Risso's Dolphins


by Amy West

Snubnose, Gray porpoise, Whitehead Grampus, Bean Pot. These monikers all describe the same whitish-gray dolphin found off continental shelves worldwide, the Risso’s dolphin (Grampus griseus). Its characteristic stocky and blunt head, so unlike the smiling beak found on a bottlenose dolphin, and the scratches crisscrossing a Risso’s body make this large cetacean fairly easy to identify when you encounter one.

That isn’t that common, though. Seeing these dolphins, which can reach nearly 13 feet, close to land is rare. But in a few places where the continental shelf drops off near the coast, creating habitat for deepwater fish and the dolphin’s favorite prey, squid, you might catch a glimpse of Risso’s cruising in groups of 10-50. On the Pacific Coast these places include Monterey Bay, the Gulf of the Farallones and a swath of Southern California from Point Conception to San Diego.

Close to shore in these zones you may find Risso’s swimming among Pacific white-sided dolphins, evident by their distinct white belly, or Northern Right Whale dolphins, which have no dorsal fin and could be mistaken for a porpoising sea lion when viewed from a distance. Scientists lump Risso’s into the blackfish group—signifying tall dorsal fins and blunt heads like their brethren pilot and killer whales.

Sarah Allen, a National Park Service marine ecologist and author of the Field Guide to Marine Mammals of the Pacific Coast, observed Risso’s dolphins in the Monterey Bay when doing ship surveys 20 years ago—long before Humboldt squid, now a favored food source for the dolphins, moved to the area. Despite their enduring presence in the bay, “These dolphins are just hard to study,” she says. “We really don’t know much about them.” It’s expensive to tag them, she explains, and since these mammals typically inhabit deeper waters, scientists know very little about their habits, reproductive lives or range.

It’s challenging to study any marine mammal, but life history with holes is particularly disturbing because the dolphin slaughter in Taiji, Japan, which captured the attention of many worldwide in the award-winning documentary The Cove, includes Risso’s.

One thing scientists do know about these marine animals is a unique aspect of their ability to locate prey by echolocation, or bouncing sound off objects for guidance. “Normally marine mammals direct their sonar straight ahead, but Risso’s direct it downward,” says Allen. They can also be cooperative fishers, and have been spotted herding fish.

If you happened to be something this dolphin likes to chomp on, you would find teeth only on this predator’s lower jaw. Getting that close would also reveal another unique feature: a groove in its head from its mouth to forehead. The groove’s function is a mystery; however, the linear scars lining their body are not. These markings become white once they heal and increase as the dolphin ages. It’s believed they arise from fighting with other Risso’s and from attacks by orcas and sharks.

A Risso’s dolphin named Rocky made the news in 2005 after washing up on a Delaware beach. On his way to becoming the first Risso’s dolphin to be rehabilitated, Rocky burned through 10 tons of squid and nearly $200,000 in expenses before his marine mammal caretakers released him off Hampton Bays in New York eight months later.

Though recognizing this dolphin is fairly easy above water, they also have some signature sounds underwater. These clicks and whistles are most likely the last thing a squid “hears” before a Risso’s dolphin gobbles it up.

Find great photos of Risso's at Arkive.

Photo credit: Mike Baird, Flickr/Creative Commons