Risso’s dolphins, scientifically known as Grampus griseus, are marine mammals sometimes referred to as Grampus or gray dolphins. Their unique appearance and specialized diet make them a distinct presence in marine ecosystems worldwide.
Distinctive Appearance
Risso’s dolphins have a robust body, reaching 8.5 to 13 feet and weighing between 660 and 1,100 pounds. They possess a blunt, rounded head that lacks a distinct beak. A vertical crease is often visible on their bulbous forehead, known as the melon.
Risso’s dolphins’ skin coloration undergoes a transformation with age. Calves are born dark gray or brownish-black, which gradually lightens to a pale gray or almost white as they mature. This change is largely attributed to the accumulation of numerous white scars across their bodies. These scars, which can be scratches, splotches, or circular marks, are a defining characteristic, sometimes covering so much of an adult’s body that it appears entirely white.
These white markings originate from two sources: interactions with other Risso’s dolphins and encounters with prey. During social interactions, dolphins may rake their teeth against each other, causing linear scars. Circular scars result from grasping cephalopod prey like squid and octopus, which can leave suction cup marks. The healing tissue in these areas lacks pigmentation, making the scars noticeable.
Habitat and Diet
Risso’s dolphins are found in temperate and tropical waters across the globe, inhabiting all major oceans. While they can occasionally be seen closer to shore, they prefer deeper, offshore waters, particularly along continental shelves and slopes where depths range from 1,300 to 3,300 feet. They thrive in water temperatures between 50 and 70 degrees Fahrenheit.
Their diet consists of cephalopods, primarily squid, but also octopus and cuttlefish. Risso’s dolphins are night feeders, as their prey migrate closer to the water’s surface at night. They can dive nearly 2,000 feet and hold their breath for up to 30 minutes while foraging.
The dentition of Risso’s dolphins is adapted for their specialized diet. Unlike most dolphin species, they have only two to seven pairs of peg-like teeth in the front of their lower jaw, and usually none in their upper jaw. This limited number of teeth is suited for grasping and manipulating soft-bodied cephalopod prey, which they swallow whole.
Behavior and Social Structure
Risso’s dolphins are social animals, commonly observed in groups known as pods. These groups range in size from 10 to 30 individuals, though aggregations of hundreds or thousands have been reported in areas with abundant food resources. Within larger pods, smaller, stable subgroups often form, sometimes segregated by age and sex. For example, adult females and their calves may form separate nurseries.
These dolphins exhibit active surface behaviors. They are often seen breaching, leaping clear of the water, and may slap their pectoral flippers or tails on the surface. They also spy-hop, raising their heads vertically out of the water to survey surroundings. While some groups can be shy, young dolphins are known for energetic displays, including surfing on waves.
Risso’s dolphins communicate using vocalizations. They produce clicks for echolocation, which helps them locate prey, and use whistles and pulsed sounds for communication within their pods. Their social interactions can be physically assertive, which is thought to contribute to the extensive scarring on their bodies. They have also been observed associating with other cetacean species, such as bottlenose dolphins and gray whales.