Humans have developed large, energy-hungry brains that require us to consume far more calories than our closest animal relatives. However, the same does not seem to be true for our water intake.
Compared to monkeys, a surprising new study found that our bodies produce much less fluid daily.
The researchers found that, on average, people processed 3 liters, or about 12 cups, of water a day. Chimpanzees, bonobos and gorillas living in a zoo, on the other hand, pass almost twice as long.
The results were somewhat unexpected. Because humans have 10 times more sweat glands than chimpanzees and are, on the whole, much more active than monkeys, you would expect us to lose more water every day, no less.
However, even when outdoor temperatures, body size and activity levels are taken into account, people still need less water to maintain a healthy balance.
“Compared to other monkeys, the people in this study had a substantially 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 body fluids, allowing them to travel from the rainforest to drier regions. Exactly how this was done remains unclear.
“Even the mere possibility of being left without water for a while would have been a great advantage, as the first humans began to make a living in dry landscapes and savannas,” says study 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 in rainforest sanctuaries, using double-labeled water containing deuterium and oxygen-18 as trackers. This was able to tell the 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 people who drank the same double-labeled water. These people come from a range of lifestyles, including farmers, hunter-gatherers and sedentary office workers.
Even among a small sample of adults in rural Ecuador who consume a remarkable amount of water for cultural reasons (over 9 liters per day for men and almost 5 liters per day for women), the overall water-energy ratio fits people elsewhere about 1.5 milliliters for each calorie consumed.
In fact, it is worth noting that the same ratio is evident in human breast milk. Monkey’s breast milk, on the other hand, has a 25% lower water-energy ratio.
Such findings suggest that the human body’s thirst response has been somehow “regulated” over time, which means we could crave less water per calorie than our cousin monkeys.
In the rainforest, monkeys take most of their water from plant food, which means they can spend days or weeks without drinking at all directly. However, humans can only survive about three days without water, possibly because our food is not nearly as moist.
This inevitably forces us to drink fluids more often than monkeys, which means we can’t stray too far from our connections with lakes and streams (or running water).
Pontzer refers to this as an “ecological leash” and argues that natural selection has given humans a longer lead so that we can travel further without water, allowing early hominins to spread to drier environments, where heat stress is higher. and finding food requires more work.
There is, however, another way in which our bodies could have been changed to conserve water. Unlike monkeys, humans have external noses, which are thought to reduce water loss when we breathe.
These prominent muzzles first appear in the fossil record about 1.6 million years ago, with the appearance of Standing man, and since then, such prominent noses have continued to move away from the flatter muzzles of monkeys.
More space in the nasal passages gives water the ability to be cooled and condensed, allowing fluids to be reabsorbed instead of expiring fluid into the air. In addition to our thirst response, these new noses could have been crucial in enabling people to be more active in arid environments.
“There is still a mystery to be solved, but clearly people are saving water,” says Pontzer.
“Discovering the exact way we do this is where we go next and it will be a lot of fun.”
The study was published in Current biology.