Primates, Humans Too, Expend Half the Daily Calories as Other Mammals of Similar Size

New research shows that primates - humans included - burn half as many calories in a day compared to other mammals their size.

The markedly slower metabolisms of humans and other primates help explain their slow aging process and high longevity, according to research published in the Proceedings of the National Academy of Sciences.

Some animals with faster metabolisms are able to reproduce not long after being born and go on to have lots of offspring before dying relatively young.

This is not the case for primates such as humans, apes, monkeys, tarsiers, lorises, and lemurs, all of which have long childhoods, reproduce infrequently and live long lives.

Using non-invasive techniques over a 10-day period, researchers examined the daily energy expenditures for 17 primate species to test whether the primates' slow pace of life is a result of their slow metabolism.

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

Pontzer and his colleagues contend that primates' slow rate of growth, reproduction, and aging correlate to their slow rate of energy expenditure. The researchers argue that this is an indication that "evolution has acted on metabolic rate to shape primates' distinctly slow lives."

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

Surprisingly, the researchers found that primates in captivity burn just as many calories in a day as their wild counterparts, which suggests that physical activity contributes less to energy expenditure than previously believed.

The researchers plan to take what they've learned and explore it more in depth with a focus specifically on humans.

"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."