Cetaceans are carnivorous mammals that live permanently in the water. The group is made up of the whales, dolphins and porpoises.
Whales have a long, streamlined shape with no hair or fur on their bodies, so that they can swim quickly and easily. They also have a thick layer of fat called blubber to keep them warm in cold waters. Like all other mammals, whales must breathe air. Although they are able to hold their breath underwater for long periods of time, they have to come to the surface to breathe.
Whales are the largest animals in the world. In fact, the enormous blue whale, which can measure over 30 metres long, is probably the biggest animal that has ever lived. Whales are able to grow so large because their huge weight is supported by the water. If a land animal grew to this size, its legs would collapse under the weight of its own body.
Despite their huge size, most great whales feed on tiny creatures called zooplankton, which float through the oceans in great masses. The whales take in mouthfuls of water and filter out the zooplankton through plates of horn-like material, called baleen, in their mouths. Some will also feed on schools of tiny fish using this method.
Other whales such as the sperm whale or the beluga have many small, peg-like teeth. These whales feed mainly on fish or squid.
The sperm whale preys on giant squid. It is able to dive to depths of over 3000 metres and stay underwater for over an hour in search of its deep-dwelling prey.
There is little light underwater, so most whales find their way using echolocation, in the same way as bats. They communicate with one another using clicks, squeals and moans.
Dolphins also live in water all the time, but they are smaller than the whales. They all have teeth and feed mostly on fish or squid, which they are able to chase through the water at speed. The largest member of the dolphin family, the orca or killer whale, also feeds on sea mammals such as seals or even whales.
Dolphins range in size from just over one metre, to the orca which can be nine metres in length. They have a backward-curving dorsal (back) fin, and many have beak-like snouts. Most dolphins live in the oceans, but a few live in large rivers.
Like most whales, dolphins use echolocation to navigate through the water. This also helps them to find shoals of fish or squid. The few species of dolphin that live in the long rivers of India, China and South America, are almost blind and rely completely on echolocation to find their way through the muddy waters.
Most dolphins are social animals. They often form large groups which can number several hundred. Dolphins will frequently move between groups, but some species, such as the orca, form small, stable families.
Dolphins are intelligent, inquisitive animals. Some kinds will often swim beside boats or come up to investigate us if we enter the water. Dolphins are famous for their acrobatics, leaping and twisting up out of the water. This may be a way of taking in air quickly while swimming at top speed, signaling, or maybe just having fun.
Porpoises are smaller than dolphins, and have blunt snouts. They have small, flattened teeth rather than the pointed teeth of dolphins. Porpoises usually live alone or in small groups. Most are shy animals that stay below the surface of the water as much as possible. This means that they are rarely seen by humans.
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