Dolphins - What Type of Animal is a Dolphin?

Dolphins are part of the mammalian order Cetacea, which contains about 90 species of whales, dolphins and porpoises. Of these, the gigantic blue whale is the largest and the freshwater or river dolphin the smallest. A big blue whale may attain a length of 31m (104ft) and weigh up to 125 tons, while the La Plata dolphin will rarely exceed 1.75m (5.7ft) in length.

Cetaceans are the mammals most completely adapted to life in the water. Whales, dugongs and manatees are the only mammals which spend their entire life in water (seals and sealions etc, return to dry land to mate and breed). The body of the whale is streamlined; tapering towards the tail. Whales cannot move about on land and, having no breastbone, they will usually die if stranded on the seashore because the weight of the body causes excess pressure on the lungs. In addition, the skin of the whale needs to be moist at all times, so exposure to air over a long period can cause serious damage to the outer layers of skin.

The largest member of the dolphin family is the killer whale (Orcinus orca), which grows to a length of 9m (29.5ft) and weighs about 10 tons. Killer whales hunt in packs and will kill fish, squid and marine mammals such as seals and whales - including other dolphins! The killer whale is found in all the oceans of the world wherever suitable prey species exist.

Many kinds of dolphins and porpoises are sociable animals and live in schools varying in size from a family group to around 1,000 individuals. Some dolphins, especially the common dolphin seem to enjoy playing around the bows of ocean-going ships. They use the pressure valve built up around the bows and "freewheel" along just in front of the ship. Dolphins are fast swimmers and can maintain a steady speed of up to 20mph of required.

Dolphins are born after a gestation period of 12 months. They are weaned at 9 months and become able to breed at the age of 8 years. The female dolphin will have one baby each year until she stops all breeding at the age of about 14 years. Life expectancy is 25 - 35 years.