A new study out of New York’s Hunter College sheds some light on why it’s not only difficult for humans to shed pounds, but also why humans and primates tend to live so much longer than other mammals of similar size. According to their research, we burn about 50% fewer calories per day than comparably sized mammals. The slower metabolism partly explains why we take so much longer to reach maturity, and why we live so long.

Dr. Herman Pontzer lead the study, which involved measuring the caloric output of 17 different primate species over a period of 10 days. They examined animals in zoos, the wild and sanctuaries, using a reportedly non-invasive measurement technique.

“The results were a real surprise,” says Pontzer. “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.”

Healthcare professionals have long believed that diet and genetics play a much larger role in calorie expenditure than physical activity could ever hope to, and Pontzer’s research in non-human primates supports that. Despite moving around much less, animals in captivity burned about the same number of calories per day as their wild counterparts.

As scientists learn more about the link between activity and energy expenditure, it may help us better understand obesity and metabolic diseases. The relationship between the two is becoming increasingly complex. Having too much body fat is obviously not good for longevity, but it’s possible that the way we burn and store energy plays a role in how long we live. Dogs and cats, for example, typically carry very little body fat and rarely live longer than their late teens.

Pontzer has already begun work on a more detailed study of of the relationship between energy expenditure, activity and aging among humans and apes. “Humans live longer than other apes, and tend to carry more body fat,” says Pontzer. “Understanding how human metabolism compares to our closest relatives will help us understand how our bodies evolved, and how to keep them healthy.”