Humans have developed large, energy-hungry brains that require us to consume far more calories than our immediate family members. However, the same does not seem to be our water intake.
Compared to monkeys, a surprising new study found that our bodies struggle through much less fluids daily.
Researchers found that humans processed an average of 3 liters, or about 12 cups, of water per day. In contrast, chimpanzees, bonobos, and gorillas pass through a zoo almost twice as much.
The results were somewhat unexpected. Since humans have ten times as many sweat glands as chimpanzees, and are generally much more active than monkeys, you would expect us to lose more water every day, not less.
Even when people take into account outside temperature, body size and activity levels, people still need less water to maintain a healthy balance.
“Compared to other monkeys, 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 developed a way or ways to conserve their body fluids so that they could travel from the rainforest to drier regions. Exactly how this was achieved remains unclear.
“Even just being able to go without water a little longer would have been a great advantage because early humans began to make a living in arid, savannah landscapes,” explains lead author and evolutionary anthropologist Herman Pontzer of the Duke University.
In the study, researchers tracked the daily water turnover of 72 monkeys, in both zoos and rainforest shrines, with double-labeled water containing deuterium and oxygen-18 as trackers. It was able to tell researchers how much water was obtained through food and drink and lost through sweat, urine and the GIT.
The results were then compared with 309 modern people who drank the same double-labeled water. These people come from different lifestyles, including farmers, hunters and sedentary office workers.
Even among a small sample of adults in rural Ecuador, who drink a significant amount 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 in people elsewhere, about 1.5 milliliters for every calorie consumed.
In fact, it is worth noting that the same ratio occurs in human breast milk. The breast milk of monkeys, on the other hand, has a ratio of water to energy that is 25 percent lower.
Such findings suggest that the thirst response of the human body is somehow ‘re-established’ over time, meaning that we may require less water per calorie than our cousins.
In the rainforest, monkeys get most of their water from plant foods, which means they can go days or weeks without drinking directly. However, humans can only survive three days without water, possibly because our food is not nearly as wet.
This inevitably requires us to drink fluids more frequently than monkeys, which means we can not stray too far from our ties with lakes and streams (or running water).
Pontzer refers to it as an ‘ecological leash’, and he claims that natural selection has given humans a longer lead, so that we can continue to drive without water, so that early hominins can expand in drier environments where heat stress is greater and around finding food requires more work.
However, there is another way our bodies have changed to save water. Unlike monkeys, humans have an external nose, which presumably reduces water loss when we breathe.
These prominent snouts first appeared in the fossil record about 1.6 million years ago, with the rise of Homo erectus, and since then such prominent noses have continued to deviate from the flatter snouts of monkeys.
More space in the nasal passages allows water to cool and condense, allowing reabsorption of fluids instead of exhaling the fluid into the air. In addition to our thirst response, these new noses would have been crucial in enabling people to be more active in arid environments.
“There is still a mystery to be solved, but it is clear that people are saving water,” says Pontzer.
“Determining exactly how we do it is where we’re going, and it’s going to be really fun.”
The study was published in Current biology.