Humans have developed large, energy-hungry brains that require us to consume far more calories than our closest animal relatives. The same, however, does not seem to apply to our water intake.
Compared to monkeys, a surprising new study found that our bodies experience far less fluid on a daily basis.
The researchers found that, on average, humans processed 3 liters, or about 12 cups, of water per day. Chimpanzees, bonobos and gorillas that live in a zoo, on the other hand, go through almost twice that.
The results were somewhat unexpected. Since humans have ten times more sweat glands than chimpanzees and, in general, are much more active than monkeys, we would be expected to lose more water each day, not less.
However, even considering external temperatures, body size and activity levels, humans still need less water to maintain a healthy balance.
“Compared to other monkeys, the humans in this study had substantially less water turnover and consumed less water per unit of metabolized energy in the food,” write the authors.
This suggests that early hominins somehow developed a way or ways to conserve their body fluids, allowing them to travel from the rainforest to more arid regions. Exactly how this was achieved is still unclear.
“Even being able to run a little more without water would have been a great advantage, since the first humans started to make a living in dry savanna landscapes,” explains the study’s lead author and evolutionary anthropologist Herman Pontzer of Duke University.
In the study, the researchers tracked the daily water volume of 72 monkeys in zoos and rainforest sanctuaries, using double-labeled water containing deuterium and oxygen-18 as trackers. This was able to 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 to 309 modern humans who drank the same double-labeled water. These humans 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 a notable amount of water for cultural reasons (more than 9 liters per day for men and almost 5 liters per day for women), the overall ratio of water to energy is still equals humans elsewhere, about 1.5 milliliters for each calorie consumed.
In fact, it is important to note that this same proportion is apparent in breast milk. Monkeys’ breast milk, on the other hand, has a 25% lower water-to-energy ratio.
These findings suggest that the human body’s reaction to thirst has somehow “readjusted” over time, which means that we may want less water per calorie than our monkey cousins.
In the rainforest, monkeys get most of the water from plant foods, which means they can go days or weeks without drinking directly. Humans, however, can only survive about three days without water, possibly because our food is not so wet.
This inevitably requires that we drink fluids more often than monkeys, which means that we cannot get too far from our connections with lakes and streams (or running water).
Pontzer refers to this as an “ecological collar” and argues that natural selection gave humans a greater advantage so that we could travel further without water, allowing early humans to expand into drier environments, where thermal stress is greatest and finding food takes more work.
There is, however, another way in which our bodies may have changed to conserve water. Unlike monkeys, humans have external noses, which seems to reduce water loss when we breathe.
These prominent snouts first appeared in the fossil record about 1.6 million years ago, with the appearance of Homo erectusand ever since, those prominent noses have continued to diverge from the apes’ flatter snouts.
More space in the nasal passages gives the water the opportunity to be cooled and condensed, allowing the reabsorption of fluids instead of exhaling the liquid into the air. In addition to our response to thirst, these new noses may have been crucial in enabling humans to be more active in arid environments.
“There is still a mystery to be solved, but clearly humans are saving water,” says Pontzer.
“Finding out exactly how we do it is where we’re going next, and it’s going to be a lot of fun.”
The study was published in Current Biology.