Herman Pontzer from Washington University, who made the discovery, thinks that orangutans have evolved to live life in the slow lane because they can't be sure of a steady food supply. They mostly eat fresh fruit and, being large animals, they need lots of it. But rainforests are chaotic places where ripe fruit can disappear quickly, unpredictably and for a long time. If orangutans aren't getting any fuel, they have to minimise the amount of energy they spend, so that they don't starve to death. And they're very good at it.
This has many implications for zookeepers who care for captive orangutans. These are animals that eat as much as they can when food is around and burn off their calories very slowly - they're easy to overfeed and prone to obesity, even in enclosures with plenty of opportunities for exercise. Learning about the energy needs of these great apes will allow keepers to plan more appropriate diets for them.
It's a lesson that the staff of the Great Ape Trust sanctuary are taking to heart. The sanctuary, a sprawling 230-acre campus in Des Moines, is where Pontzer carried out his research. He studied four of the resident orangutans, including Azy, an adult male; Katy and Knobi, , adult females; and a young male called Rocky.
Pontzer tracked their activity with a technique commonly used in humans. The method involves 'doubly-labelled water', made of rarer and heavier versions of the normal hydrogen and oxygen atoms. These heavy atoms can be tracked as they make their way through the body, whether they end up in the animal's urine or in the carbon dioxide it breathes out. In fact, the amount that ends up in these two waste products is related. So by taking regular urine samples, Pontzer could work out how hard the orangutans were breathing out, and thus how much energy they were using up.
It turned out that they were using very little indeed. All the orangutans, including Rocky the youngster, were in the bottom 1% of all mammals in terms of the calories they burn every day. Adjusting for their size, they use less energy than humans from industrial societies, where inactive lifestyles are common. They even use less than macaque monkeys on a strict diet or lemurs undergoing temporary hibernation.
You might think that captive orangutans might be more lethargic than their wild cousins, but that's not the case for Azy and his chums. Their enclosure comes with climbing frames, rope and easy access to a three-acre forest, where they spend much of their days. The four apes spent around the same amount of time feeding, resting and sleeping as their wild counterparts, and they walked and climbed over similar distances. They may be more familiar with their surroundings and know the easiest routes, but even if they spent twice as much energy moving around, they'd still burn fewer daily calories than almost all other mammals.
Of course, Pontzer only studied four orangutans but these individuals are hardly sluggish members of the species. They're as active as their wild peers and they get in about as much exercise as human farmers who lead physically demanding lives. They have lively existences, but they use their energy very efficiently. When they're at rest, their metabolic rates are slightly lower than expected for their size and much lower than in humans and chimps. The odds of finding four humans that use similar levels of energy, even in a Western population, are 1 in 10,000.
The orangutan's extremely sparing use of energy is just part of an entire lifestyle that takes place in the slow lane. It grows more slowly than any primate except for humans, and it breeds less frequently than any ape. It lives alone so it doesn't have to share any food it managed to find. It even moves with incredible efficiency - rather than jumping across an opening in the treetops, it can rock the tree it sits on so they sway across the gap. All of these traits make orangutans "consummate low-energy specialists" that can survive in an environment where their food of choice is hard to come by.
But not all orangutans are the same. Carel van Schaik, who has spent many years studying orangutans, points out that there are two species or orangutan - one that lives in Sumatra and one that lives in Borneo. "The northern part of Sumatra is less subject to the dramatic lean periods seen in most of Borneo, so we might see some interesting differences between the two orangutan species in physiology," he says. Unfortunately, the individuals at the Great Ape Trust are all hybrids so they have nothing further to say on the matter.
Van Schaik also thinks that energy-saving adaptations might be a common feature among Borneo's mammals. There's some evidence that these creatures have a tendency to become smaller. And while orangutans are still relatively large, those from the most unreliable regions seem to have smaller brains for their size. Van Schaik also wants to see if gibbons share the same traits, especially because they lead slow lives with little risk of predators and they share the orangutan's problem of unpredictable food supplies.
By Ed Yong
Reprinted with permission from Discover