Porpoise

Porpoises Temporal range: 15.970–0Ma PreЄ Є O S D C P T J K Pg N Miocene to Recent
Phocoena phocoena, harbour porpoise near Denmark
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Cetacea
Suborder:	Odontoceti
Superfamily:	Delphinoidea
Family:	Phocoenidae Gray, 1825
Genera
See text

Porpoises (/ˈpɔrpəs/; also called mereswine) are small cetaceans of the family Phocoenidae; they are related to whales and dolphins. They are distinct from dolphins, although the word "porpoise" has been used to refer to any small dolphin, especially by sailors and fishermen. The most obvious visible difference between the two groups is that porpoises have shorter beaks and flattened, spade-shaped teeth distinct from the conical teeth of dolphins.

The name derives from French pourpois, possibly from Medieval Latin porcopiscis (porcus pig + piscis fish; cf. classical porcus marīnus ("sea hog").^[1]

Porpoises, divided into six species, live in all oceans, and mostly near the shore. Freshwater populations of the finless porpoise also exist. Probably the best known species is the harbour porpoise, which can be found across the Northern Hemisphere. Like all toothed whales, porpoises are predators, using sounds (echolocation in sonar form) to locate prey and to coordinate with others. They hunt fish, squid, and crustaceans.

Taxonomy and evolution

Porpoises, along with whales and dolphins, are descendants of land-living ungulates (hoofed animals) that first entered the oceans around 50 million years ago (Mya). During the Miocene (23 to 5 Mya), mammals were fairly modern. The cetaceans diversified, and fossil evidence suggests porpoises and dolphins diverged from their last common ancestor around 15 Mya. The oldest fossils are known from the shallow seas around the North Pacific, with animals spreading to the European coasts and Southern Hemisphere only much later, during the Pliocene.^[2]

Suborder Odontoceti toothed whales

• Infraorder Delphinida
  • Superfamily Delphinoidea
    • Family Phocoenidae - porpoises
      • Genus †Haborophocoena^[3]
        • H. toyoshimai
      • Genus Neophocaena
        • N. phocaeniodes - Finless porpoise
      • Genus †Numataphocoena^[4]
        • N. yamashitai
      • Genus Phocoena
        • P. phocoena - harbour porpoise
        • P. sinus - vaquita
        • P. dioptrica - spectacled porpoise
        • P. spinipinnis - Burmeister's porpoise
      • Genus Phocoenoides
        • P. dalli - Dall's porpoise
      • Genus †Septemriocetus^[5]
        • S. bosselaersii

• • • • Genus †Piscolithax
        • P. aenigmaticus
        • P. longirostris
        • P. boreios
        • P. tedfordi

Recently discovered hybrids between male harbour porpoises and female Dall's porpoises indicate the two species may actually be members of the same genus.^[6]

Physical characteristics

A harbour porpoise at an aquarium. In the wild, porpoises rarely jump out of the water.

Porpoises tend to be smaller but stouter than dolphins. They have small, rounded heads and blunt jaws instead of beaks. While dolphins have a round, bulbous "melon", porpoises do not. 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 and large whales. Some species have small bumps, known as tubercles, on the leading edge of the dorsal fin. The function of these bumps is unknown.^[6] Because porpoises have a spongy material within their loose, layered skin which beats rhythmically with the motion of the water to strongly reduce water drag, porpoises can swim faster than any other creature in the sea.^{[citation needed]}

These animals are the smallest cetaceans, reaching body lengths up to 2.5 metres (8.2 ft); the smallest species is the vaquita, reaching up to 1.5 metres (4.9 ft). In terms of weight, the lightest is the finless porpoise at 30 to 45 kilograms (66 to 99 lb), and the heaviest is Dall's porpoise at 130 to 200 kilograms (290 to 440 lb). Because of their small size, porpoises lose body heat to the water more rapidly than other cetaceans. Their stout shape, which minimizes surface area, may be an adaptation to reduce heat loss. Thick blubber also insulates them from the cold. The small size of porpoises requires them to eat frequently, rather than depending on fat reserves.^[6]

Life history

Porpoises bear young more quickly than dolphins. Female Dall's and harbour porpoises often become pregnant with a single calf each year, and pregnancy lasts for about 11 months. Porpoises have been known to live 8–10 years, although some have lived to be 20.^[6]

Behavior

"Rooster tail" spray around swimming Dall's porpoises

Porpoises prey on 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, referred to as pods. 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 (34 mph). Porpoises tend to be less acrobatic and more sexually aggressive than dolphins.^[7]

Humans and porpoises

Accidental entanglement (bycatch) in fishing nets is the main threat to porpoises today^{[8][citation needed]}. One of the most endangered cetacean species is the vaquita, having a limited distribution in the Gulf of California, a highly industrialized area. In some countries, porpoises are hunted for food or bait meat.^{[citation needed]}

Porpoises are rarely held in captivity in zoos or oceanaria, as they are generally not as capable of adapting to tank life or as easily trained as dolphins.^{[citation needed]}

See also

• List of cetaceans by population
• Unihemispheric slow-wave sleep

References

External links

Media related to Phocoenidae at Wikimedia Commons

• Whale Trackers - An online documentary series about whales, dolphins and porpoises.