Lie down on the couch, television blaring, a bowl of chips on your stomach and a Big Gulp of cola on your chest. If you need anything, yell for it. Wait a few hours. Guess what? You're still burning more energy than a fully active orangutan.

According to a new study of orangutan energy expenditure, our close evolutionary cousins use less energy, pound for pound, than almost any other mammal. The findings could help illuminate how humans became so energy-intensive.

"Such an extremely low rate of energy use has not been observed previously in primates," wrote researchers led by Washington University anthropologist Herman Pontzer Aug. 3 in Proceedings of the National Academy of Sciences.

The researchers studied four orangutans – Azy, Knoby, Katy and a four-year-old named Rocky – at the Great Ape Trust, an Iowa-based center for non-invasive studies of great ape behavior, communication and culture.

To measure energy use, the researchers dosed the orangutans' water with traceable molecules of hydrogen and oxygen, then measured how the concentration changed in the orangutans' urine. Lost oxygen could be attributed to carbon dioxide produced as a body burns fat, carbohydrates and protein. This provided a true-to-life snapshot of energy use, far more detailed than at-rest metabolic rates produced by most primate metabolism studies.

Over the course of two weeks, the orangutans used even less energy than predicted. They used far less energy per unit of body weight than humans, or macaque monkeys on calorie-restricted diets, or hibernating lemurs. Excepting egg-laying mammals like duck-billed platypuses, or marsupials like kangaroos – whose odd reproductive habits and evolutionary histories make them metabolic outliers – the orangutans appeared to use less energy than every mammal except three-toed sloths.

A sloth can spend its adult life in a single tree, moving so slowly that moss grows on it. But orangutans are quite active, and the Great Ape Trust orangutans are no different. Inside their multi-acre, open-air woodland habitat, their activity patterns resembled those seen in the wild.

The findings suggest "a physiological adaptation for minimizing energy throughput previously unknown in apes," wrote the researchers. And whatever the mechanism may be, the findings also raise the question of why the orangutans require so little energy.

The researchers think it's an evolutionary adaptation to boom-and-bust cycles in the availability of the endangered orangutans' South Asian rain forest fruit fare. This fits with their extremely slow growth and reproduction rates. By contrast, humans – which last shared a common ancestor with orangutans between 12 million and 16 million years ago – are fast-growing and fast-breeding.

Improvements in hunting and gathering "may ultimately explain why human reproductive rates diverged," the researchers wrote.

Image: Flickr/Chi King.

See Also:

Citation: "Metabolic adaptation for low energy throughput in orangutans,” by Herman Pontzer, David A. Raichlen, Robert W. Shumaker, Cara Ocobock and Serge A. Wich. Proceedings of the National Academy of Sciences, Vol. 107 No. 31, August 3, 2010.

Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecological tipping points.