DURHAM, N.C. — Although humans are constantly losing water from sweat, urination, and even respiration, our bodies conserve more water, making us the “water-saving” primates, scientists say. How do they know? A recent study comparing humans and apes indicates that humans are more efficient at replacing water lost from the body. Researchers at Duke University compared the daily amount of water lost and replaced was measured in humans and apes, calculating that apes use 30% to 50% more water per day than humans.
Water levels must be maintained in the body for healthy blood volume, as well as keeping the volume of other important fluids within a homeostatic range. Scientists believe this water-saving ability allowed our evolutionary ancestors to hunt for food farther away from water sources. “Even just being able to go a little bit longer without water would have been a big advantage as early humans started making a living in dry, savannah landscapes,” says lead author Herman Pontzer, associate professor of evolutionary anthropology at Duke, in a statement.
Fluid levels in the body are maintained by various methods of homeostasis. “Water coming in has to equal water coming out,” Pontzer explains. For example, when water is lost by sweat, the body sends a signal to the brain which indicates thirst. When we take in an excess amount of water, the kidneys are stimulated to produce more urine to rid the body of the excess fluid.
Comparing water retention between humans and apes
For the study, researchers recruited 309 participants, which included hunters, farmers, and office personnel. They also studied 72 apes from zoos and nature reserves. They were able to calculate and compare the amount of water taken in and the amount of water lost by the GI tract, urination, and sweat. Results showed that on average, humans process about 12 cups or 3 liters per day, whereas an ape processes up to 6 liters or double the amount of water per day.
Due to the fact that apes do not possess the ability to sweat like humans, the results are surprising. “Humans have 10 times as many sweat glands as chimpanzees do,” says Pontzer. Humans can lose over half a gallon of water by sweating for an hour, the authors note.
Additionally, apes such as chimps, gorillas, orangutans, and bonobos do not expend much energy in their daily lives. “Most apes spend 10 to 12 hours a day resting or feeding, and then they sleep for 10 hours. They really only move a couple of hours a day,” Pontzer says.
These factors were accounted for in the study as researchers also incorporated differences in body size, activity levels, number of calories used, and even climate. This led to the conclusion that humans are the more efficient water-savers regardless of habitat or physical activity levels. These results suggest the reduction in water usage is an evolutionary change that enables humans to use water more efficiently.
Pontzer explains that during this time of evolution, humans likely could not survive long without water, like humans today. “You probably don’t break that ecological leash, but at least you get a longer one if you can go longer without water. The next step is to pinpoint how this physiological change happened,” he says.
The data suggest that the human body adapted so that they did not crave as much water per calorie as the ape family. Other studies have shown that human breast milk contains 25% less water than the breast milk of ape mothers when comparing the water-to-calorie composition.
An additional hypothesis centers around the nose structure of humans and apes. Prior to 1.6 million years ago, human fossils reveal flattened noses like those of our ape cousins. In the sinus passages of the nose, water vapor is cooled and then condensed back into liquid during respiration, helping to prevent water loss. Scientists believe the evolved human nose which protrudes from the face has helped humans conserve more water.
“There’s still a mystery to solve, but clearly humans are saving water. Figuring out exactly how we do that is where we go next, and that’s going to be really fun,” said Pontzer.
This study is published in the journal Current Biology.