Like whales and porpoises, dolphins are mammals, not fish. Mammals, unlike fish, feed their young with milk that is produced in the mother's body. Also unlike fish, dolphins have lungs and are warm-blooded-that is, their body temperature always stays about the same, no matter what the temperature of their surroundings. Many scientists believe that dolphins rank among the most intelligent animals, along with chimpanzees and dogs.
Dolphins, whales, and porpoises are members of a group of mammals called cetaceans. Dolphins and porpoises look a lot alike. Their chief differences occur in the mouth and teeth. Dolphins have a beak like mouth and cone-shaped teeth. Porpoises have a rounded snout and flat or spade-shaped teeth. Whales are way bigger than most dolphins and porpoises. Scientists apply the term dolphin to two families of cetaceans, marine dolphins and river dolphins.
There are 32 species of marine dolphins. They are found in nearly all oceans, and most of them live only in salt water. Many species of marine dolphins remain near land for most of their lives, but some live in the open sea. River dolphins live in fresh or slightly salty water. The various species of dolphins usually range from 4 to 30 feet long and weigh from 100 pounds to 10 short tons (9 metric tons). The most familiar types are the bottle-nosed dolphin and the common dolphin. The bottle-nosed dolphin is the most widely known species. Its short beak gives this dolphin an expression that looks like a smile. Most performing dolphins in aquariums and zoos are bottle-nosed dolphins. Members of this species measure up to 13 feet long and can weigh as much as 600 pounds . They are gray, but their backs are darker than their undersides.
Bottle-nosed dolphins show great friendliness toward people, and they often swim alongside ships. They also get used to life in captivity. Bottle-nosed dolphins live in temperate to tropical waters. Most of them stay within 100 miles of land. Many live in bays and protected inlets, where the water is relatively shallow. Bottle-nosed dolphins are found year-round off the coast of Florida. They range as far north as Japan and Norway and as far south as Argentina, New Zealand, and South Africa.
The common dolphin has several unique features. For example, a dark band around the eyes extends to the end of the long, narrow beak. Common dolphins also have black backs, white undersides, and prominent gray and yellowish-brown stripes on their sides. These dolphins grow from 6 to 8 feet long and weigh up to 165 pounds. Common dolphins live in temperate to tropical waters. They often swim in large schools and are frequently seen in the open ocean. Common dolphins sometimes follow ships for many miles. As they do so, these playful dolphins may leap out of the water and turn somersaults.
Other species include killer whales, which are the largest dolphins. Killer whales can reach a length of 30 feet or longer and may weigh up to 10 short tons (9 metric tons). Members of another species, known as pilot whales or black fish, grow 15 to 20 feet long. Pilot whales have gray to black backs and sides. These dolphins are different from other large dolphins because of their bulging foreheads. Among the most numerous species of dolphins are spinner dolphins, which sometimes spin on their sides when they leap out of the water.
Many kinds of dolphins have distinguishing colors or other markings. For example, Risso's dolphins are brown and gray, and most of them have many unusual white streaks. White-sided dolphins have gray, white, and yellow stripes on their sides. Spotted dolphins are named for their white spots. Striped dolphins have black stripes on their sides.
All dolphins have torpedo like bodies, which enable the animals to move through the water quickly and easily. They have a pair of paddle-shaped forelimbs called flippers, but no hind limbs. Most species of dolphins also have a dorsal fin on their back. This fin, along with the flippers, helps balance the animal when it swims. Powerful tail fins, called flukes, shoot dolphins through the water.
The skin of dolphins is smooth and rubbery. A layer of fat, called blubber, lies beneath the skin. The blubber keeps dolphins warm and acts as a storage place for food. It is lighter than water, and so it probably also helps dolphins stay afloat.
Like all other mammals, dolphins have lungs. The animals must surface regularly to breathe air and usually do so once or twice a minute. A dolphin breathes through a blowhole, a nose on top of its head. The animal closes its blowhole by means of powerful muscles most of the time while underwater.
Dolphins have a highly developed sense of hearing. They can hear a wide range of low and high-pitched sounds, including many that are beyond human hearing. Dolphins also have good vision, and the entire surface of their bodies has a super sense of touch. All these senses function well both above and below the surface of the water. Dolphins have no sense of smell.
Dolphins have a natural sonar system called echolocation, which helps them locate underwater objects in their path. A dolphin locates such objects by making a series of clicking sounds. These sounds leave the animal's body through the melon, an organ on top of the head. The melon consists of special fatty tissue that directs the sounds forward. Echoes are produced when the sounds reflect from an object in front of the dolphin. By listening to the echoes, the animal determines the location of the object.
Most kinds of dolphins have a large number of teeth. Some species have more than 200. Dolphins use their teeth only to grasp their prey, which are mainly fish and octopus like animals called squids. Dolphins swallow their food whole and usually eat the prey headfirst.
Most dolphins mate in spring and early summer. The males are called bulls, and the females are called cows. The courtship behavior may involve head-bumping. The pregnancy period for most species of dolphins lasts from 10 to 12 months. The females almost always give birth to one baby, called a calf, at a time.
After the calf is born, it immediately swims to the surface for its first breath of air. A newborn dolphin is about a third as long as its mother. Female dolphins, like all female mammals, have special glands that produce milk. The calf sucks the milk from its mother's nipples. The females nurse and protect their young for more than a year.
Male dolphins take no part in caring for the young. Most species of dolphins live at least to 25 years of age. Some pilot whales reach 50 years of age.
Sharks are the chief natural enemies of dolphins. Some dolphins die after swimming into extremely shallow water and stranding themselves on the shore. The animals cannot live long out of water because their bodies become overheated. Scientists do not know why dolphins beach themselves. Most beached dolphins die from natural causes.
Most knowledge about the living habits of dolphins comes from aquariums and zoos. Killer whales seem to have the closest-knit groups, most of which consist of from several to 17 or 18 animals. Bottle-nosed dolphins live in groups of about 12. Among some species the groups combine and form herds of 100 to 1,000 dolphins. Adult males and young dolphins move among groups of females with their calves. The animals in such groups play and hunt for food together. They also help other members of the group that are in trouble. Dolphins sometimes use their backs or flippers to keep an ill or injured dolphin near the surface.
Dolphins communicate with one another in a variety of ways. For example, they may use a complex series of whistles and clicks called phonations. The animals make these sounds in air-filled sacs connected to their blowholes. Dolphins also communicate by slapping their flukes on the surface of the water.
Dolphins swim by moving their flukes up and down. This action differs from that of most fish, which propel themselves through the water by swinging their tail fins from side to side. Dolphins use their flippers to make sharp turns and sudden stops. Killer whales and some smaller species of dolphins can swim at speeds of 20 to 25 miles per hour. But they can maintain those speeds for only a short time.
Most dolphins swim much slower. Dolphins do not usually dive deeply, though they have the ability to do so. Some dolphins have been trained to dive more than 1,000 feet. When a dolphin dives, its lungs collapse and its heart rate slows down. These actions allow the animal's body to adjust to the increasing pressure as the dolphin dives deeper underwater.
The attraction between dolphins and people goes back thousands of years. Ancient Greek artists decorated coins, pottery, and walls with pictures of dolphins, and the animals appear in Greek and Roman mythology. The ancient Greeks considered the common dolphin sacred to the god Apollo. For centuries, sailors have regarded the presence of dolphins near ships as a sign of a smooth voyage. On the other hand, hunters of several nations, including Sri Lanka and Japan, kill thousands of dolphins annually. The dolphins provide meat eaten by people and animals, and the oil from their bodies is used as a lubricant.
In addition, millions of dolphins have drowned in fishing nets that were intended to catch cod, mackerel, salmon, and other kinds of fishes. Tuna fishing crews have been responsible for the largest number of these deaths among dolphins. For some unknown reason, some species of dolphins often swim over large schools of yellow fin tuna. As a result, nets meant to catch tuna trap many dolphins as well. In 1972, the United States government passed a law limiting the number of dolphins that could be killed yearly by tuna fishing crews. Improved fishing technology also greatly reduced the number of dolphins killed unintentionally by human beings. In 1990, leading U.S. tuna-canning companies announced that they would refuse to accept tuna caught in nets that also kill dolphins.
Since the mid-1900's, hundreds of dolphins have been trained to perform in shows presented by aquariums, zoos, and amusement parks. Scientists conduct various types of research on dolphins to understand their complex communication systems. Most trained dolphins in amusement parks, aquariums, and zoos are bottle-nosed dolphins, though many pilot whales, spotted dolphins, and killer whales also perform in shows. These playful animals sometimes invent new behavior patterns by watching other dolphins perform.
Trained dolphins jump through hoops, throw balls through nets, or "walk" backwards on the water by using their powerful flukes. Some leap 15 to 20 feet out of the water to ring a bell or to take a fish from a trainer's mouth.
Research on dolphins has mostly concentrated on dolphins' echolocation and communication systems. For example, dolphins that have been blindfolded with suction cups use echolocation to detect even small differences in the shape, size, and thickness of objects.
Scientists have also studied the diving ability of dolphins. Certain sounds that dolphins make when communicating with one another apparently are associated with specific situations. For example, some zoologists believe dolphins make a particular sound when they are in trouble, though these distress calls vary. Eventually, researchers hope to learn the exact nature of the information that dolphins apparently transmit among themselves.
Whale is a huge sea animal that looks much like a fish. But whales are not fish. They belong instead to the group of animals called mammals. Other mammals include chimpanzees, dogs, and human beings. Like these mammals, whales have a highly developed brain and are among the most behaviorally complex of all animals.
Most whales are enormous. One kind, the blue whale, is the largest animal that has ever lived. Blue whales can grow up to 100 feet long and weigh over 150 short tons (135 metric tons). But some kinds of whales are much smaller. Belugas and narwhals, for example, grow only 10 to 15 feet long.
Like other mammals, whales give birth to live young and feed them with milk produced by the mother's body. Most fish, however, lay eggs and do not feed their offspring. Whales are also warm-blooded--that is, their body temperature remains about the same regardless of the temperature of their surroundings. Almost all fish are cold-blooded. Their body temperature changes with changes in the temperature of the water.
Whales have a highly streamlined shape, which enables them to swim with a minimum of resistance. Their shape resembles that of fish. But a whale's powerful tail fins, called flukes, are horizontal instead of vertical like the tail fins of a fish. A whale propels itself by moving its flukes up and down. Most fish swim by swinging their tail fins from side to side.
A whale's backbone, ribcage, and shoulder blades resemble those of other mammals. The absence of hind legs, however, distinguishes the whale from most other mammals. Two small bones buried in the hip muscles are all that remain of the whale's hind legs.
Whales have smooth, rubbery skin that slips easily through the water. Most mammals are covered with hair, which holds warm air next to the body. Whales, however, do not have a coat of hair to provide them with insulation. A few bristles on the head are all the hair that whales have.
Like other mammals, whales have lungs. They must therefore come to the surface regularly to breathe. Baleen whales usually breathe every 5 to 15 minutes, but they can go as long as 40 minutes without breathing. A sperm whale can hold its breath up to 2 hours.
Whales have no sense of smell, but they have good eyesight and a well-developed sense of taste. All whales also have well-developed senses of touch and hearing. Their keen hearing provides them with much information about their surroundings. They can hear an extremely wide range of sounds, including low- and high-pitched sounds far beyond the range of human hearing. Whales can also tell from what direction a sound is coming underwater.
Whales live in groups called herds, pods, or schools. Toothed whales appear to be more socially organized than baleen whales. However, scientists do not know enough about the social behavior of baleen whales to make a definitive comparison. Toothed whales, especially female offspring of the same mother, often live together for many years or for life. Many species of dolphins swim in herds of 1,000 or more. Most dolphin schools consist of animals of both sexes and a variety of ages. Other species, such as sperm whales, form all-male groups and groups consisting of several females and young calves. Most adult killer whales live in pods with their young.
Most kinds of baleen whales migrate between polar and tropical regions. The cold waters of the Arctic and Antarctic are rich with plankton. The whales spend the summer in these areas, feeding and storing up blubber. As winter approaches, the polar waters freeze over and the whales move to warmer seas. There they mate and the females that are already pregnant give birth. The warm waters may provide a comfortable environment for the babies. In the tropics, adult whales live mostly off their blubber because food is scarce. Mother whales convert part of their blubber into milk for the babies. By late spring, whales return to the polar feeding area.
The life span of whales ranges from 15 years for the common porpoise to 60 or more years for killer, bowhead, and sperm whales. Humans and large sharks account for many whale and dolphin deaths. Killer whales, which are dolphins, occasionally attack young whales, smaller dolphins, and weak or diseased baleen whales. Whales that escape human actions and large predators die of parasite infections, other diseases, or old age.