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 the case with our water intake.
Compared to apes, a surprising new study has found that our bodies process far less fluids on a daily basis.
The researchers found that, on average, humans processed 3 liters, or about 12 cups, of water a day. Chimpanzees, bonobos, and gorillas that live in a zoo, on the other hand, go through almost twice as long.
The results were somewhat unexpected. Since humans have 10 times more sweat glands than chimpanzees and are generally much more active than apes, we are expected to lose more water every day, not less.
However, even when outside temperatures, body size, and activity levels are taken into account, humans still need less water to maintain a healthy balance.
“Compared to other apes, the humans in this study had substantially less water turnover and consumed less water per unit of dietary energy metabolized,” the authors write.
This suggests that early hominins somehow developed a way or ways to conserve their bodily fluids, allowing them to travel from the rainforest to more arid regions. It is unclear exactly how that was achieved.
“Even just being able to go a little longer without water would have been a huge advantage when the first humans began to make a living in dry savanna landscapes,” explains study lead author and evolutionary anthropologist Herman Pontzer of the University of Duke.
In the study, the researchers tracked the daily water turnover of 72 apes, both in zoos and jungle 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 to 309 modern humans who drank the same double-labeled water. These humans came from a variety of lifestyles, including sedentary farmers, hunter-gatherers, and 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 total proportion of water Energy still equaled humans elsewhere. approximately 1.5 milliliters for each calorie consumed.
In fact, it’s worth noting that this same ratio is evident in human breast milk. Apes’ breast milk, 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 somehow ‘readjusted’ over time, meaning that we may desire less water per calorie than our ape cousins.
In the jungle, apes get most of their water from plant foods, which means they can go days or weeks without drinking anything directly. Humans, however, can only survive for about three days without water, possibly because our food is not so wet.
This inevitably requires us to drink fluids more often than apes, which means we can’t stray too far from our ties to 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 expand to drier environments where heat stress is higher and finding food requires more work.
However, there is another way that 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 first appear in the fossil record approximately 1.6 million years ago, with the appearance of Homo erectus, and ever since, such prominent noses have continued to diverge from the flatter snouts of the apes.
More space within the nasal passages gives the water a chance to cool and condense, allowing reabsorption of fluids rather than exhaling the fluid 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 still a mystery to be solved, but clearly humans are saving water,” says Pontzer.
“Finding out exactly how we do it is the next step, and that’s going to be a lot of fun.”
The study was published in Current biology.