Porpoise

Porpoises Fossil range: ?Latest Oligocene - recent
Phocoena phocoena, Harbour Porpoise
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Mammalia Order: Cetacea Family: Phocoenidae Gray, 1825
Genera
Neophocaena Phocoena - Harbor porpoise Phocoenoides - Dall's porpoise

The porpoises are small, marine cetaceans of the family Phocoenidae. Porpoises belong to the "toothed whales" suborder Odontoceti, along with whales and dolphins.

Although scientifically, the term porpoise should be reserved for members of the family Phocoenidae, historically the term has been often applied in common venacular to any small cetacean, especially by sailors and fishermen (Grzimek 2004).

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.

Description

As mammals, cetaceans are warm-blooded, breathe air, and feed their young on milk. Porpoises all give birth to live young. Like mammals, porpoises have hair at least some stage in their development, but are nearly hairless, like other cetaceans. They also share with other cetaceans a fusiform (spindle-shaped) body with anterior limbs in the form of flippers, and a flat, notched tail with horizontal flukes that lacks bony support.

Like other members of the Odontoceti suborder, porpoises have only one blowhole and have the main bones of the upper jaw thrust upward and back over the eye sockets (Grzimek 2004). An outstanding ability of the Odontoceti is to sense their surrounding environment through echolocation.

In common use, the term porpoise is often applied to dolphins. There is no strict definition of the term dolphin (Grzimek 2004). Generally, the term has been used either for members of the family Delphinidae or for any member of the families Delphinidae and Platanistoidea.

Porpoises tend to be smaller but stouter than dolphins. They have small, rounded heads and blunt jaws instead of beaks like the Delphinidae. While dolphins in the Delphinidae typically have a round, bulbous "melon", porpoises do not. Their teeth are flattened and 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 Delphinidae and Phocoenidae lack dorsal fins.) Some species have small bumps, known as tubercles, on the leading edge of the dorsal fin. The function of these bumps is unknown (Read 1999).

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). In terms of weight the lightest is the Finless Porpoise at 30-45 kg (65-100 lb) and the heaviest is Dall's Porpoise at 130-200kg (280-440 lb). Porpoises are intelligent and friendly mammals. 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 (Read 1999).

Porpoises, along with whales and dolphins, are descendants of land-living mammals and are related to hoofed animals. They entered the water roughly 50 million years ago.

Recently-discovered hybrids between male Harbour porpoises and female Dall's Porpoises indicate that the two species may actually be members of the same genus (Read 1999).

Lifecycle and behavior

Porpoises are relatively r-selected compared with dolphins: that is, they rear 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. Although the lifespan of most species is not known, specimens older than in their mid-teens have rarely been found.

"Rooster tail" spray around swimming Dall's Porpoises

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).

Unlike the delphinids, which often leap out the water and bow-ride on the wave of a ship, phocoenids rarely are observed doing this. Porpoises tend to be less acrobatic and more wary than dolphins.

Human impact

Accidental entanglement (bycatch) in fishing nets is the main threat to porpoises today. 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.

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.

See also

• Dolphin
• Whale

References

ISBN links support NWE through referral fees

• Grzimek, B., D. G. Kleiman, V. Geist, and M. C. McDade. 2004. Grzimek's Animal Life Encyclopedia. Detroit: Thomson-Gale. ISBN 0787657883.

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