Risso’s  Dolphin

Risso’s dolphins are deep-water marine mammals. They are one of the larger members of the dolphin family and the only species of the genus Grampus. These dolphins are very active on the ocean’s surface. They often leap out of the water, slap its surface or pop their heads out in an action known as spy-hopping.

They are named after a nicard and naturalist, Antoine Risso, who studied the animal. Based on his study, the French naturalist and zoologist Georges Cuvier described it in 1812. ‘Grampus’ is another common name used for Risso’s dolphin.

Description

Length: 120 – 156 in (300 – 400 cm). Males and females are of the same size.

Weight: 660 – 1100 lb (300 – 500 kg).

Body and Coloration: Baby Risso’s dolphins have a greyish-brown color on the top and a cream-color at the bottom. There is a white anchor-shaped area between the mouth and the chest. Their body color is brownish grey when they are juveniles. As they grow older, their body color becomes paler due to scarring. The scars are caused by other Risso’s dolphins, parasites, or squid bites.

These dolphins have rounded heads emerging vertically from the upper jaw. They have a prominent dorsal fin, prompting distant observers to mistake them for an orca or a great white shark. Sometimes they are considered part of a subfamily called ‘blackfish.’

Range and Distribution

Risso’s dolphins live in the temperate and tropical waters of the Indian, Atlantic, and Pacific oceans. They can also be found in the Persian Gulf, the Mediterranean, and the Red Sea. Their range is to the Gulf of Alaska and Southern Greenland in the North and Tierra del Fuego in the South. 

Habitat

They stay in deep, tropical, temperate, and offshore waters mostly at depths of 400 – 1000 meters. Their favorite living place is the mid-temperate waters of the continental shelf and slope between the latitudes of 30⁰ and 45⁰. They prefer to live in deep water where they can dive to at least 1000 feet and hold their breath for 30 minutes.

Diet

These dolphins are carnivorous whose favorite food is squid (cephalopod), but they also eat prawns, shrimp, and cuttlefish.

Behavior

• Risso’s dolphins are generally found in groups of 10 to 30 animals. They are solitary individuals, sometimes in pairs or loose aggregations of hundreds and thousands. Occasionally, they also associate with other dolphins and whales.
• If threatened by a predator, they are seen moving in and out of the water in a series of high-speed leaps. 
• Risso’s dolphins migrate to cooler regions during the summer months. However, their migration patterns are yet to be found.
• They are night feeders because their primary prey, squids, come to the water’s surface at nighttime.

Lifespan

On average, Risso’s dolphins have a lifespan of around 35 years.

Adaptations

• They can hold their breath for 30 minutes, enabling them to dive deep into the water, indicating strong lung capacity.  These dolphins can delve to at least 1000 feet but mostly take shorter dives where they need to hold their breath for just a few minutes.
• Risso’s dolphins have 2-7 pairs of peg-like teeth in the front of the lower jaw, which helps them to capture prey.  

Mating and Reproduction

The mating season of Risso’s dolphins varies based on their geographic location. Their gestation period is 13 to 14 months. They gain reproductive maturity when they are 8-10 years of age and reach a length of about 8.5 to 9 feet. 

Breeding and calving occur year-round, and the peak breeding and calving season varies based on location. The ones residing in the Japanese water give birth between summer and fall, whereas the ones in California waters give birth between fall to winter. The newborns are 3.5-5.5 long and weigh about 45 pounds. They give birth to a single baby who is born independent.

Predators

Predators of the Risso’s dolphin include killer whales and sharks.

Conservation Status

The IUCN has placed the Risso’s dolphin on the “Least Concerned” or “LC” list. As per the census record, there are 60,000 individuals in the United States, 175,000 in the eastern and 85,000 in the western Pacific. Their population is currently not under threat.

Interesting Facts

• A Risso’s dolphin named “Pelorus Jack” is famously known for guiding boats across Cook Strait in New Zealand that sparked the first government protection for a Cetacean in 1904. This happened after someone fired a rifle at the curious dolphin. 
• These dolphins hybridize with bottlenose dolphins in oceans and also in captivity. 
• Because of their large dorsal fin, Risso’s dolphins are often confused with other female and juvenile killer whales at sea.
• They perform a variety of acrobatic behavior at the sea surface like skyhopping, breaching, and slapping its tail flukes or pectoral flippers.