Dolphins (Delphinidae) are aquatic mammals related to whales and porpoises and famous for their intelligence, apparent compassion and joy. There are almost 40 species of dolphin in 17 genera. Dolphins vary in size from 1.2 metres (4 feet) and 40 kilograms (88 pounds) (Maui's Dolphin), up to 9.5 metres (30 feet) and 10 tonnes (the Orca). However, the average length for most North American species is 13.89 feet in length. Most species weigh about 50 to 200 kilograms (110 to 440 pounds).
Dolphins are found worldwide, mostly in the shallower seas of the continental shelves, and 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 (The Miocene epoch is a period of time that extends from about 23 to 5.3 million years before the present).
Dolphins have a spindle-like shaped body that tapers at both ends, adapted for fast swimming. Their head contains the 'melon' (an oval shaped oily, fatty lump of tissue found at the centre of the forehead ), which is used for echolocation (non-human animals emitting sound waves and listening to the echo in order to locate objects or navigate).
In many species of dolphin, the jaws are elongated, forming a distinct beak however for some species like the Bottlenose Dolphin, there is a curved mouth that looks like a fixed smile. Teeth can be very numerous (up to 250) in several species. The dolphin brain is large and has a highly structured cortex, which often is referred to in discussions about their high intelligence.
Dolphins are often regarded as one of the most intelligent animal species. Dolphins often leap above the water surface, sometimes performing acrobatic figures. Scientists are not quite certain about the purpose of this behaviour, but it may be to locate schools of fish by looking at above water signs, like feeding birds.
Dolphins can also communicate with other dolphins to join a hunt, attempting to dislodge parasites, or simply doing 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. Dolphins have even been seen harassing other creatures, like seabirds and turtles. Dolphins also seem to enjoy riding waves and are frequently seen 'surfing' coastal swells and the bow waves of boats.