Who exerts less energy, a person lounging in an armchair or an orangutan swinging through the forest? Surprisingly, it's the ape. Researchers have found that, for their size, orangutans expend less energy than nearly every placental mammal measured. The discovery could help explain how animals have adapted to cope with lean times.
When it comes to expending energy, not all animals are created equal. Placental mammals use more energy than marsupials, for example, when performing the same task. Creatures with greater body mass expend more energy, and placental mammals tend to be larger than marsupials. But the equation doesn't always hold. Lizards, for example, exert less energy than birds when they both drink water even though lizards tend to be heavier.
To get a better sense of what other factors influence an animal's energy expenditure, biological anthropologist Herman Pontzer of Washington University in St. Louis and colleagues collected data on orangutans (Pongo pygmaeus and P. abelii). These animals are a bit of an energy curiosity: They live in the food-scarce Southeast Asian rainforest, yet they are very active.
The team studied orangutans at the Great Ape Trust, an ape sanctuary in Des Moines. The apes drank "doubly labeled water," a harmless liquid identical to water except that its hydrogens and oxygens are heavier than normal. That difference allowed researchers to track those molecules through the ape's system and to measure, indirectly through their urine, how much energy the animal expends. "All we'd have to do was say, 'Hey, Penelope, we'd like a urine sample,' " Pontzer says.
Although the orangutans at the Great Ape Trust spend their days running, climbing up trees, and playing with their friends, they expended about 30% less energy than expected for their mass, Pontzer's team reports online today in the Proceedings of the National Academy of Sciences. That makes orangutans the world's most energy-efficient primate measured, second only to the three-toed sloth as the most energy-efficient placental mammal for its size.
This energy efficiency is likely due to a low metabolic rate, the researchers found, more than 20% lower than expected for their size. As a result, an orangutan running through the forest burns anywhere from 500 to 1000 calories less than a human sitting in front of a TV.
The findings could explain why apes are able to survive on limited food, says primatologist Carel Van Schaik of the University of Zurich in Switzerland. "[The adaptation] will reduce overall energy needs, thus making it easier to meet them in lean times."