Why do kangaroos hop instead of run?

The only big animal that bounces

Lots of small animals hop: frogs, rabbits, grasshoppers and hopping mice. But kangaroos are the only large animals that get around mainly by hopping. A big red kangaroo can be taller than an adult human, and it still bounces everywhere on two back legs.

Kangaroos belong to a family of animals called macropods, which means "big foot". The family also includes wallabies, wallaroos and tree-kangaroos. Their back feet are long and strong, and their back legs are packed with muscle. Their front legs are much smaller and are mostly used for feeding, grooming and leaning on when they move slowly.

So why hop, when almost every other large animal walks or runs? The answer is hidden inside those long back legs.

Springs inside the legs

Running the way we do costs a lot of energy. Every time your foot lands, your muscles have to work to push you forward again. The faster you go, the more energy you burn.

A kangaroo's legs work more like a pogo stick. Along the back of each leg runs a long, stretchy tendon, the tough cord that joins muscle to bone. When the kangaroo lands, its weight stretches these tendons, and they store energy, just like a spring being squashed. As the kangaroo pushes off, the tendons snap back and hand much of that energy back again.

Because the tendons do so much of the work, a hopping kangaroo can recycle energy from one bounce to the next. Its muscles only have to top up what is lost, instead of doing everything from scratch.

Faster without getting more tired

In 1973, two scientists, Terence Dawson and Richard Taylor, trained red kangaroos to hop on a treadmill while they measured how much oxygen the animals breathed in. Oxygen use shows how much energy an animal is burning.

What they found was a surprise. For most animals, going faster means burning more energy. But once the kangaroos were hopping, going faster barely changed how much energy they used. Instead of taking more hops, a faster kangaroo mostly takes longer hops. The springy tendons store more energy on each bigger bounce, so speed comes almost for free.

This is perfect for life in the outback, where food and water can be far apart. A kangaroo can cover long distances at a steady bounce without wearing itself out. In short bursts, the biggest kangaroos can go faster than 50 kilometres an hour, and one leap can carry them about 8 metres.

The tail is a fifth leg

When a kangaroo is grazing and moving slowly, it does something strange. It leans forward onto its front paws and its tail, then swings both back legs forward together. Scientists call this pentapedal movement, which means moving on five "legs".

For a long time people thought the tail was only a prop for balance. In 2014, researchers measured the forces with special plates in the ground and found that the tail actually pushes the kangaroo along. In slow walking, the tail does about as much pushing as the front and back legs put together.

When a kangaroo is hopping fast, the heavy tail swings up and down to balance the body, a bit like the long pole a tightrope walker carries.

Tiny babies, big bouncers

Every bouncing kangaroo started life very small. A newborn joey is about the size of a jellybean. It is born after only about a month and crawls through its mother's fur into her pouch, where it keeps growing.

Red kangaroo joeys dive back into the pouch for safety until they are around eight months old. Soon after, they are hopping beside their mother, and their tendons are already starting to work like springs.

Kangaroos are so well suited to Australia that they appear on the country's coat of arms, next to the emu. Next time you see one bounding across a paddock, remember: it is not just jumping, it is running on springs.

📚 Words to know

Tendon

A tough, stretchy cord that joins a muscle to a bone. Kangaroo tendons store energy like springs.

Macropod

The animal family that kangaroos, wallabies and wallaroos belong to. The name means "big foot".

Marsupial

A mammal whose babies finish growing in a pouch, like kangaroos, koalas and wombats.

Pentapedal

Moving on five limbs. A slow-moving kangaroo uses its tail as a fifth leg.

Elastic energy

Energy stored in something that is stretched or squashed, like a rubber band or a spring.

🧪 Try this: be a kangaroo spring

1. Find a safe, open space. Jump on the spot ten times with stiff, straight legs. Notice how hard the landings feel.
2. Now jump ten more times, letting your knees and ankles bend as you land, and spring straight back up. Does it feel easier and bouncier?
3. Your tendons (especially the Achilles tendon at the back of your ankle) stretched and sprang back, just like a kangaroo's. Theirs are much longer, so they store far more energy.
4. Challenge: measure how far you can go in three bouncy jumps with a tape measure. A red kangaroo can cover about 8 metres in one.