Whales, Dolphins and Sea Mammals

Mammals that live in the sea

The ocean is full of fish — but some of its biggest, cleverest animals are not fish at all. Whales and dolphins look fish-shaped and live underwater, yet they are mammals, just like you, a dog or an elephant. Animals like these are called marine (sea) mammals, and they also include seals, sea lions and porpoises.

So how can an animal live in the sea and still be a mammal? It comes down to four things every mammal does.

The four mammal clues

An animal is a mammal if it:

1. Breathes air with lungs
2. Is warm-blooded (keeps a steady warm body temperature)
3. Gives birth to live young (not eggs)
4. Feeds its babies milk

Whales and dolphins tick every box. A baby dolphin is born alive underwater and drinks its mother's milk. The whole family stays warm even in cold seas. To compare them with other groups, see animal groups: mammals, birds and reptiles, and explore the ocean further in oceans and sea life.

Breathing through a blowhole

Because they have lungs, sea mammals cannot breathe underwater. Instead they swim up to the surface and breathe through a blowhole — a nostril on the top of the head. When a whale surfaces, it blows out warm, used air in a big spray, then takes a fresh breath and dives again.

Why a blowhole on top? Having the breathing hole on top of the head means the animal only needs to lift the very top of its head out of the water to breathe, so it barely has to slow down. When it dives, strong muscles close the blowhole tight, like a trapdoor, to keep water out.

Whales can hold their breath for a very long time. A dolphin surfaces every few minutes, but a deep-diving sperm whale can stay under for nearly an hour.

Staying warm in cold water

The sea can be freezing, so sea mammals have a thick layer of fat called blubber under their skin. Blubber works like a cosy coat, trapping body heat inside. It also stores energy for times when food is hard to find. This is why seals and whales can swim happily in icy polar waters where most animals would quickly freeze.

Finding food with sound

Deep water is dark, so many sea mammals find food using echolocation. The animal makes clicking sounds, and the sound waves bounce off objects and come back as echoes. By listening to the echoes, a dolphin can tell where a fish is, how big it is and how fast it is moving — even when it cannot see it. This is a bit like the sonar used by submarines.

Two ways to eat

Sea mammals feed in two main ways:

• Toothed whales and dolphins have teeth and hunt fish and squid one by one. They use echolocation to track their prey.
• Baleen whales, like the giant blue whale, have no teeth at all. Instead they have rows of comb-like plates called baleen. They gulp a huge mouthful of seawater full of tiny shrimp-like creatures (krill), then push the water out through the baleen and swallow the food left behind.

Why no teeth on the biggest whales? Filtering lets a blue whale — the largest animal that has ever lived — eat tonnes of tiny food every day without ever needing to chase it.

Observe and investigate

1. Hold your breath: time how long you can comfortably hold your breath. Then look up how long a sperm whale can hold its — you will see why blubber, slow heartbeats and big lungs make whales such amazing divers.
2. Make an echo: stand a good distance from a large wall or building and shout. Listen for the echo bouncing back. That returning sound is exactly what a dolphin uses to "see" with sound.
3. Spot the spray: if you ever visit the coast or watch a wildlife film, look for the misty "blow" when a whale surfaces. That spray is the whale breathing out before its next dive.

Next time you see a dolphin leap, remember: it is a warm-blooded, air-breathing mammal that just happens to call the ocean home.