Orangutans spend all day exercising, slowly swinging from tree to tree, munching on low-fat plants, but they're still kind of pudgy. It turns out that your average orang, for all its constant motion, burns fewer calories per day than a human couch potato.

Scientists figured this out by convincing captive orangutans to drink radio-labeled water, a technique that has been used to measure metabolism in humans for decades:

Since the orangutans possess a rudimentary understanding of spoken commands or requests, the researchers could ask them to drink water that had been modified to contain heavier isotopes of hydrogen and oxygen and then to urinate in cups for analysis.

“It’s that easy,” said Herman Pontzer, a professor of biological anthropology at Washington University in St. Louis and lead author of the study of the energetics of orangutans that will be published in the Proceedings of the National Academy of Sciences. “It’s easier than working with 3-year-olds.” [NYT]

The average 120-pound female orangutan burns just 1600 calories a day, despite continual exertion and an extremely muscular build. A moderately active woman burns several hundred more calories a day, despite having less muscle. The researchers say that orangs evolved an extra-thrifty metabolism to cope with an uncertain food supply.

[Photo credit: flickr user Paolo Camera, distributed under Creative Commons.]