How many legs does a kangaroo have?

Kangaroos have a third leg, say scientists - masquerading as a tail.

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Red kangaroo

 Yes – the kangaroo actually has three legs.

This discovery was made in 2014 by a team from Canada’s Simon Fraser University.

They corralled red kangaroos through a chamber that measured the downward forces they exerted as they walked.

When they aren’t hopping, kangaroos walk by putting their arms on the ground and moving their back legs in front of them.

As they do this, they push down with their tail to propel themselves forward.

This tail exerts just as much force as any of the other limbs, making it effectively a third leg. Not only that, it’s the most important of the three limbs in terms of momentum – so kangaroos walk with their tails.

The ancestors of today’s kangaroos were marsupials that lived in trees and used their tails for climbing and gripping.

But after 15 million years living on the ground as grass-grazing ruminants, the kangaroo’s tail has evolved into a specialised limb.

It contains almost as many vertebrae as the human spine, and is full of powerful muscles.

When boxing, kangaroos lean on their tails, and can use them as weapons – a single swipe could break someone’s leg.

There are other candidates for species that naturally have an odd number of legs, but none are very convincing.

Most starfish have five limbs, but these are arms, not legs, because they’re not used for locomotion.

Snails and slugs are called gastropods (‘stomach feet’) because they only have one foot they crawl around on.

But this is an integral part of their bodies rather than a limb, so they don’t count either.

Finally, almost all of the Japanese spider crabs that are caught have nine limbs, but this is because they’ve lost one: they’re all born with 10. 

Extracted from QI: The Third Book of General Ignorance (Faber & Faber, £14.99).

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