Humans have developed large, energy-hungry brains that require us to consume far more calories than our closest animal relatives. However, the same doesn’t seem to hold true for our water intake.
Compared to monkeys, a surprising new study has shown that our bodies pass much less fluids on a daily basis.
Researchers found that people processed an average of 3 liters or about 12 cups of water per day. Chimpanzees, bonobos, and gorillas living in a zoo, on the other hand, pass nearly twice as often.
The results were somewhat unexpected. Since humans have ten times more sweat glands than chimpanzees, and are generally much more active than monkeys, you would expect us to lose more water every day, not less.
But even when we factor in outside temperatures, body size and activity levels, people still needed less water to maintain a healthy balance.
“Compared to other monkeys, the humans in this study had significantly lower water turnover and consumed less water per unit of metabolized food energy,” the authors write.
This suggests that early hominins somehow evolved a way or ways to conserve their bodily fluids, allowing them to travel from the rainforest to more arid regions. Exactly how that was achieved remains unclear.
“Even if we could go without water for just a little bit longer, that would have been a big advantage, as early humans began to make a living in arid savannah landscapes,” explains lead author and evolutionary anthropologist Herman Pontzer of Duke University.
In the study, researchers tracked the daily water turnover of 72 monkeys, both in zoos and rainforest reserves, with doubly labeled water containing deuterium and oxygen-18 as trackers. This could tell researchers how much water was gained through food and drink and lost through sweat, urine and the gastrointestinal tract.
The results were then compared with 309 modern humans drinking the same doubly labeled water. These people came from a variety of lifestyles, including farmers, hunter-gatherers, and sedentary office workers.
Even among a small sample of adults in rural Ecuador who drink remarkably large amounts of water for cultural reasons (more than 9 liters per day for men and almost 5 liters per day for women), the total water / energy ratio was still corresponds to that of people elsewhere, about 1.5 milliliters for every calorie consumed.
In fact, it’s worth noting that the same ratio can be seen in human breast milk. The mother’s milk of monkeys, on the other hand, has a water-to-energy ratio that is 25 percent lower.
Such findings suggest that the human body’s thirst response has been somehow ‘modified’ over time, meaning we may need less water per calorie than our cousins.
In the rainforest, monkeys get the most water from plant food, which means they can spend days or weeks without drinking directly. However, humans can only survive without water for about three days, possibly because our food isn’t nearly as wet.
This inevitably requires us to drink liquids more often than monkeys, which means we can’t stray too far from our ties to lakes and streams (or running water).
Pontzer calls this an “ ecological leash, ” and argues that natural selection has given humans a longer lead so we can travel further without water, allowing early hominins to expand into drier environments where heat stress is greater and finding food more work. necessary.
However, there is another way that our bodies could have changed to conserve water. Unlike monkeys, humans have external noses, which are thought to reduce water loss while breathing.
These prominent snouts first appear in the fossil record about 1.6 million years ago, with the emergence of Standing man, and such prominent noses have continued to diverge from the flatter snouts of monkeys ever since.
More space in the nasal passages allows water to be cooled and condensed, allowing reabsorption of liquids instead of exhaling the liquid into the air. In addition to our thirst response, these new noses could have been crucial in allowing humans to be more active in arid environments.
“There is one more mystery to be solved, but it is clear that people are saving water,” says Pontzer.
“Figuring out exactly how we do that is where we’re going, and it’s going to be a lot of fun.”
The study is published in Current Biology