We humans and our primate kin are weird mammals. We grow slowly, bear few young, and live exceptionally long lives. Now there's an explanation: primates simply expend less energy.
Herman Pontzer of Hunter College, New York, and colleagues have compared the average daily energy expenditure of primates with that of other mammals. His team studied animals living at the Lincoln Park Zoo in Chicago.
Pontzer found that primates expend 50 per cent less energy than other mammals of equivalent mass during an average day. "What's more, he says 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.
Only the tiny mouse lemur breaks the mould, expending more energy while active each day than the average primate. Pontzer suggests this is because the lemur spends significant amounts of its time in a low-energy rest state called torpor, which the experiments did not measure.
The finding offers a completely new way to understand why primates have slower life histories than other mammals of equivalent body size, says primatologist Erin Vogel of Rutgers University in New Jersey.
No one knows quite how primates expend so little energy. "We just do not know enough yet," says Vogel."
It is also unclear how primates first came to expend so little energy. Pontzer thinks that the slower metabolism may have evolved to help primates cope with food shortages. For instance, orang-utans suffer frequent famines. "Orang-utans experience extended periods of low fruit availability," says Vogel. "There are months when caloric intake is less than expenditure – and they burn body fat stores." A slow metabolism might help them survive.
Modern humans are an exception, perhaps because of our greater intelligence. Most of us rarely go without food, and a new analysis shows that is even true of hunter-gatherers, who had been thought to be more prone to famine than people in farming societies (Biology Letters, doi.org/qvn).
Young primates also take a long time to make the switch to solid food, Vogel says, and may need extra time to learn how to survive in challenging environments. "Slowing life-history down would be advantageous and provide juveniles with extended periods to learn how to survive," she says. "If they can slow down their expenditure, this could buffer them against an uncertain environment."
Burn those calories
Surprisingly, Pontzer's captive primates expended just as much energy per day as those in the wild. Pontzer says this suggests that physical activity is not as closely linked to energy expenditure as is generally thought.
"Exercising more might not mean burning more energy," says Pontzer. "It might be that bodies get used to the lifestyles that they live, and work their way toward this predetermined energetic set point. People with traditional lifestyles, working their butts off, don't burn any more calories than me or you."
Journal reference: PNAS, DOI: 10.1073/pnas.1316940111
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