Primates may have their slow metabolisms to thank for their long life span, a new study suggests.

The findings, published in the Proceedings of the National Academy of Sciences, show that humans and other primates burn 50 percent fewer calories each day than other mammals. Scientists credit primates’ long lives to their slow metabolism to explain why primates have long lives and grow up slowly compared to other mammals.

"The results were a real surprise," Herman Pontzer, an anthropologist at Hunter College in New York and the lead author of the study, said in a news release. "Humans, chimpanzees, baboons and other primates expend only half the calories we'd expect for a mammal. To put that in perspective, a human -- even someone with a very physically active lifestyle -- would need to run a marathon each day just to approach the average daily energy expenditure of a mammal their size."

Researchers conducted the study by collecting data from primates in zoos and sanctuaries. Using a non-invasive technique known as “doubly labeled water,” they were able to track the amount of carbon dioxide produced in the body, which helped measure the number of calories the primates burn over a period of 10 days.

The primates’ metabolic rate was slow, leading researchers to conclude that primates have relatively longer lives because of it. But scientists remain puzzled on why primates expend little energy and how they evolved that way. "We just do not know enough yet," primatologist Erin Vogel of Rutgers University in New Jersey told New Scientist.

Pontzer says the primates’ slow metabolism may have evolved to help the animals deal with food shortages. "Orangutans experience extended periods of low fruit availability," Vogel said. "There are months when caloric intake is less than expenditure -- and they burn body fat stores."

Researchers also found that captivity did not play a role in the primates’ metabolic rate. That is, primates in captivity burned just as many calories as those living in the wild. This suggests that physical activity may have a lesser impact than previously thought on total energy expenditure.

"Exercising more might not mean burning more energy," Pontzer said. "It might be that bodies get used to the lifestyles that they live, and work their way toward this predetermined energetic set point. People with traditional lifestyles, working their butts off, don't burn any more calories than me or you."