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Orangutans are more efficient than we are

Azy, a male orangutan, was one of the primates tested in an energy-efficiency study at the Great Ape Trust of Iowa.
Azy, a male orangutan, was one of the primates tested in an energy-efficiency study at the Great Ape Trust of Iowa.AP file
/ Source: Discovery Channel

Orangutans are more economical than any other primate, including humans. A new study found that orangutans need less food fuel than we do for the same, or greater, levels of activity.

In general, birds use more energy than mammals, which require more energy than marsupials, which in turn need more fuel than reptiles. But among mammals, orangutans merit an Energy Star label.

Orangutans "require less food than humans, pound-for-pound," lead author Herman Pontzer told Discovery News. When they do eat, orangutans nibble mostly on ripe fruit, along with smaller portions of leaves and seeds.

Even in captivity, this diet doesn't diminish an orangutan's get-up-and-go.

"They wake up early, after a long night's sleep," explained Pontzer, an assistant professor of anthropology at Washington University in St. Louis. "Then they spend the day socializing, exploring their indoor or outdoor enclosures. ... They also regularly engage in games with researchers."

Taken together, all of these activities add up to the same level of exercise performed by humans in physically demanding agricultural lifestyles, according to the study, which is published in the latest Proceedings of the National Academy of Sciences.

Peeing in a cup
Pontzer and his team documented the activities and daily energy expenditure of four orangutans housed at the Great Ape Trust in Des Moines, Iowa. The intelligent orangutans were taught to urinate in a paper cup each day throughout the study so that the scientists could analyze the waste material's contents.

When deuterium oxide, an isotopic form of water known as "heavy water," is drunk, it allows researchers to monitor carbon dioxide production, which in turn provides a very accurate measure of calories burned. For this study, the heavy water was mixed with sugar-free iced tea, a drink orangutans happily gulp as a treat.

The researchers additionally documented what the orangutans were eating. The scientists also measured the resting metabolic rate for each primate.

Relative to their body mass, orangutans were found to have the lowest daily energy expenditure of nearly all other mammals, with sloths being one of the few exceptions. A reason is that orangutans have a very low resting metabolism.

"Low energy requirements make it easier to survive periods of food shortage," said Pontzer.

Feast or famine
In the wild, orangutans live in the rainforests of Borneo and Sumatra where food availability is highly variable and unpredictable, Pontzer added.  Like fresh fruit from the garden, the pickings are often feast or famine.

"But the trade-off," he added, "is that low-energy throughput means less energy is available to do things like grow and reproduce. So orangutans grow slowly and reproduce slowly, which is evolutionarily risky because an orangutan might die before it passes on its genes."

Human mothers can have a child every two to four years, but orangutans in the wild only reproduce every seven to eight years.

Leslie Aiello, president of the Wenner-Gren Foundation, which has funded other primate research, believes this study "is exciting work because Pontzer and his team were able to directly measure the daily energy expenditure in orangutans rather than relying on proxies for estimation."

"(The findings) help to explain the slow orangutan life history," she added. "And as more comparable information becomes available on other ape species, they will help us to understand the energetic trade-offs that were involved in human and ape evolution."

Orangutans are highly endangered, with many lost due to human activities such as logging, mining and the illegal pet trade. Pontzer hopes the study will highlight "how much information we lose about our closest relatives and our own evolutionary history if we let them go extinct."