Sea Wonder: Risso’s Dolphin

Sometimes called gray dolphins, the Risso’s dolphin (Grampus griseus) is part of a subfamily of whales called blackfish, which includes false killer whales, pilot whales, pygmy killer whales, and melon-headed whales. They don’t have the long beaks we usually associate with dolphins, they are quite active, and they are often seen associating with other species of whales and dolphins.

Description

In 1812, French anatomist Georges Cuvier formally described Risso’s dolphins and and named them after naturalist Antoine Risso. This species is one of the largest kinds of dolphins. They have distinctly bulbous heads and a tall, curved dorsal fin located mid-way down their backs. They can reach lengths of about 13 feet and weigh as much as 1,100 pounds when fully grown. Risso’s dolphins are born with dark gray skin that lightens as they age, with scars from teeth raking, a social behavior between dolphins that involves scratching each other with their teeth. Risso’s dolphins have notably fewer teeth than other dolphin species

Diet & Habitat

Four Risso’s dolphins jumping out of the water together in Monterey Bay National Marine Sanctuary. Photo: Douglas Croft under NOAA permit #20519

Risso’s dolphins are opportunistic feeders, meaning they eat whatever is available and easiest to catch, with preferences for fish, squids, and cuttlefish. They generally feed at night when their preferred prey moves closer to the ocean’s surface, though Risso’s dolphins can dive at least 1,00 feet and hold their breath as long as 30 minutes.

Risso’s dolphins live in temperate, subtropical, and tropical oceans worldwide. They seem to prefer deeper, open ocean waters but may venture near the coast depending on each population’s home range and where their prey travel. Throughout the National Marine Sanctuary System, we can find Risso’s dolphins near almost all of the oceanic marine sanctuaries, including: Channel Islands, Cordell Bank, Florida Keys, Greater Farallones, Hawaiian Islands Humpback Whale, Monterey Bay, Olympic Coast, and Stellwagen Bank national marine sanctuaries.

Life History

Risso’s dolphins are quite social, usually forming groups of 10 to 30 animals. However, scientists and observers have seen them socializing with other species of whale and dolphin and gathering in huge groups of hundreds or thousands of animals for short periods of time, usually where food is plentiful. Their social interactions often lead to high energy behaviors like jumping out of the water, slapping their fins on the surface, and bobbing their heads over and under the water. Scientists still have much to learn about reproduction in Risso’s dolphins, but we know they fertilize internally, mate when food is plentiful and environmental conditions are appropriate, and give birth to one live calf after a pregnancy of a little more than a year.

Risso’s dolphin calves usually weigh about 45 pounds and are three to five feet long when they are born. They nurse from their mothers for most of their first year, if not longer, learning to hunt and eat solid foods as they grow. Risso’s dolphins can live to be at least 35 years old and likely reach maturity when they are fully grown, which likely happens before they turn 10 years old.

Threats & Conservation

Risso’s dolphins receive protection in U.S. waters under the Marine Mammal Protection Act and are not considered threatened or endangered throughout their range. However, the species, like many others in the ocean, faces threats like entanglement in fishing gear, decreased availability of prey, noise pollution, and environmental changes associated with coastal development and climate change. In some cultures, people hunt Risso’s dolphins for their meat and oil.