Dolphin (aquatic mammal)

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Introduction

Dolphin (aquatic mammal), fast-swimming mammal belonging to the order Cetacea, which also includes whales and porpoises. Sleek and powerful swimmers, dolphins are found in seas throughout the world; some inhabit freshwater rivers and lakes. Characteristic features of most dolphins are long snouts with rows of sharp teeth, and rounded foreheads with a nostril on top, known as the blowhole.

There are at least 40 species of dolphins. Dolphins resemble fish in many ways, but they exhibit a number of true mammalian characteristics: They are warm-blooded, breathe air, and nurse their young on milk. Dolphins and porpoises have a similar appearance, but dolphins can be distinguished from porpoises by their more prominent snouts and conical teeth. Porpoises have blunt snouts, chisel-shaped teeth, and a stouter body than dolphins.

Sailors have long considered the presence of dolphins cruising alongside the bows of ships as a good omen and a promise of fair weather. Dolphins also figure prominently in folklore, often appearing in works of art, on coins and currency, and on stamps. Ancient Greek coins depicted the son of Poseidon seated on a dolphin, and the ancient Roman author Pliny the Elder wrote a story of a dolphin that carried a poor man’s son to school each day. Many classical writers described how dolphins harnessed to chariots helped maidens in distress. A dolphin was on the coat of arms of the dauphin, the title given the eldest son and heir of the king of France.

Dolphins throughout the world are threatened by habitat destruction and pollution. Many cultures have hunted dolphins for food and for the oil found in small quantities in the animal’s head. (The oil was once widely used as a lubricant.) Although the hunting of dolphins greatly declined in the 20th century, many dolphins are still killed inadvertently when they become entrapped in huge nets used to catch tuna.

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Physical Description

Dolphins range in size from the small tucuxi dolphin, which grows to about 1.2 m (4 ft) in length and weighs about 50 kg (110 lb), to the killer whale, which can grow to 9.8 m (32 ft) in length and weigh over 5,500 kg (12,100 lb). In most dolphins males grow slightly larger than females.

Dolphins are commonly black, brown, or gray in color and are often marked with patterns of white or light colors. The dolphin body is streamlined and adapted to a life underwater. Two finlike flippers near the front of the body help stabilize and steer the animal as it swims. The flippers contain bones that are similar in arrangement to the bones in the human hand. Horizontal tail flukes, composed of tough connective tissue, move up and down to propel the animal through the water.

Many dolphins have a dorsal fin made of connective tissue in the middle of the back, which helps the animal maintain balance. The dorsal fin of each dolphin has a unique shape, enabling scientists to use this feature to identify individual dolphins in the wild. The dorsal fin of some ocean dolphins is hooked rather than triangular in shape. In some species the fin is tall—the dorsal fin of a male killer whale can reach a height of 1.8 m (5.9 ft). The dorsal fin of river dolphins is small, sometimes barely forming a ridge on the back.

Most dolphins have a prominent snout and, in some species, the jaws contain as many as 250 conical teeth. Some river dolphins have molar teeth in addition to conical teeth. Dolphin teeth are adapted for ripping and tearing prey but not for chewing. Scientists use growth layers in the teeth to determine a dolphin’s age.

The eyes of a dolphin contain elastic lenses that expand and contract, enabling the animal to focus its vision both above and below water. Special glands protect the dolphin’s eyes from stinging salt water. Dolphins often use echolocation (use of reflected sound to perceive surroundings) to navigate and forage for food. They have a bulging forehead, called the melon, which contains a fatty substance that aids in echolocation. Clicks made by the dolphin project from the melon as a sonar beam. The beam bounces off nearby objects, and the dolphin analyzes the reflected sound waves to locate prey and avoid obstacles.

Dolphins breathe through a blowhole located on top of the head. A muscular plug closes the blowhole to keep water out of the animal’s lungs during a dive. When dolphins come to the water’s surface to breathe, they make a short, explosive exhalation through the blowhole, then take a longer inhalation before submerging again. In this process they may empty and refill their lungs in less than one-fifth of a second.

Dolphins have a layer of blubber (body fat) that lies just beneath the skin. This layer of fat insulates the animal, enabling it to maintain a constant body temperature even in the coldest waters. The thickness of blubber varies among species according to their need for insulation. For instance, hourglass dolphins living in the frigid waters around Antarctica have a much thicker layer of blubber than do rough-toothed dolphins inhabiting the warm waters in the Gulf of Mexico.

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Behavior

Studies of dolphins in captivity have taught scientists much about dolphin physiology and social behavior. In addition, since the late 1980s advances in the use of satellite tracking systems have broadened the opportunities for scientists to observe how dolphins behave in their natural habitat.

Some studies of captive dolphins from the 1960s suggested that the mental abilities of dolphins might rival those of primates, including humans. But scientists now agree that these early studies may have been overly optimistic. Scientists have yet to develop intelligence tests for dolphins that can be comparable to intelligence tests used for humans, but more recent studies indicate that dolphins probably match or slightly exceed the intelligence of dogs.

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Swimming and Diving

Dolphins are among the fastest animals in the sea. Several species of trained dolphins have been recorded swimming up to 40 km/h (25 mph), and killer whales in the wild can reach swimming speeds of up to 50 km/h (30 mph). These are maximum speeds that can only be sustained for short periods of time, however, and average or sustained swimming speeds are considerably slower and vary in different species. Many dolphins frequently jump clear of the water, and some species, such as spinner dolphins, often execute intricate flips and barrel rolls in the air.

Some dolphins can dive to depths of 60 m (200 ft) or more. During a dive, the animal’s heart rate slows from 60 beats per minute to 30 beats per minute. Oxygen-carrying blood is shunted away from the extremities and channeled toward the life-sustaining heart, lungs, and brain. These adaptations enable the animal to conserve oxygen while underwater.

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