I 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.

II 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.

III 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.

A 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.

B Socialization

      Dolphins are social animals that travel in schools varying in size from small herds of 2 to 5 members to immense schools of 1,000 or more. A school of bottlenose dolphins typically numbers about 10 to 20 individuals in coastal areas. Killer whale groups can range from around 8 to more than 50 animals, and herds of several hundred have been reported. Spotted dolphin schools can include up to 2,500 animals.

      Scientists have observed dolphins displaying three types of caregiving behaviors: standing by, excitement, and supporting. In standing-by behavior, dolphins remain in the vicinity of an injured or ill companion without offering aid. In excitement behavior, dolphins swim swiftly in circles around an injured dolphin, responding aggressively toward threats to the injured animal. For instance, they may bite harpoon lines or charge boats that come too close to the animal. In supporting behavior, one or more dolphins will help a sick or injured dolphin by supporting it at the surface to prevent the animal from drowning.

C Communication

      Dolphins communicate with one another using a complex set of whistles, screeches, and clicks. These sounds are produced in the upper nasal passages just beneath the blowhole. In many dolphin species each individual produces a unique signature whistle. The loudness of the whistling may communicate the emotional state of the dolphin.

      Some studies of dolphins in captivity suggest that dolphins can understand some elements of human language. These language studies are controversial, however. Critics argue that the studies only show that dolphins are excellent mimics and exhibit strong auditory skills, but that these abilities do not necessarily equate to intelligence.

      Because dolphins rely on sound to communicate and find food, scientists are concerned about the increase of underwater noise pollution from ship motors, oil drilling and production, and undersea machinery. Increasingly loud sounds are used to develop sonar images to study the structure of the seafloor, survey fish populations, or probe for submarines in warfare activities. Scientists believe that the high intensity of noise levels in some ocean areas may interfere with the transmission of dolphin calls or the ability to locate food. In particularly noisy areas, tissues associated with hearing, sound production, and air intake may be injured. In some cases, noise pollution can even kill dolphins.

D Hunting

      Dolphins are predatory animals, feeding largely on fish, squid, and in the case of killer whales, other marine mammals, sea turtles, and birds. Dolphins that dive for food in deep, dark oceans or live in muddy rivers or lakes with low visibility rely almost entirely on echolocation to navigate and forage for food.

      Dolphins often work together to hunt prey. For example, a group of dolphins may swim in circles around a school of fish, making it difficult for the intended prey to escape. At intervals several of the dolphins will dash across the circle, catching and devouring the trapped fish. In the coastal saltwater marshes of South Carolina and Georgia, dolphins have been observed chasing schools of fish up onto the muddy banks, then propelling themselves onto the banks to grab the stranded fish.

E Reproduction and Life Span

      Male and female adult dolphins partake in extensive courting, which includes behaviors ranging from tender caressing to playful roughhousing. After a gestation period of 12 to 16 months, depending on the species, a single calf is born. The mother nurses her newborn underwater for about 6 months to 2 years. Some dolphin species, including the striped dolphin and the pantropical spotted dolphin, live for 50 years or more. For other species, such as the bottlenose dolphin and common dolphin, 20 to 25 years is the maximum life span.

IV Types of Dolphins

      Scientists recognize at least 40 species of dolphins, and although there are different ways to classify these species, most authorities categorize dolphins into two groups: ocean dolphins and river dolphins. Ocean dolphins make up the family Delphinidae, which is the largest cetacean family, consisting of about 36 species. Members of this family include the bottlenose dolphin, the species that scientists have studied most extensively; and the spinner dolphin, named for its habit of leaping vertically out of the water and twirling. The largest member of this family is the killer whale, named for its voracious appetite for fish, seabirds, sea turtles, and some aquatic mammals. These and most other ocean dolphins are found in open seas, but several freshwater species inhabit rivers and lakes in Asia and South America. The small, graceful tucuxi dolphin, for example, has been sighted more than 2,000 km (more than 1,240 mi) up the Amazon River.

      The river dolphins are categorized into four families, each containing a single species. Three of the four species of river dolphins are found only in fresh water. One species, the Franciscana dolphin, inhabits freshwater rivers and coastal marine waters from northern Argentina to southern Brazil.