Risso’s Dolphin Facts

(Grampus griseus)

HOW DO THEY LOOK? PHYSICAL CHARACTERISTICS.

It is a very robust dolphin, devoid of a snout. If only its dorsal fin appears above the surface of the ocean, it can be confused with a bottlenose dolphin (Tursiops truncatus) or with an orca (Orcinus orca), since the shape of its fin resembles that of orcas and Tursiops truncatus.

Weight and size.
The length of its body is on average 3 meters, but some individuals reach up to 4.3 meters. It has a body mass of 300 to 500 kilograms.

Morphology.
This dolphin has a compact and almost square head, similar to that of pilot whales (Globicephala Family), and it also lacks a beak. The anterior part of the body is sturdy but narrows towards the tail. Its dorsal fin with a sickle shape is large and curved, located in the center of the back. The pectoral flippers are long and thin and bent backward in a semi-straight line. The caudal peduncle is very narrow.

Skin Coloration.
It varies according to the age of the dolphin. Young specimens have a metallic gray or brown skin in the dorsal area and whitish in the belly. The snout has an ocher color, and between the pectoral flippers, there is a light spot with an anchor shape. As they grow, the coloration fades, and the adults appear to be almost white. They have 2 to 7 pairs of teeth in the lower jaw

Distinctive characteristics.
Its superior jaw is devoid of teeth, and on the melon, there is an external vertical groove that extends from that organ to the mouth. Another distinctive sign of the species is the presence of scar marks on the back and sides due to a lack of repigmentation and a very slow healing process.

WHERE DO THEY LIVE? DISTRIBUTION AND HABITAT.

This dolphin has a wide distribution in temperate, tropical and subtropical waters of the oceans of the world between the latitudes 60° north and 55° south. In the northern hemisphere, Scotland is the limit of its distribution, and in the southern hemisphere, it does not travel beyond the Cape of Good Hope, Australia, and New Zealand. Its distribution zones include the Gulf of Alaska, the Gulf of Mexico, the Persian Gulf and the Red Sea.

The countries that host the species include: Australia, Bahamas, Barbados, United Kingdom, United Arab Emirates, Peru, Philippines, Papua New Guinea, Madagascar, Mexico, Chile, China, Canada, United States, Cook Islands, Brazil, Cayman Islands, Argentina, Costa Rica, Haiti, India, Indonesia, Japan, Jamaica, Mozambique, Denmark, Ecuador, New Zealand, Taiwan and France.

It prefers to inhabit deep waters of the continental slope and seldom ventures to visit the coastal areas. It avoids polar waters and likes those with temperatures between 15° to 20° Celsius but can thrive in waters up to 10° Celsius. It dwells on steep slopes, edges of the continental shelf, mountains and submarine canyons.

WHAT DO THEY EAT? DIET AND EATING HABITS.

The daily diet of the Risso dolphin is composed of fish, crustaceans such as krill and cephalopods such as squid, octopus, and cuttlefish. Thier favorite food is Argonauta argo, a species of pelagic octopus.

Usually, they seek their food at night when the prey travels to the surface of the ocean, but they can submerge to depths less than 1,000 meters (their usual range is 600-800 meters) for 30 minutes to get food. Occasionally they feed in cooperation with other dolphins.
Their teeth only serve to hold their prey firmly, as this dolphin swallows its victims whole.

HOW DO THEY REPRODUCE? MATING AND REPRODUCTION.

It is very likely that Risso’s dolphin reproduces throughout the year but with regional variations. In southern Africa, for example, births occur mainly during December.

The breeding practice is not known, but the hypotheses focus on polygamy and multiple mating during the same season (polyandry). The gestation period lasts between 13 and 14 months; Subsequently, only one offspring of about 20 kilograms of weight is born.

The mother cares the calf until it leaves the pod, a few time after being weaned at 12-18 months of age.

Females mature sexually at the age of 8 to ten years, but males reach maturity around 10 and 12 years of age. Mothers give birth every 2.4 years.

HOW DO THEY BEHAVE? BEHAVIOR

Thanks to research, it is known that Risso’s dolphins spend almost 4 percent of their time resting, 5 percent feeding, 13 percent socializing and 77 percent traveling. Their migration patterns are unknown, but not all populations usually migrate.

Their pods have 10-50 members and organize according to their age and gender, being the adult female-male relationships the strongest bonds, but female groups are much larger than male ones. The latter is due to a cooperative behavior characterized by the support in caring the offspring among the female mothers. They are also able to form groups with other cetaceans.

At times, they show aggression to their companions hitting with the flippers and the tail, and they have been seen chasing bottlenose dolphins (Tursiops truncatus). They communicate through clicks, whistles, grunts and squeals and use echolocation to get an image of their surroundings.

WHAT IS THEIR CONSERVATION STATUS? THREATS AND CONSERVATION.

Conservation Status: “Least Concern.”

This dolphin has a “Least Concern” conservation status in the Red List of the International Union for the Conservation of Nature (IUCN). Nevertheless, this does not mean that they lack threats to its survival, as these include:

– Intentional catch to obtain meat for the human consumption, bait for fishing and oil.
– Bycatch.
– Reduction of their prey and food availability.
– Habitat Loss.
– Climate change that modifies its distribution and populations.

The Risso’s dolphin is still unthreatened, so it is essential to maintain its populations and improve their living conditions by reducing the impact of anthropogenic factors.

 

 

References

Jefferson, Webber, Pitman. Marine Mammals of the World: A Comprehensive Guide to Their Identification. Academic Press, 2015. Page 210.

http://www.cms.int/reports/small_cetaceans/data/g_griseus/g_griseus.htm

Gerardo Ceballos. Mammals of Mexico. JHU Press, Oct 9, 2014

 

 

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