Humans have developed large, high-energy brains that require us to consume many more calories than our closest animal relatives. However, it does not seem the same for our water intake.
Compared to apes, a surprising new study has found that our bodies bounce much less fluid daily.
The researchers found that, on average, humans processed 3 liters of water a day. In contrast, the chimpanzees, bonobos and gorillas that live in the zoo spend almost twice as much.
The results were a bit unexpected. Because humans have ten times more sweat glands than chimpanzees and are generally much more active than apes, we are expected to lose more water every day, no less.
Yet, even when external temperatures, body size, and activity levels are taken into account, humans still needed less water to maintain a healthy balance.
“Compared to other apes, the humans in this study produced 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 more arid regions. It is still unclear how exactly this was achieved.
“Even being able to go a little further without water would have been a great advantage as the first humans began to make a living in dry, savannah landscapes,” explains the study’s lead author. and evolutionary anthropologist Herman Pontzer of Duke University.
In the study, researchers tracked the daily water turnover of 72 monkeys at both zoos and rainforest sanctuaries, using trackers of doubly marked water containing deuterium and oxygen-18. This was able to tell researchers how much water was gained through food and drink and lost through sweat, urine and gastrointestinal tract.
The results were compared with 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 remarkable amount of water for cultural reasons (more than 9 liters a day for men and almost 5 liters a day for women), the global water-energy ratio still coincides with humans elsewhere, approximately 1.5 milliliters for each heat consumed.
In fact, it is worth noting that this same proportion is evident in human breast milk. Monkey breast milk, on the other hand, has a 25% lower water / energy ratio.
These findings suggest that the human body’s response to thirst has “re-tuned” over time, meaning we might want less calorie water than our ape cousins.
In the rainforest, monkeys get most of their water from plant foods, which means they can spend days or weeks without drinking directly. Humans, however, can only survive about three days without water, possibly because our food is not nearly as moist.
This inevitably forces us to drink liquids more often than apes, which means we can’t get too far away from our ties to lakes and streams (or running water).
Pontzer refers to this as an “ecological strap,” arguing that natural selection has given humans a longer advantage so that we can travel more without water, allowing early hominins to expand into drier environments where stress by heat it is larger and finding food requires more work.
However, there is another way our bodies could have changed to conserve water. Unlike apes, humans have external noses, which are believed to reduce water loss when we breathe.
These prominent snouts appear in the fossil record about 1.6 million years ago, with the appearance of Right man, and since then, such prominent noses have continued to diverge from the flatter muzzles of apes.
More space within the nasal passages gives water the ability to cool and condense, allowing fluid to be reabsorbed instead of exhaling the liquid into the air. In addition to our response to thirst, these new noses could have been crucial in enabling humans to be more active in arid environments.
“There’s still a mystery to be solved, but it’s clear that humans save water,” Pontzer says.
“Finding out exactly how we do it is where we’re going next, and that’s going to be a lot of fun.”
The study was published in Current biology.