Porpoises are mostly ocean-dwelling marine mammals, although some species can also live in freshwater rivers. They are often confused with dolphins. In casual conversation many people incorrectly use the terms dolphin and porpoise to mean the same thing. Both porpoises and dolphins came from a common ancestor, ancient relative, however they have been distinct families for about eleven million years.
Porpoises have a blunt snout, as opposed to the beak and elongated snout of dolphins. Their dorsal, back, fins are triangular. They have thick, stocky bodies that help them to conserve heat in cold waters. There are several differences between the skulls of porpoises and dolphins, but the most obvious is in the teeth. Porpoises have between sixty and 120 almost triangular, spade-shaped teeth, while dolphins have cone-shaped teeth. Most members of this family lack a melon. The melon is a fatty organ on the forehead. This gives their heads a tapered rather than a bulging look.
Porpoises range in weight from 90 to 485 pounds (40 to 220 kilograms) and in length from 4 to 7 feet (1.2 to 2.2 meters). The smallest porpoise is the vaquita (vah-KEE-tah), which lives in the Gulf of California in Mexico. Dall’s porpoise and the spectacled porpoise are the two largest porpoises. In all species except the spectacled porpoise, females are larger than males.
Porpoises range in color from black to gray to tan. Generally, their backs are dark and their bellies are lighter. Some, such as the spectacled porpoise and Dall’s porpoise, have quite distinctive black and white markings. Others, such as the finless porpoise, are a single dull color.
Porpoises are found along the coasts of large parts of North and South America (except the tropics and subtropics), Europe, and in the Mediterranean and Black Sea. They are also found off the coast of Siberia and northern Japan.
Porpoises live in a variety of ocean habitats. The spectacled porpoise lives in cold, open ocean in the Southern Hemisphere. Another Southern Hemisphere porpoise, Burmeister’s porpoise, lives in warmer, shallow waters along the coast of South America. This porpoise can also live in freshwater rivers. The finless porpoise and the vaquita also like shallow warm water. The harbor porpoise and Dall’s porpoise both live in cold water habitats.
Porpoises are carnivores, meat eaters. They eat mainly fish. The type of fish they prefer depends on the habitat in which they live. They also eat squid and octopus. Some also eat shrimp and mollusks (hard shelled animals like clams). Many porpoises migrate seasonally in order to follow the fish they feed on. Their natural predators, animals that hunt them for food, are some sharks, killer whales, and bottlenosed dolphins.
Porpoises use echolocation (eck-oh-loh-KAY-shun) to help find food. They make sounds (scientists disagree about how this is done) that are sent out into the environment. When the sounds bounce back, the echo is passed through special tissue in the lower jaw to the inner ear. From the time it takes to collect the echoes, their strength, and their direction, the animal can construct a “sound picture” of its environment. This system is extremely sensitive and allows the animal to locate very small objects.
BEHAVIOR AND REPRODUCTION
Except for the finless porpoise on the Yangtze (yang-see; or Chang) River in China, which seems to have become used to heavy boat traffic, porpoises tend to avoid boats. This makes them difficult to study. They rise to the surface to breathe quietly without showing much of their bodies. Rarely do they leap above the surface of the water. Generally porpoises live in small groups of no more than ten individuals. When larger groups occasionally gather, it may be to feed or follow schools of fish.
Very little is known about the reproduction of vaquita and spectacled porpoises. Other porpoises become mature at three to five years, and have a single calf every year after that. Pregnancy lasts about eleven months and mothers nurse their young, feed them breast milk, for more than a year. Porpoises live about fifteen years.
PORPOISES AND PEOPLE
Because porpoises are shy and avoid boats, they have very few interactions with people. Until the mid 1940s, they were hunted for food and oil, but now intentional hunting occurs only occasionally in Greenland and in the Black Sea. They are, however, often caught and drowned in fishing gear.
The vaquita is the least abundant porpoise. There may be only a few hundred individuals remaining. The vaquita is considered Critically Endangered, facing an extremely high risk of extinction in the wild. It lives in northern Mexico in the Gulf of California where there is a lot of commercial fishing. The main threat to its survival is being accidentally killed by becoming entangled in fishing nets.
The harbor porpoise is listed as Vulnerable, facing a high risk of extinction. It is a protected species in the United States and Canada. Threats to its survival include pollution and accidental death in fishing gear. Little is known about the population levels of the other four species of porpoises.