How do you measure an orangutan's energy use? First, you feed it heavy water. Then you need to get it to pee in a cup. Luckily, captive orangutans at the Great Ape Trust in Iowa are pretty agreeable. "We walked around with some little paper Dixie cups and just held them under the ape and asked them if they would pee in the cup for us," says Washington University anthropologist Herman Pontzer.
Pontzer thinks orangutans have evolved super-slow metabolisms because, in the wild, they survive on fruit that can be scarce for much of the year. I'd be interested to know, though, how similar the captive orangutans really are to wild ones. The researchers say that the captive orangutans have "activity levels similar to orangutans in the wild." But surely there are other factors that could affect the metabolism of these animals, which are close relatives to humans. The types of activity they get, their stress levels, the regularity of their feeding, and their degree of interaction with other orangutans must all be different in captivity from in the wild. Just think of all the factors (according to your average women's interest magazine, anyway) that can affect human metabolism!
Even if the captive apes have a slower metabolism than their wild counterparts, it's still an impressive feat. The only mammal with a slower measured metabolism is the tree sloth. Like an orangutan, a sloth hangs out in trees a lot (often upside-down, in the case of the sloth) and has goofily long arms. Both animals are big dozers.
The sloth moves at an especially unhurried pace: somewhere around 2 meters per minute. So much for cardio!