The Dusky Dolphin is a dolphin found in coastal waters in the Southern Hemisphere. Its specific epithet is Latin for "dark" or "dim". It is very closely genetically related to the Pacific white-sided dolphin, but current scientific consensus is that they are distinct species. The dolphin's range is patchy with major populations around South America, southwestern Africa, New Zealand and various oceanic islands with some sightings around southern Australia and Tasmania. The dusky dolphin prefers cool currents and inshore waters, but can also be found offshore. It feeds on a variety of fish and squid species and has flexible hunting tactics. The dusky dolphin is known for its remarkable acrobatics, having a number of aerial behaviors. The status of the dolphin is unknown but it has been commonly caught in gill nets.
The dusky dolphin is small to medium in length compared with other species in the family. There is significant variation in size among the different population areas. The largest dusky dolphins have been encountered off the coast of Peru, where they are up to 210 cm (6 feet) in length and 100 kg (210 pounds) in mass. The size for dusky dolphins in New Zealand have been recorded to be a length range of 167–178 cm and a weight range of 69–78 kg for females and a length range of 165–175 cm and a weigh range of 70–85 kg for males.
There is almost no sexual dimorphism in this species, although males have more curved dorsal fins with broader bases and greater surface areas. The back of the dolphin is dark grey or black, and the dorsal fin is distinctively two-toned—the leading edge matches the back in color, but the trailing edge is a much lighter greyish white. Dusky dolphins have a long, light grey patch on their foreside leading to a short, dark grey beak. The throat and belly are white, and the beak and lower jaw are dark grey. There are two blazes of white color running back on the body from the dorsal fin to the tail. Right between the white areas remains a characteristic thorn-shaped patch of dark color, by which the species can easily be recognized. Aside from that, dusky dolphins may be confused with other members of their genus when observed at sea. It can be distinguished from the common dolphins, which have a more prominent and longer beak and yellow flank markings. The skull of a dusky dolphin has a longer and narrower rostrum than that of a hourglass dolphin or Peale's dolphin of similar age and size.
Dusky dolphins prefer cool, upwelling waters as well as cold currents. They largely live in inshore waters and can be found up to the outer continental shelf and in similar zones in offshore islands.
Dusky dolphins prey consume a variety of fish and squid species. Common fish species eaten include anchovies, lantern fish, pilchards, sculpins, hakes, horse mackerel, hoki and red cod. They are generally coordinate hunters. These dolphins have very flexible foraging strategies that can change depending on the environment. In certain parts of New Zealand, were deep oceanic waters meet the shore, dusky dolphins forage in deep scattering layers at night. They arrive at the hunting site individually but form groups when in the layer. The dolphins use their echolocation to detect and isolate an individual prey. Groups of foraging dolphins tend to increase when the layer is near the surface and decrease when it descends.
Dusky dolphins perform a number of aerial displays. Displays include leaps, backslaps, headslaps, tailslaps, spins and noseouts. The dolphins also perform head-over-tail leaps which is has been called the most "acrobatic" of the displays. A headfirst re-entry is performed when a dolphin leaps entirely water and positions its back in a curve while it flips the tail to land back in the water head-first. "Humping" is similar except the snout and tail remain in the water when the dolphin is the arch. Leaps, head-over-tail leaps, backslaps, headslaps, tailslaps and spins are often done over and over again. Young dusky dolphins apparently are not born with the ability to perform the leaps and must learn to master each one. Calves appear to learn the leaps in the following order: noisy leaps, head first re-entries, coordinated leaps and acrobatic leaps. Adult dusky dolphins may perform different leaps in different contexts and calves may independently learn how to perform leaps, but learn when to perform these when interacting others.