Known for their intelligence, playfulness, and friendliness toward humans, dolphins can often be seen swimming alongside boats or performing at aquariums. Dolphins look like large fish, but they are actually small whales. Like whales and other mammals, they breathe through lungs instead of gills and give birth to live young, among other typical traits.Dolphins are closely related to another group of small whales, the porpoises. Because dolphins and porpoises look so much alike, the two are often confused with one another. In fact, the names dolphin and porpoise are sometimes used as if they are one and the same. In general, however, dolphins can be told apart from porpoises by their larger size and their snouts. A dolphin has a long, sharp snout that is flattened like a beak. A porpoise's snout is short and blunt (see porpoise). The name dolphin is also sometimes used for a large fish known as the mahimahi or dorado. As a fish, the mahimahi differs from the mammals more commonly thought of as dolphins.The larger dolphins, including the bottle-nosed species, can reach a length of 13 feet (4 meters). Most dolphins, however, are 7 to 10 feet (2 to 3 meters) long. They have smooth, rubbery skin and are usually colored in some mixture of black, white, and gray. A layer of fat beneath the skin, known as blubber, helps keep them warm in the water. Dolphins have two flippers, which are fins that serve as forelimbs, as well as a triangular fin on the back.
Like other mammals, dolphins are warm-blooded. This means that they keep their body temperature about the same even when the temperature of their surroundings changes. Because a dolphin breathes through lungs, it must come up to the water's surface to breathe. A single nostril, called a blowhole, on the top of a dolphin's head lets air move in and out.
Dolphins can be found throughout the world in either salt water or freshwater. The most widespread species, or kinds, are the common and bottle-nosed dolphins. Both live in warm and temperate oceans and seas worldwide. River dolphins are found only in South America and Asia. They prefer freshwater.Bottle-nosed dolphins and other species used to be hunted in large numbers for their meat and for other reasons. Many countries have banned dolphin hunting, but the practice continues in some places. Nets set in the ocean to catch tuna and other fish are another threat to dolphins. Dolphins swim into these nets and become trapped. To prevent dolphins from being injured or drowning in the nets, some of them are now made with exits that allow dolphins, but not fish, to escape. Another danger is pollution of the world's oceans. The dumping of chemicals and other wastes into the water may poison the dolphins' food supply and make the dolphins sick.
Dolphins are social animals, meaning that they generally live in schools, or groups. Sometimes these schools are made up of as many as 1,000 individuals. Dolphins use a range of sounds to communicate with each other and often act together to achieve a goal, such as trapping fish to eat. They have been known to help an injured dolphin by pushing it to the surface so it can breathe.Dolphins also use sounds to find objects underwater. A dolphin lets out whistles and squeaks that are too high-pitched to be heard by humans. These sounds reflect off solid surfaces and then travel back to the dolphin's sensitive ears. The time it takes for a reflected sound to return tells the dolphin how far away something is. This process is called echolocation. It is especially helpful for river dolphins, many of which live in muddy water.Dolphins are intelligent animals with great learning abilities. They have been trained to perform impressive tricks in aquariums, including high leaps above the water. Their speedy and graceful swimming adds to their appeal as performers.Reproduction in dolphins usually takes place in the warm months. After a pregnancy of about 10–12 months, a female dolphin gives birth to a single offspring. A newborn dolphin, called a calf, looks like a smaller version of its parents. Just after birth the mother nudges the calf toward the surface of the water so that it can take its first breath of air. Dolphin mothers provide milk for their young for a year or longer. The length of time a dolphin lives varies by species, but some have survived more than 30 years.
