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.
Unlike land mammals, dolphins sleep with only part of their brains at any time, past research has suggested. Half of their brains rest, while the other half remains "awake," and dolphins regularly switch which side is active.
"After being awake for many hours or days, humans and other animals are forced to stop all activity and sleep," said researcher Brian Branstetter, a marine biologist at the National Marine Mammal Foundation in San Diego. "Dolphins do not have this restriction, and if they did, they would probably drown or become easy prey."
To see just how mindful dolphins are with just half a brain, researchers tested their ability to scan the environment. Dolphins use echolocation to map the world, a biological form of sonar where they emit clicks and listen for their echoes to probe murky, dark surroundings.
The researchers set up a portable floating pen outfitted with eight modules, each consisting of an underwater sound projector and microphone. When a dolphin scanned any of these modules using echolocation clicks, they could respond with sounds mimicking echoes of those clicks from remote surfaces. Essentially, these modules could behave as "phantom targets" -- illusions that acoustically simulated physical objects.
The scientists had two dolphins -- a female, Say, and a male, Nay -- continuously scan these modules. If they detected phantom targets, they were trained over the course of a year to press a paddle to get fish. The dolphin Say often gave victory squeals whenever she succeeded.
The scientists found these dolphins could successfully use echolocation with near-perfect accuracy and no sign of deteriorating performance for up to 15 days. The researchers did not test how much longer the dolphins could have continued. Branstetter indicated that "Dolphins can continue to swim and think for days without rest or sleep, possibly indefinitely."
These findings suggest that dolphins evolved to sleep with only half their brains not only to keep from drowning, but also to remain vigilant. "These majestic beasts are true unwavering sentinels of the sea," Branstetter said.
Future research can help verify whether the dolphins stayed awake and alert for multiple days by sleeping with half their brains. This would require monitoring their brains for electrical activity via electroencephalogram, or a EEG.
"Research with freely moving humans who wear portable EEG equipment has been conducted; training a dolphin to wear a similar portable EEG backpack that is capable of withstanding and functioning in an ocean environment presents much greater challenges," Branstetter said. "However, these hurdles are not insurmountable. Also, we are interested in investigating if dolphins can perform more complex cognitive tasks without rest, like problem-solving or understanding an artificial language." (Adapted from Encarta encyclopedia and Internet sources)