Porpoises tend to be smaller but stouter than dolphins. They have small, rounded heads and blunt jaws instead of beaks. Their teeth are spade-shaped, whereas dolphins have conical teeth. In addition, a porpoise's dorsal fin is generally triangular, rather than falcate (curved) like that of many dolphins and large whales.
These animals are the smallest cetaceans, reaching body lengths up to 2.5 m (8 feet); the smallest species is the Vaquita, reaching up to 1.5 m (5 feet).
Porpoises are predators of fish, squid, and crustaceans. Although they are capable of dives up to 200 m, they generally hunt in shallow coastal waters. They are found most commonly in small groups of fewer than ten individuals. Rarely, some species form brief aggregations of several hundred animals. Like all toothed whales they are capable of echolocation for finding prey and group coordination. Porpoises are fast swimmers—Dall's porpoise is said to be one of the fastest cetaceans, with a speed of 55 km/h (15 m/s). Porpoises tend to be less acrobatic and more wary than dolphins. When a porpoise swims upside down, it is searching for a mate.
The Harbour porpoise (Phocoena phocoena) is one of 6 species of porpoise, and so one of about eighty cetacean species. The Harbour porpoise, as its name implies, stays close to coastal areas or river estuaries and as such is the most familiar porpoise to whale watchers. This porpoise often ventures up rivers and has been seen hundreds of miles from the sea.
The species is sometimes known as the Common porpoise in texts originating in the United Kingdom, though this usage appears to be dying out.
Porpoises, along with whales and dolphins, are descendants of land-living mammals, most likely of the Artiodactyl order. They entered the water roughly 50 million years ago.
In many countries, porpoises are hunted for food or bait meat. Additionally, bycatch (accidental entanglement) in fishing nets is responsible for a decline in porpoise numbers. One of the most endangered cetacean species is the Vaquita, having a limited distribution in the Gulf of California, a highly industrialized area.
Porpoises are rarely held in captivity in zoos or oceanaria, as they are generally not as capable of adapting to tank life nor as easily trained as dolphins.
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