Commerson's dolphins are small oceanic dolphins. They are named after French naturalist Dr. Philibert Commerson, who first described them in 1767 after sighting them in the Strait of Magellan. These dolphins have a black head, dorsal fin, and fluke, with a white throat and body. Their black and white pattern varies with age, sex, and geographic location. The appearance of these dolphins resembles that of a porpoise, but the conspicuous behaviour of Commerson's dolphins is typical of a dolphin. Males and females are easily distinguished by the different shape of the black blotch on the belly. It is shaped like a teardrop in males but is more rounded in females.
Commerson's dolphins have two geographically-isolated but locally-common subspecies. The main subspecies is found around the tip of South America and the other subspecies is found around the Kerguelen Islands in the Indian Ocean. The main subspecies occur inshore in various inlets in Argentina including Puerto Deseado, in the Strait of Magellan and around Tierra del Fuego, and near the Falkland Islands. Dolphins of the second subspecies reside near the Kerguelen Islands in the southern part of the Indian Ocean and prefer shallow waters.
Commerson's dolphins are very social animals. They typically live in groups ranging from just a few individuals to more than 100. Sometimes these dolphins feed alone or more commonly they can be observed in cooperative feeding. Commerson's dolphins are very active. They are known for their high-speed swimming, reaching up to 11 - 13 km/h. These dolphins are often seen swimming rapidly on the surface and leaping from the water. They also spin and twist as they swim and may surf on breaking waves when very close to the shore. They will bow-ride and swim behind fast-moving boats. Commerson's dolphins are also known to swim upside-down, which is thought to improve the visibility of their prey.
Little is known about the mating system in Commerson's dolphins. Mating occurs in the spring and summer. The gestation period lasts for about 11 months, after which one calf is born. There is no information about the length of the nursing period in the wild. It is known that in captivity calves begin eating solid food at 2 months of age and take whole fish at 4 months. Females reach breeding age at 6 to 9 years. Males reach sexual maturity at about the same age.
Main threats to Commerson's dolphins are pollution and humans. The proximity of the dolphin to the shore makes accidental killing in gillnets and other fishing gear a common occurrence. At least 5 - 30 dolphins die each year in Tierra del Fuego as by-catch in nets set to the shore.
According to IUCN, the Commerson's dolphin is abundant and widespread throughout its range but no overall population estimate is available. However, a survey in 1984 estimated there to be 3,400 individuals in the Strait of Magellan. Currently, Commerson's dolphins are classified as Least Concern (LC) on the IUCN Red List.