The smallest members of the group of animals called whales are the porpoises. Porpoises are often confused with their close relatives, the dolphins. Like dolphins, porpoises are mammals that are great swimmers and divers. Porpoises and dolphins also have a similar appearance, but a few physical differences help tell them apart. A porpoise is usually smaller and chubbier than a dolphin, and it lacks the dolphin's beaklike snout. Scientists know less about porpoises then they do about dolphins because porpoises do not survive as well in zoos or aquariums. (See also dolphin.)

Where porpoises live

Most porpoises live in the oceans and seas north of the equator. The harbor porpoise is one of the most common species, or types. It lives in the northern parts of the Atlantic and Pacific oceans and is often seen along the coasts of Europe and North America. South of the equator, porpoises are found along the coast of South America and in the area of the East Indies. Porpoises are most common in coastal waters, but some live farther out. A few species, or types, of porpoise live in rivers as well.

Physical features

Compared to other whales, porpoises are fairly small. They reach a maximum length of about 7 feet (2 meters). The skin is smooth, usually with a color pattern combining black, white, and gray. Porpoises have two flippers that they use as forelimbs. All porpoises except one type, the black finless porpoise, have a triangle-shaped fin on the back.

Porpoises are mammals and not fish. Among other things, this means that they are warm-blooded—their body temperature stays about the same regardless of the temperature of the environment. Beneath a porpoise's skin is a layer of fat known as blubber, which helps the animal stay warm in cold waters. Another trait that identifies a porpoise as a mammal rather than a fish is the way it breathes. Fish can get the oxygen they need from the water through gills, but porpoises have lungs and need to breathe air at the water surface. They have a single nostril, called a blowhole, on the top of the head.

Behavior

Porpoises generally swim together in small groups of two to four. Larger groups are usually seen only in good feeding grounds. For porpoises, this means an area with plenty of fish, shrimps, and squid. Some types of porpoise, such as Dall's porpoise, are very active and enjoy swimming and jumping around boats. Others, such as the harbor porpoise, are shy. They avoid boats and rarely leap above the water.

Porpoises use sounds to communicate with each other and to find objects underwater through a process called echolocation. In echolocation, a porpoise makes a sound that reflects off a solid surface and back to the porpoise's sensitive ears. Based on how long it takes the sound to return, the porpoise can tell how far away the object is.

Most porpoises have their young in the warm part of the year. A female porpoise carries the young for about 10 to 12 months before giving birth. She almost always has only a single offspring, which is called a calf. The mother provides the calf with milk, sometimes for a year or longer. Harbor porpoises usually live for about six to ten years, but other types live longer.

Survival issues

Porpoises declined in number during the 20th century. For years they were hunted in great numbers, especially for their meat. Porpoise hunting is now restricted in many places. Today the biggest threat to porpoises may be huge nets placed in the oceans to catch other fish. Thousands of porpoises have drowned after becoming tangled up in the nets.