitat in the north Atlantic, is one of several hosts for Risso's dolphins.