Why do we have two legs?

By Martyn Robinson 10 May 2010
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An Australian Museum expert explains why the number of legs varies between different animals.

QUITE CLEARLY WE – and the birds – survive with just two walking legs. Shouldn’t that be good enough for all other animals? We can walk upright and the elevation provides good vision of problems or opportunities ahead. We can move swiftly, stepping or jumping over barriers and keeping our front legs (arms or wings) free for other purposes like flying (birds and bats) or feeding and answering mobile phone calls (us).

But two legs are also unstable, as any one-year-old child can demonstrate. It takes energy to keep upright, and it’s easy to trip. The loss or breakage of one leg can be very serious – even fatal for non-flying birds like penguins or emus.

Four legs might not give you the best view but they are more stable and faster (how many legs does the fastest land animal, the cheetah, have?). A broken or missing limb can still be a problem, but not as serious as it is for bipeds. You often find three-legged mammals and reptiles that live to a ripe old age within wild populations.

Limbs to spare

What’s good at four is even better at six or eight. Insects don’t trip. With their often brief, dangerous lives they risk losing limbs, but have more to spare and in some cases can regrow the missing parts.

Is coordination a problem for insects, spiders and especially centipedes (one pair of legs per body segment) and millipedes (two pairs per segment)? No. Insects use their six legs in two sets of three – like two tripods, which are very stable. Each ‘step’ forward uses the two outside legs on one side and the middle leg on the other side.

Spiders, centipedes and other multi-leggers use a wave or ripple step where the first and third pair of legs reaches forward while the second and fourth pair pushes back. These movements are then switched so the second and fourth reach forward and the first and third push back, and the pattern is repeated all the way along the body in centipedes and millipedes – very stable and fast. And just think how swiftly a huntsman spider can move once it knows you’re trying to catch it.

So that just leaves snakes and other legless animals to think about – but that’s another story.

Martyn Robinson is a researcher at the Australian Museum in Sydney. This is an edited version of an article first published by the museum’s Explore Magazine.