Herman Pontzer, one of the researchers said, “You and I sitting in front of our computers use more energy each day than these orangutans that are walking around, and climbing around and socializing around their big enclosures.”
The researchers used slightly altered water to measure metabolic rate. Non-radioactive isotopes such as deuterium and oxygen-18 were used in place of hydrogen and oxygen to make a water-like liquid. The animals consumed the liquid, and the uncommon isotopes were measured in the elimination. This technique, the doubly-labeled water method, has been used in over 200 species of animals, including humans.
Orangutans were studied at an animal park in Des Moines called the Great Ape Trust. The subjects were found to use about 30 percent less energy than expected relative to their mass (Males range from 150-200 lbs. and females 50-100). The activity level of the orangutans studied in the animal park was deemed similar to that of wild ones. Wild orangutans spend most of their time in trees looking for food and eating. They also can spend twelve hours a day sleeping. The researchers have suggested that orangutans have evolved to have a slower metabolic rate to survive in a habitat which at times provides little-to-no fruit due to seasonal fluctuations.
The orangutans cover a fairly wide range of trees in order to find enough to eat. They actually spend little time on the ground, and are not adept at walking. It is also thought they could be the least prolific, or one of the least prolific mammals reproductively.