The Risso’s dolphin is a widely ranging dolphin that lives in the open ocean in areas with depths of at least 1500-3300 feet (450-1000 m). Though they are born grayish brown, by the time they reach adulthood, they can appear to be almost completely white, as a result of scarring that covers their heads and bodies. This dolphin is relatively large, reaching lengths over 13 feet (4 m) and weights of up to 1100 pounds (500 kg). There are only a few species in the dolphin family that grow to be larger than the Risso’s dolphin (e.g., killer whale, long-finned pilot whale, etc.).
Risso’s dolphins feed primarily on squid of all sizes (including the humboldt squid) and are therefore excellent divers, spending much of their feeding time at depth, hunting for their preferred prey. Scientists believe that much of the white scarring on the heads of these animals may be a result of aggressive interactions with large-bodied squids. Risso’s dolphins feed mostly at night, a probable result of the migration of their preferred prey to shallower waters in the dark. Scarring is also a likely result of interactions among adult males and between males and females.
This species, like all mammals, reproduces via internal fertilization and gives birth to live young, which nurse from their mothers for at least several months.
Though Risso’s dolphins are offered some or complete legal protection throughout much of their range, they are still hunted for food and for fertilizer in some places and are captured accidentally in fisheries targeting other species throughout most of their range. Current population trends are not known, but scientists believe the Risso’s dolphin to be common and to be a species of least concern. Further study of this species and monitoring of its populations is required to determine whether or not they are stable.