Humans and other primates—with the exception of the tiny mouse lemur—have a signficant edge over other large mammals. According to new research, we require half the amount of energy to get through our day as a mammal of equal size. And it's thanks to this energetic advantage that we enjoy such long lives.

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It also explains why, while other mammals take just weeks or months to grow up, it takes humans years and years.

A team of scientists studied energy expenditures of 17 primate species in zoos, sanctuaries and in the wild. They measured metabolic rate using an established method called "doubly labeled water," which involves dosing the study subject with a special isotope of oxygen and watching how the creature's system processes it into carbon dioxide over time. After collecting this data from primates, the researchers compared it to known values for other mammals, taken from previously published studies. Controlling for body size, primates burn about 50 percent fewer calories than other animals, they found.

The team has no idea why primates burn so little energy. "What's more, the difference is not easily explained by differing activity levels: a human would need to run a whole marathon every day to be on an even energetic footing with mammals that aren't primates," the lead author told the New Scientist.

Surprisingly, the researchers also found that the primates housed at zoos and sanctuaries burned through the same amount of energy as those in the wild, the New Scientist adds. It's a hint that there might be some energetic sweet spot that our bodies condition themselves to, regardless of what sort of lifestyle we lead.