The porpoises are small cetaceans of the family Phocoenidae; they are related to whales and dolphins. They are however distinct from dolphins, although the word "porpoise" is often used to refer to any small dolphin, especially in North America. A key difference is the shape of the teeth and of the head.

Porpoises, divided into six species, live in all oceans, mostly near the shore. Probably best known is the Harbour Porpoise, which can be found across the northern hemisphere.

Anatomy

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 curved like that of many dolphins.

These animals are the smallest cetaceans, reaching body lengths up to 2.5 m; the smallest species is the Vaquita, reaching up to 1.5 m.

Behaviour

Porpoises are predators hunting mainly fish, often also squid and crustaceans. Most common are small groups of up to ten individuals, which in some species may join forming aggregations of several hundred animals. Different click and whistle sounds are used for communication. Like all toothed whales they are capable of echolocation by means of ultrasound. Porpoises are fast swimmers—Dall's porpoise is said to be one of the fastest cetaceans with a speed of 55 km/h. Porpoises tend to be less acrobatic and more wary than dolphins.

Taxonomy

• Order Cetacea
  • Suborder Mysticeti
    • Family Eschrichtiidae: Gray Whale
    • Family Balaenopteridae: rorquals
    • Family Balaenidae: right whales
    • Family Neobalaenidae: Pygmy Right Whale
  • Suborder Odontoceti
    • Family Platanistoidea: river dolphins
    • Family Delphinidae: oceanic dolphins
    • Family Phocoenidae
      • Genus Neophocaena
        • Finless Porpoise, Neophocaena phocaenoides
      • Genus Phocoena
        • Vaquita, Phocoena sinus
        • Harbour Porpoise, Phocoena phocoena
        • Burmeister's Porpoise, Phocoena spinipinnis
        • Spectacled Porpoise, Phocoena dioptrica
      • Genus Phocoenoides
        • Dall's Porpoise, Phocoenoides dalli
    • Family Monodontidae: Beluga and Narwhal
    • Family Physeteridae: Sperm Whale
    • Family Kogiidae: Pygmy Sperm Whale and Dwarf Sperm Whale
    • Family Ziphiidae: beaked whales

Evolution of porpoises

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. See evolution of cetaceans for the details.

Human impact

In many countries, porpoises are hunted for food or bait meat. Additionally, bycatch in fishing gear is responsible for a decline of the stocks. One of the most endangered cetacean species is the Vaquita, having a limited distribution in a highly industrialized area.

Porpoises are rarely held in captivity in zoos or oceanaria, in contrast to dolphins, which are far more popular.