Risso's Dolphin

Risso's Dolphin (Grampus griseus) is a marine mammal within the dolphin family and represents the sole species of the genus Grampus. Closely related species include the pilot whales (Globicephala spp.), pygmy killer whales (Feresa attenuata), melon-headed whales (Peponocephala electra), and false killer whales (Pseudorca crassidens). Reaching lengths of approximately 10 feet (3 meters), Risso's dolphins are discernible by their distinctive white scarring patterns. These dolphins are geographically widespread, inhabiting cold to temperate marine environments, favoring areas along continental shelves due to their dietary preferences. Their primary prey consists of cephalopods, which they can pursue at significant depths, as they are capable of diving nearly 600 meters (2,000 feet). Typically, Risso's dolphins form social pods ranging from 10 to 50 members, fostering strong social bonds within these groups.

Like many marine species, Risso's dolphins are susceptible to human-induced environmental disruptions. Recognized threats include pollution (both acoustic and plastic), and the risk of entanglement in fishing gear. Although protected under 🇺🇸 United States laws, they continue to be hunted in certain regions globally.

Antoine Risso, whose research facilitated the initial scientific description by Georges Cuvier in 1812, lends his name to the genera and species. The holotype specimen is preserved at the Muséum National d'Histoire Naturelle, consisting of a skin and skull collected in Brest, France. The species name "Delphinus griseus Cuvier" proposed in 1812, along with subsequent nomenclatural reformulations, are now acknowledged as synonyms post-publication in the Catalog of Whales (Hershkovitz, 1966).

Risso's dolphins possess a robust anterior body with a substantial dorsal fin, tapering to a narrow tail. The species exhibits a characteristic bulbous head with a vertical crease. Younger dolphins present a dorsal grey to brown coloration with a ventral cream hue, featuring a distinctive anchor-shaped white area. With age, these nonwhite areas darken, then lighten, primarily due to non-repigmenting scars accumulated through social interactions and competition. This scarring, abundant in older individuals, has a potential role in deterring challenges among males.

The species is widely distributed, spanning the 🇮🇳 Indian, Pacific, and Atlantic Oceans, along with specific seas such as the Baltic, 🌊 Persian Gulf, and the Mediterranean, potentially influenced by ecological factors prompting range expansions. Their preferred habitats are often located along steep continental slopes with water depths between 400 and 1,000 meters, and temperatures from 10°C to 20°C. Risso's dolphins have been recorded diving to 600 meters to forage, primarily feeding on squid during nighttime.

Social organization within Risso's dolphins is complex. They typically travel in structured pods and occasionally form larger "super-pods" of several thousand individuals. These groups often comprise younger dolphins or those segregated by sex and age. Long-term group fidelity is notable among adult males, whereas younger dolphins exhibit a higher tendency for group changes. Females show strong affinity to pods formed around maternal care, although their post-weaning associations remain ambiguous.

Echolocation is a primary tool for hunting, enabling Risso's dolphins to target prey at varying depths. Tagging studies conducted in regions such as the Azores reveal their strategic planning for successive deep and shallow dives based on prey availability. Surface behavior is highly interactive, involving tail slaps, pectoral fin displays, and spy-hopping—potentially serving as a social or mating display.

Reproductive cycles require 13–14 months of gestation. Sexual maturity occurs at 8–10 years for females and 10–12 years for males, with observed longevity extending to 39.6 years. While Risso's dolphins have been held in captivity, hybridization with bottlenose dolphins has been suggested as a rare occurrence without distinctive evolutionary advantages.