squid, octopuses and fishes
to 12.5 feet (3.8 m), 650 pounds (300 kg) to 1,100 pounds (500 kg)
bottlenose dolphin, common dolphin, Pacific white-sided dolphin, orca; Order: Cetacea; Family: Delphinidae
tropical and temperate waters worldwide, excluding the Arctic Ocean and Southern Ocean
Like a battered boxer, the bulky, blunt-headed Risso's dolphin bears lots of scars. Its Latin species name griseus refers to the skin's ghostly gray-white mottling—an effect enhanced with age by extensive scarring made by the teeth of its own kind or by the beaks and tentacles of squid, its preferred prey.
Famous French anatomist Georges Cuvier formally described the species in 1812 and named it for naturalist Antoine Risso. One of the largest kinds of dolphin, the Risso's dolphin performs an array of acrobatic behaviors including breaching, spyhopping and slapping flukes or flippers on the sea surface.
Most common along the continental slope or in the open ocean, Risso's dolphins dwell over deep water in order to hunt vertically migrating squid, octopuses and other cephalopods at night. The marine mammals may seek the glow from their preys' bioluminescent bodies. The stomachs of stranded dolphins in the Mediterranean contained only the remains of cephalopods from 25 different species.
Risso's dolphins typically gather in groups of a few up to 30 related individuals of similar age and sex but have been known to form "super-pods" a few thousand strong. Recent research suggests this species has a unique form of social organization for dolphins, with the most stable groups segregated by age and sex. Risso's dolphins can be seen swimming alongside several other dolphin species and with gray whales.
In Monterey Bay
Somewhere between 11,000 and 16,000 Risso's dolphins live along California and the Pacific Northwest coast. In recent decades, Risso's dolphins have been spotted more regularly in Monterey Bay and can be seen with Pacific white-sided dolphins and northern right whale dolphins. Some evidence suggests that Risso's dolphins are particularly abundant in the Monterey region during winter and come closer to shore during periods of higher surface water temperature. Risso's dolphins are the largest dolphin species to visit Monterey Bay, other than the orca.
Further south, Risso's dolphin numbers have increased in the Southern California Bight, a coastal region from Point Conception to the Mexico border. This species has become common around Santa Catalina Island and may have displaced pilot whales along some stretches of the coast.
Historically, Risso's dolphins died in large numbers after being caught in purse seine nets along with tuna. This species benefited from regulation that led to more dolphin-safe tuna fishing practices. Unintended bycatch of dolphins in gill net and longline fisheries poses a potential threat.
In Sri Lanka, Risso's dolphins are the second most commonly caught cetacean, with around 1,300 per year taken to provide meat for people and for fish bait. Hunting of Risso's dolphins also occurs in Japan, Indonesia, the Solomon Islands and the Lesser Antilles.
- The Risso's dolphin lacks teeth in the upper jaw and has no more than seven pairs of conical teeth in the lower jaw.
- Risso's dolphins have prominent dorsal fins, sometimes prompting observers at a distance to think they're orcas or white sharks instead.
- A Risso's dolphin named Pelorus Jack became famous for guiding boats across Cook's Strait in New Zealand and sparked the first government protection for a cetacean after someone fired a rifle at the curious dolphin in 1904.
- This species has been known to hybridize with bottlenose dolphins in captivity and on occasion in the wild.