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Getting your energy back

Why Does a Kangaroo Hop?

A hopping kangaroo goes faster and faster without getting more tired, because its legs give the energy back.

It lands and the tendons stretch Energy isstored in thestretch The tendons snap back Most of thepush camefree
Every landing pays for the next jump.

  1. Step 1 of 5

    The legs work like springs.

    Thin ankles, hugefeet Tendons that stretch andsnap back

    Behind a kangaroo's ankle are huge rubbery tendons. Landing stretches them, exactly like squashing a spring, and they snap back on the way up — so much of each hop costs no muscle at all.

  2. Step 2 of 5

    Going faster barely costs more.

    A running animal Faster costs steadilymoreA kangaroo Faster costs almost thesame

    For most animals, twice the speed means far more effort. A hopping kangaroo just hops further each bounce, so its energy use stays almost flat as it speeds up.

  3. Step 3 of 5

    Breathing comes free with it.

    Lands — the guts push up Air is pushedout Rises — the guts drop Air is drawnin

    Every landing squashes the guts up against the lungs and pushes air out; every rise pulls it back in. The breathing is driven by the bouncing, so it costs nothing extra.

  4. Step 4 of 5

    The tail is a third leg.

    Two back legs forhoppingA tail that pushes likea legSmall front legs forfeedingFeet far longer thanthe shin

    Standing still, a kangaroo props on its tail and back feet like a tripod. Walking slowly, the tail actually takes its weight and pushes — it does as much work as a whole leg.

  5. Step 5 of 5

    It cannot walk backwards.

    Forwards Big feet and a stifftail push it alongBackwards A direction it simplycannot go

    Those enormous feet and stiff tail simply will not go into reverse. A kangaroo can only hop forwards — which is one reason it is on the Australian coat of arms.

    A newborn kangaroo is about the size of a jellybean. It crawls up into the pouch by itself and stays there for months.

The short version

A kangaroo hops because its tendons store and return energy — so speeding up costs it almost nothing extra.

Try it yourself

Bounce gently on the spot without stopping. After a few, notice you are barely pushing — the bounce is doing it.

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