The graceful, playful activities of dolphins are familiar to everyone. Yet science still has a lot to learn about these intelligent and fascinating mammals.
Dolphins are small whales that live in all the world’s oceans. Four species of river dolphins live in rivers.There are at least forty species of ocean and river dolphins.The bottlenose dolphin is probably the best-known species and is often kept in dolphinaria (aquariums for dolphins). In the wild, bottlenose dolphins live in warm seas, usually near the coast. The spinner dolphin is another common warm-water species. It prefers deep waters and, as with many dolphins, it enjoys leaping out of the water and then diving back in.When a spinner dolphin makes a leap, it spins around, which is how it got its name. Many species of dolphins look quite similar, even when they are not closely related.That is partly because dolphins need to be streamlined to chase after their prey. Many dolphins also have similar patterns of spots or patches that are usually various shades of gray. The largest member of the dolphin family is the killer whale, or orca. Unlike most dolphins, the orca has a blunt head,without a pointed beak. Next largest are two species of pilot whales.They also have a blunt head and grow nearly as long as killer whales but feed on much smaller prey.
A dolphin has powerful muscles that move its flat tail up and down in the water.The tail movements push the dolphin along. Dolphins breathe air but, unlike people, their nose is nowhere near their mouth. Instead, like other whales, they have a single nostril, or blowhole, behind their head. Dolphins need to go back to the surface regularly to breathe. Dolphins have front flippers but no hind flippers.Most species also have a fin on their back called a dorsal fin. Most dolphins have their mouth stretched out into a long beak, which contains many pointed teeth.This mouth shape helps dolphins grab the slippery fish and squid on which most of them feed. Dolphins also typically have a dome-shaped front to their head. Under the dome is an oil-filled structure called a melon.The melon dolphins make. Sound is extremely important to dolphins because sound travels better through water than light does. Dolphins have no voice box but produce sound by moving air in the tube leading up to their blowhole. Dolphins communicate with each other by making noises, such as whistles.
They also produce high-frequency clicking sounds, which are outside the range of human hearing.These clicks bounce off objects such as reefs or nearby fish. By listening to echoes, a dolphin can figure out where objects are and how large they are; the dolphin can “see” with its ears by this process, called echolocation. Dolphins have good eyesight, even in dim light. They see well in both air and water.They can also taste, but not smell, because their blowhole is always closed under water.
Habitats and Food
The world’s oceans are a vast habitat. Some dolphin species live near coasts, while others inhabit deeper water. Some live worldwide, while others live in only one ocean. Sometimes various populations of the same species lead quite different lifestyles. Deepwater dolphins sometimes make long journeys to look for food, often traveling in large groups. They use ocean-bottom features as landmarks to help them navigate. Dolphins living near coasts tend to stay in the same home area, but within that area they have regular patterns of movement. For example, dusky dolphins living off Argentina spend the day feeding several miles out to sea.At night they usually come closer to shore, where there is less danger from sharks. Most dolphins feed during the day, although spinner dolphins are mainly nighttime feeders.They also dive farther down from the surface to feed.They can therefore live in the same areas as other types of dolphins without competing for food. Dolphins eat a wide variety of fish, squid, and other animal prey.They probably eat whatever they can catch. Most species of dolphins catch prey that is swimming freely in the water. However, some dolphins nose around on the sea floor or coral reefs for food. The strong social bonds among dolphins help them cooperate for feeding. For example, a group of dolphins that comes across a shoal of fish may herd it to the surface by blowing bubbles and swimming around the fish. Once trapped at the surface, the fish are easier to catch and eat. Killer whales are unique because they kill and eat other sea mammals, including the calves of larger whales. Sometimes they hunt in groups, sometimes alone.A killer whale will even launch itself up a beach to catch an unwary sea lion.
Dolphins live complicated social lives, which vary from one species to another. Scientists still have a lot to learn about dolphin societies.Most species live in small groups, which may team up into larger groups, such as when rounding up a huge shoal of fish. Sometimes, thousands of deepwater dolphins may join forces to journey across the ocean. Dolphins do not seem to be monogamous,with one male staying permanently with one female. Even small groups of dolphins may not always be permanent,with individuals switching from one group to another.However, dolphins are intelligent animals that can recognize other individuals, even if they have not seen each other for a while. One of the whistling noises dolphins make is a so-called signature call.This call is different for each dolphin and helps other dolphins identify every individual. Sometimes two dolphins in a group may team up as buddies and stay together for months or even years. Teamwork requires team bonding. Once dolphins have fed and rested, they spend a lot of time playing, socializing with each other, and touching one another.Often, they just seem to be having fun.These playful habits are probably a reason why dolphins are also friendly toward people. Killer whales and probably pilot whales, too, live in more permanent groups than other dolphins. Killer whales live in groups containing a mother and all her offspring, including daughters, sons, grandsons, and granddaughters.Males leave the group temporarily to mate, but otherwise the family sticks together permanently, year after year. Mother dolphins give birth to a single calf at a time.Most births take place during the summer months. The mother feeds it on her milk.The calf may stay for her for months or even years, depending on the species. A female dolphin has a calf only once every two or three years at most. For a female killer whale or pilot whale, births are seven to eight years apart.
Dolphins and People
Although dolphins are popular animals, human activities have sometimes had a bad effect on them. In the past, people killed dolphins for food, although that is less common now. In the late twentieth century, nets spread in the ocean to catch tuna killed huge numbers of dolphins by accident.Nets are now designed to make it easier for dolphins to escape from or avoid them. Captive dolphins are popular, and dolphins seem to enjoy human company.However, some people think that it is unfair to keep dolphins in an unnatural environment. Certainly, dolphins in captivity die younger than wild ones. Sometimes groups of dolphins, especially pilot whales, strand themselves on beaches, where they die. Possibly they are following a leader that has fallen sick. Strandings do not necessarily have anything to do with people. But some scientists think that pollutants may affect the health of dolphins, making sickness and strandings more likely. A school of short-beaked common dolphins leaps out of the water at full speed.
DID YOU KNOW?
Two species of dolphins live in the Amazon River.
Some patterns on dolphins probably disguise them by imitating light patterns in water.
Dolphins have developed complex sound signals to stay in touch. ( Read how dolphins imitate the song of whales )