Straighter legs make for easier walking.Cary Wolinsky

It seems that some chimps surprisingly use less energy walking on two feet than they do loping around on all four limbs. The researchers who discovered the expert walking ability of one chimp say it may help to explain how the earliest humans adapted to standing upright.

Chimpanzees normally move about on all fours in a gait known as a 'knucklewalk', says David Raichlen from the University of Arizona in Tucson. "But they can also walk on two legs — when they're carrying things, reaching for fruit, that kind of thing."

Raichlen trained five adult chimps to amble on a treadmill using both kinds of locomotion, and he also roped in four humans to strut their stuff at a variety of different speeds. The researchers then measured the amount of oxygen used in each case.¹

As expected, chimps were significantly less efficient at walking than humans, using up 75% more energy, irrespective of whether they were walking on two legs or four. This is mainly down to the fact that humans walk using relatively straight legs. This means they can propel themselves along by swinging their rigid appendages, using a minimal amount of muscle. Chimps, on the other hand, generally walk with their knees and hips flexed.

But the chimp performances varied significantly. Three of them found bipedalism a real effort, burning up around a third more energy than when on all fours. But one chimp, Lucy, found upright walking remarkably easy — even easier than knucklewalking. "She walks with her knees and hip more extended," explains Raichlen.

The first hominids evolved in Africa about 6-8 million years ago and it's thought that they started walking upright straight away. Why they did so, however, is disputed among anthropologists, and a number of different ideas have been suggested. The theory that Raichlen ascribes to says that bidepalism evolved because it was a more energy-efficient way of moving between areas with trees and food as the climate changed and forests dwindled.

Raichlen notes that if the same variation existed in our early ape-like ancestors as does in today's chimps, with some finding it easier to walk than others, this could have helped to drive adaptation. "That's what natural selection would be able to work on," he says.

Visit our chimpismadefor_walking.html">newsblog to read and post comments about this story.  

• References

  1. Sockol, M. D., Raichlen, D. A. & Pontzer, H., et al. Proc. Natl Acad. Sci. doi:10.1073.pnas.0703267104 (2007).