The secret of longevity of humans and other primates have been finally revealed - they burn 50 percent less calories a day than other mammals. A new study published in the Proceedings of the National Academy of Sciences Jan. 13 shows that slow metabolism among humans and other primates may explain why they grow up so slowly and live longer than other animals.

For the study, an international group of scientists who work with primates in zoos, sanctuaries and in the wild examined the daily energy expenditure of 17 primate species to see whether a primate's slow pace of life is a result of slow metabolism. Using a technique known as "doubly labeled water", the researchers were able to track the body's production of carbon dioxide which helped measure the number of calories that the primates burned over a 10 day period.

"The results were a real surprise," said Herman Pontzer, an anthropologist at Hunter College in New York and lead author of the study. "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."

The researchers also found that primates in captivity burn as much calories a day as those in the wild suggesting that physical activity may have lesser impact on daily energy expenditure than what was previously believed.

"The environmental conditions favoring reduced energy expenditures may hold a key to understanding why primates, including humans, evolved this slower pace of life," said David Raichlen, an anthropologist at the University of Arizona and coauthor of the study.

The findings of the study are expected to provide a better understanding of health and longevity in humans. Connecting the rate of growth, reproduction and aging to daily energy output, for example, may provide more information about the processes behind body development and aging. The researchers also said that further investigation into the relationship between physical activity and daily energy expenditure can help improve understanding of obesity and other metabolic diseases.

"Humans live longer than other apes, and tend to carry more body fat," Pontzer said. "Understanding how human metabolism compares to our closest relatives will help us understand how our bodies evolved, and how to keep them healthy."