One of the characteristics that distinguish humans from their closest relatives is apparently the efficiency of our bodies in handling water, according to a study. We use 30 to 50 percent less water per day than chimpanzees, gorillas and orangutans. It is still unclear on which physiological characteristics this increased frugality is based. However, a lower water requirement presumably formed an advantage in the course of human evolution, and so our ancestors developed into the “low-flow model”, say the scientists.
Nothing works without the elixir of life: animals and humans must ensure that the physical water content remains within a certain range so that the physiological processes can run optimally. The challenge here is: through perspiration, excretions and breathing, we are constantly losing water that has to be refilled. The system for keeping the fluid balance in a healthy range is similar to maintaining a certain water level in a bathtub, explains Herman Pontzer of Duke University in Durham: “The water that flows in must correspond to the water that flows out”.
On the trail of water consumption
As part of their study, Ponzer and his colleagues have now investigated the extent to which water consumption differs between humans and their closest relatives in the animal kingdom. They used data from previous studies as a basis, which includes extensive information on 309 people with different lifestyles. Data sets from studies on the physiology of a total of 75 great apes, which contained information relevant to water consumption, were used for comparison. These were chimpanzees, bonobos, gorillas and orangutans from zoos and near-natural open-air parks. For each individual in the study, the researchers calculated on the one hand the water intake through food and drink and on the other hand the water loss through sweat, urine and digestion.
As they report, the balance of all entrances and exits and taking relevant co-factors into account: The average person processes around three liters a day. In contrast, the consumption of great apes was almost twice as high. When the researchers included differences in climate, body size and factors such as activity level and calorie consumption per day in the calculations, the basic frugality of humans became apparent: consumption is between 30 and 50 percent less than that of great apes, the researchers write.
Surprisingly economical
This appeared surprising against the background that humans among primates have a pronounced ability to sweat. “Humans have ten times as many sweat glands per square centimeter of skin as chimpanzees,” says Pontzer. In addition, the great apes lead a comparatively leisurely life. “They spend a lot of time resting and sleeping ten hours. You only move a few hours a day, ”says Pontzer. But apparently they still use more water than humans.
The results therefore suggest that in the course of human evolution, adaptations have occurred that have reduced the amount of water our body needs on a daily basis. Against the background of the assumptions about human evolutionary history, this also appears plausible: Increased water economy could have enabled our hunter-gatherer ancestors to move further away from bodies of water in search of food. “Even the possibility of going a little longer without water would have been a great advantage when early humans began to live in dry savannah landscapes,” says Pontzer.
Does the nose play a role?
However, there are still some unanswered questions about the background to the results. One explanation the data suggests is that our bodies’ needs and thirst responses have been rebalanced so that we need less overall water per calorie compared to our ape relatives. Corresponding studies in humans and monkeys are now necessary to confirm this hypothesis. The researchers also want to pursue another lead: our nose may play an important role in saving water. Because compared to great apes, we have much larger versions. In the nasal passages, water can be recovered by cooling and condensing water vapor from the exhaled air, the researchers explain.
Fossil evidence suggests that the enlargement of the nose began in Homo erectus about 1.6 million years ago. Thus, a nose that protruded more could have helped early humans regain moisture with every breath. “There are still some unanswered questions, but at least it is now fundamentally becoming apparent that humans are water savers. Finding out how we do it exactly is our next step, ”says Pontzer.
Source: Duke University, technical article: Current Biology, doi: 10.1016 / j.cub.2021.02.045