WASHINGTON U.-ST. LOUIS (US)—Orangutans living in a large indoor/outdoor habitat use less energy, relative to body mass, than nearly any eutherian mammal ever measured, including sedentary humans.

All this despite activity levels similar to orangutans in the wild.

“It’s like finding a sloth in your family tree,” says Herman Pontzer, assistant professor of anthropology at Washington University in St. Louis. “It’s remarkably low energy use.”

The research appears online Aug. 2 in the early edition of the Proceedings of the National Academy of Sciences.

Pontzer spent two weeks studying daily energy expenditure of orangutans in the Great Ape Trust, a 230-acre campus in Des Moines, Iowa.

The study revealed an extremely low rate of energy use not previously observed in primates, but consistent with slow growth and low rate of reproduction in orangutans.

This may be an evolutionary response to severe food shortages in the orangutan’s native Southeast Asian rainforests of Borneo and Sumatra—highly random environments that often experience crashes in the availability of ripe fruit, the food on which orangutans depend.

Orangutans have adapted over time by becoming consummate low-energy specialists, decreasing their daily energy needs to avoid starvation in food-poor times.

Pontzer thinks the research also may shed light on the evolved energy use of other primates and human foragers.

More news from Washington University in St. Louis: http://news-info.wustl.edu/