Is walking upright more energy efficient?

ScienceDaily reports on a new study that gives some evidence for the hypothesis that walking on two legs is more energy efficient than than dragging your knuckles while walking.

Study Identifies Energy Efficiency As Reason For Evolution Of Upright Walking

A new study provides support for the hypothesis that walking on two legs, or bipedalism, evolved because it used less energy than quadrupedal knucklewalking.

When I read that last night, while rather tired, my first thought was that that it sounded inplausible - if walking upright is more energy efficient, then why don't most mammals walk on two legs. Rereading it today, I of course realized that walking upright, like a Home Sapiens, is more energy efficient than walking on your legs and knuckles, like our fellow apes does. That doesn't mean that it's necessarily more efficient than walking on four legs, like the majority of mammals do.

Bipedalism marks a critical divergence between humans and other apes and is considered a defining characteristic of human ancestors. It has been hypothesized that the reduced energy cost of walking upright would have provided evolutionary advantages by decreasing the cost of foraging.

"For decades now researchers have debated the role of energetics and the evolution of bipedalism," said Raichlen. "The big problem in the study of bipedalism was that there was little data out there."

The researches collected metabolic, kinematic and kenetic data from five chimpanzees and four adult humans walking on a treadmill. The chimpanzees were trained to walk quadrupedally and bipedally on the treadmill.

Humans walking on two legs only used one-quarter of the energy that chimpanzees who knuckle-walked on four legs did. On average, the chimpanzees used the same amount of energy using two legs as they did when they used four legs. However, there was variability among chimpanzees in how much energy they used, and this difference corresponded to their different gaits and anatomy.

Interesting result, and it certainly explains why bipedalism became the movement of choice for Homo Sapiens and our ancestors. Or at least, it does so, once we moved down from the trees - I would expect that if we looked at movement in trees, the energy use would be somewhat reversed.

The study is published in PNAS, but unfortunately it's behind a pay-wall. The abstract can be found here though.