| Dolphins are aquatic mammals related to whales and porpoises . The name is from Ancient Greek word, delphis meaning "with a womb", viz. "a 'fish' with a womb".
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There are almost 40 species of dolphin in 17 genera. They vary in size from 1.2 m (4 ft) and 40 kg (88 lb) ( Maui's Dolphin ), up to 9.5 m (30 ft) and 10 tonnes (the Orca). Most species weigh about 50 to 200 kg (110 to 440 lb). They are found worldwide, mostly in the shallower seas of the continental shelves, and all are carnivores, mostly eating fish and squid.
The family Delphinidae is the largest in the Cetacea, and relatively recent: dolphins evolved about 10 million years ago, during the Miocene .
Dolphins are widely believed to be amongst the most intelligent of all animals. A typical statement would be that dolphins are roughly as intelligent as a two-year-old human. However, experts in comparative psychology or animal cognition would be reluctant to make any such estimate, as quantitative comparisons of intelligence between species are notoriously difficult to make in principle.
Straightforward comparisons of species' relative intelligence are complicated by differences in sensory apparatus, response modes, and nature of cognition; furthermore, the difficulty and expense of doing experimental work with a large marine animal mean that even such tests as can meaningfully be done have still not been done, or have been carried out with inadequate sample size and methodology. See the Dolphin intelligence article for more details.
Dolphins often leap above the water surface, sometimes performing acrobatic figures (e.g. the spinner dolphin ). Scientists aren't quite certain about the purpose of this behavior, but it may be to locate schools of fish by looking at above water signs, like feeding birds. They could also be communicating to other dolphins to join a hunt, or attempting to dislodge parasites. Perhaps they just do it for fun. Play is a very important part of dolphins' lives and they can often be observed playing with seaweed or playfighting with other dolphins. They have even been seen harassing other creatures, like seabirds and turtles. Frequently dolphins will accompany boats, riding the bow waves.
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They are also famous for their willingness to occasionally approach humans and playfully interact with them in the water. In return, in some cultures like in Ancient Greece they were treated with welcome; a ship spotting dolphins riding in their wake was considered a good omen for a smooth voyage. There are many stories of dolphins protecting shipwrecked sailors against sharks by swimming circles around the swimmers.
Dolphins are social animals, living in pods (also called "schools") of up to a dozen animals. In places with a high abundance of food, schools can join temporarily, forming an aggregation called a superpod ; such groupings may exceed 1000 dolphins. The individuals communicate using a variety of clicks, whistles and other vocalizations. They also use ultrasonic sounds for echolocation
Membership in schools is not rigid; interchange is common. However, the animals can establish strong bonds between each other. This leads to them staying with injured or ill fellows for support.
Because of their high capacity for learning, dolphins have been employed by humans for any number of purposes. Dolphins trained to perform in front of an audience have become a favorite attraction in dolphinaria , for example SeaWorld . Dolphin/Human interaction is also employed in a curative sense at places where dolphins work with autistic or otherwise disabled children. The military has employed dolphins for various purposes from finding mines to rescuing lost or trapped persons. Such military dolphins , however, drew scrutiny during the Vietnam War when rumors circulated that dolphins were being trained to kill Vietnamese Skin Divers .
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