Humans, chimpanzees, baboons and other primates expend only half the calories we would 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," said Herman Pontzer, an anthropologist at Hunter College in New York and the lead author of the study.

These remarkably slow metabolisms explain why humans and other primates grow up so slowly and live such long lives.

Most mammals, like the family dog, live a fast-paced life, reaching adulthood in a matter of months, reproducing prodigiously and dying in their teens if not well before.

By comparison, humans and our primate relatives (apes, monkeys, lorises, and lemurs) have long childhoods, reproduce infrequently, and live exceptionally long lives, said the study published in journal Proceedings of the National Academy of Sciences.

Scientists examined daily energy expenditure in 17 primate species - from gorillas to mouse lemurs - to test whether primates' slow pace of life results from a slow metabolism.

Using a safe and non-invasive technique known as 'doubly labeled water', - which tracks the body's production of carbon dioxide - the researchers measured the number of calories that primates burned over a 10-day period.

"The results were a real surprise. This dramatic reduction in metabolic rate, previously unknown for primates, accounts for their slow pace of life," said Pontzer.

"The research also sheds light on the fact that zoo-housed primates are relatively active, with the same daily energy expenditures as wild primates," said co-author Steve Ross from Chicago's Lincoln Park Zoo.


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