Dolphin (aquatic mammal) - biology. 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. Oxygencarrying 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. A Ocean Dolphins As the largest cetacean family, the family Delphinidae includes members that are diverse in their physical appearance, size, social behavior, and range. Although these dolphins are commonly known as ocean dolphins, some members of this family inhabit freshwater rivers and lakes in addition to the ocean. A1 Bottlenose Dolphins Bottlenose dolphins frequently ride the waves made by the bows of moving ships. They are often seen along beaches and in shallow bays and estuaries in the warmer coastal waters of all continents. In rare cases, bottlenose dolphins in their natural habitat may interact with humans, even permitting themselves to be touched. Bottlenose dolphins are the species most commonly studied by scientists and trained for performances at marine amusement parks. Scientists once classified bottlenose dolphins as a single species, but recent genetic studies have identified two separate species, the common bottlenose dolphin and the Indo-Pacific bottlenose dolphin. The common bottlenose dolphin is found throughout most of the warmer oceans of the world, while the Indo-Pacific bottlenose dolphin is found in coastal ocean waters along southern Japan, Indonesia, and northern Australia. Bottlenose dolphins are grayish in color and reach a length of 4 m (13 ft) and a weight of 650 kg (1,430 lb). They have large teeth, with 20 to 25 pairs in both the upper and lower jaw. They may consume 8 to 15 kg (15 to 30 lb) of food each day, including fish, squid, and crustaceans. Bottlenose dolphins usually travel in small groups of two to five animals near shore, but sometimes as many as several hundred may be seen together in deeper waters. Offshore they often are found in the company of other dolphin species, most notably pilot whales. They can exhibit aggressive behavior toward other cetaceans, and they have been observed chasing and butting harbor porpoises out of the water. A2 Common Dolphins Scientists recognize three species of common dolphin. Short-beaked common dolphins are found in temperate and tropical waters around the world. Long-beaked common dolphins inhabit temperate and tropical waters of the Atlantic Ocean, Pacific Ocean, and Indian Ocean. The Arabian common dolphin is found in the Arabian Sea, Gulf of Aden, Persian Gulf, and South China Sea. Common dolphins are characterized by a black ring around the eye. The back is gray to black in color and the belly is white. The dark color on the back dips low onto the sides, forming a saddle-like marking. Common dolphins have slender bodies with a long beak and tall dorsal fin. Adults can grow to 2.6 m (8.5 ft) in length and weigh up to 150 kg (330 lb). Common dolphins have 41 to 54 pairs of teeth in both jaws. They feed primarily on squid and small fish that travel in schools, such as anchovies, lanternfish, hake, and deep-sea smelt. Common dolphins travel in groups that may number thousands of members. They frequently cavort in the waves made by the bows of ships, and they are spectacular jumpers commonly trained to perform at marine amusement parks. Common dolphins figure prominently in ancient folklore and art. A3 Stenellid Dolphins There are five species of stenellid dolphins (dolphins that belong to the genus Stenella). They include the Atlantic spotted dolphin, the pantropical spotted dolphin, the Clymene dolphin, the striped dolphin, and the spinner dolphin. These dolphins vary in color patterns, tooth counts, and other structural features. Some species, such as the spinner dolphin and the pantropical spotted dolphin, are long and slender with a long snout. Others, such as the Clymene dolphin and the Atlantic spotted dolphin, have stouter bodies with a much shorter snout. Most spotted dolphins live far at sea in the tropical and temperate regions of the world. They are fast swimmers and often ride the bow wave of ships. Spinner dolphins are named for their habit of leaping out of the water and spinning as their body stays vertical. Spinners commonly swim with schools of yellowfin tuna, and fishing fleets often use spinner dolphins to lead them to schools of tuna. Spinner dolphins became the focus of media attention in the late 1960s when fishing fleets increasingly used purse seine fishing nets to catch yellowfin tuna, accidentally trapping and drowning spinner dolphins in the nets. Due to pressure from animal rights activists and United States consumers, domestic and international tuna canners today refuse to accept shipments from fishing fleets that do not protect dolphins. The Atlantic spotted dolphin has a striking spotted coloration. A community of Atlantic spotted dolphins living in the Bahamas commonly interacts with human divers and boaters. These interactions have enabled scientists to conduct long-term studies of the habits of these animals. A4 Lags The six dolphin species that make up the genus Lagenorhynchus are sometimes collectively referred to as lags, a contraction of the genus name. The members of this group are found in colder waters than most dolphins, and some species, such as the whitebeaked dolphin, travel in ice-covered subarctic and arctic waters. The hourglass dolphin travels in polar waters of the Southern Hemisphere. Lags tend to have short, thick snouts, and this feature sometimes causes them to be confused with porpoises. They have 23 to 40 pairs of teeth in both jaws, depending on the species. Lags eat squid as well as a variety of fish, including small schooling species such as herring, and larger bottom-dwelling fish such as cod and haddock. Some lags, including dusky dolphins, live with common dolphins, pilot whales, and other dolphin species. When living with other species, lags tend to be very gentle toward the young of the other species. One lag species, the Pacific whitesided dolphin, is regularly exhibited in marine amusement parks for its remarkable ability to make high jumps over a suspended bar. A5 Irrawaddy River Dolphin The common name of the Irrawaddy river dolphin makes it an oddity in a family that is generally referred to as ocean dolphins. But this small dolphin has a wide distribution in both fresh and marine waters in Asia. One subspecies lives in the Irrawaddy River in Myanmar (formerly Burma), and another subspecies is found in shallow marine waters from the Bay of Bengal to Borneo and Java. Growing up to 2.5 m (8 ft) in length, the Irrawaddy river dolphin has a light gray coloration and a rounded head that lacks a beak, making the dolphin resemble a small beluga whale. It has 15 to 20 pairs of teeth in the upper and lower jaws, which it uses to feed on a wide variety of fish; cephalopods such as squid, cuttlefish, and octopus; and crustaceans. Living in coastal marine waters and in freshwater rivers places the Irrawaddy river dolphin in regular contact with humans. Certain human activities, such as fishing with gill nets or explosives, regularly kill these dolphins, and their populations have become endangered in some areas. B River Dolphins Scientists recognize four species of river dolphins--three that live only in fresh water and a fourth that lives in rivers and estuaries and migrates along the eastern South American coast during the winter months. Until recently all species of river dolphins were grouped in a single family, but scientists now place each species of river dolphin into its own family. Living in rivers and coastal waters, river dolphins come into frequent contact with humans. Many river dolphin populations are threatened by a number of human activities, including collisions with boats, deliberate and accidental killing by fishermen and hunters, and water pollution due to runoff from irrigation and other agricultural practices. B1 Amazon River Dolphin The Amazon river dolphin is classified in the family Iniidae. It is the most abundant of all river dolphins. Botos, as these strictly freshwater dolphins are called in Brazil, are widespread in the Amazon and Orinoco basins of South America. During times of flood these dolphins swim among the trees of flooded forests. Amazon river dolphins reach a length of 2.5 m (8 ft). The animals are light gray when young but gradually take on a pinkish hue as they grow older. The mouth line angles up toward the eye to form what appears to be a sly smile. Amazon river dolphins have 25 to 27 pairs of teeth in each jaw. These include sharp-pointed conical teeth in the forward part of the jaw and molar-like teeth toward the back that help the dolphin crush hard-bodied prey, such as crustaceans and catfish. B2 Indian River Dolphin The Indian river dolphin is classified in the family Platanistidae. Known locally as susu, this dolphin is restricted to the Indus, Ganges, and Brahmaputra rivers of India. These animals are nearly blind--they lack eye lenses and their eye openings are no bigger than a pinhole. As a result, they rely almost entirely on echolocation to forage for food and navigate in the murky river waters. Indian river dolphins may grow up to 2.5 m (8 ft) in length. They have plump, gray bodies with a strikingly long beak that they use to root in river bottoms for shrimp and small fish. They have 26 to 39 pairs of teeth in both jaws. The teeth in the front of the mouth overlap and are visible even when the mouth is closed. B3 Chinese River Dolphin Classified in the family Lipotidae, the Chinese river dolphin is also known as the baiji or Chinese lake dolphin. This dolphin was once found in central eastern China in the Yangtze River (Chang Jiang), Dongting and Poyang lakes, and the Qiantang River near the mouth of the Yangtze. The Chinese river dolphin reaches 2.5 m (8 ft) in length. It has a plump, mostly gray body and a long, narrow snout that turns upward at the end. Like the Amazon river dolphin, its mouth line curves near the eye, giving the appearance of a smile. It has 30 to 36 pairs of teeth in each jaw and feeds on fish, particularly an eel-like catfish, which it digs up from river-bottom mud with its long snout. Although legal protection has been extended to the Chinese river dolphin, it was declared functionally extinct in 2006, meaning its population is too small to recover. An international team of scientists found no individuals during a six-week expedition. Experts believe that any surviving members of this species are doomed by construction of the Three Gorges Dam across the Yangtze River. The dam, scheduled for completion in 2009, will destroy the dolphin's natural habitat. B4 Franciscana The Franciscana dolphin, classified in the family Pontoporiidae, is also known as the La Plata dolphin and, in Brazil, as toninha. The Franciscana is unique among river dolphins in that it lives in the freshwater Río de la Plata in South America but also finds its way into the marine coastal waters from northern Argentina to southern Brazil. One of the smallest dolphins, the Franciscana reaches only 1.7 m (5.6 ft) in length and is primarily gray in color. Its beak is extremely long, extending as much as 15 percent of the body length in adults. It has 51 to 59 pairs of small teeth in both jaws and feeds on fish, shrimp, and squid. V THREATS TO DOLPHINS During the 18th and 19th centuries dolphins were hunted commercially, especially for the small quantity of valuable oil in parts of the head. This oil was used to lubricate delicate watch mechanisms; cheaper oils have now been found from other sources. Today dolphins are less often hunted, although some cultures continue to take dolphins for food. In the second half of the 20th century, new tuna-fishing techniques threatened many populations of dolphins. Some dolphin species, including spinner, spotted, and common dolphins, commonly swim with schools of tuna. Tuna boats follow these dolphins, hoping the dolphins will lead them to the tuna. In the 1960s tuna fleets began to use purse seine nets, large nets that encircle tuna and any nearby dolphins, trapping them when the nets are pulled shut at one end. When the purse seine net is pulled aboard the tuna boat, the dolphins often die before they can be released from the net. From 1959 to 1972 an estimated 4.8 million dolphins died in this way. Conservationists fought to reduce dolphin mortality in the tuna-fishing industry. As a result of their efforts, in 1990 the United States Congress passed the Dolphin Protection Consumer Information Act (DPCIA). The DPCIA requires that tuna canners can only label their products as 'dolphin-safe' if the tuna was harvested using certain strict criteria. For instance, if there is even one instance in which a tuna-fishing boat circles dolphins with a purse seine net, the entire load of tuna captured by the boat cannot be called dolphin-safe. In recent years a number of tourist programs around the world have developed in which humans are encouraged to swim and interact with bottlenose dolphins at sea. Scientists are concerned that such human interactions can interrupt dolphin social activities or force them to leave preferred habitats. In the United States, the Marine Mammal Protection Act of 1972, amended in 1988 and 1992, prohibits feeding or harassing dolphins and all marine mammals in U.S. waters. Scientific classification: Dolphins belong to the suborder Odontoceti (toothed whales) of the order Cetacea. The common bottlenose dolphin is classified as Tursiops truncatus and the Indo-Pacific bottlenose dolphin is classified as Tursiops aduncus. The three species of common dolphin are classified in the genus Delphinus. The five species of spotted dolphins belong to the Stenella genus. The six species collectively referred to as lags belong to the genus Lagenorhynchus. The Irrawaddy river dolphin is classified as Orcaella brevirostris. The Amazon river dolphin is classified as Inia geoffrensis, the Indian river dolphin as Platanista gangetica, the Chinese river dolphin as Lipotes vexillifer, and the Franciscana as Pontoporia blainvillei. Reviewed By: Daniel K. 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