Why can't humans drink raw water, but animals drink it directly?

In nature documentaries, we often see this scene: on a vast grassland, there is a mud puddle that is not too deep. Many wild animals gather around the puddle to drink water, and some animals roll happily in the muddy water. When you see this scene, does a thought flash through your mind: is this water drinkable?

Maybe animals want to use kettles, too? Image source: documentary "The Great Rift Valley: The Beautiful Heart of Africa"

Or when you see a group of hyenas and vultures surrounding a dead wildebeest and feasting on it, have you ever wondered whether the meat is edible?

For most modern people, we now follow extremely strict dietary standards. Raw water needs to go through multiple water treatment steps from the water source to the tap, such as flocculation, sedimentation, filtration, disinfection, and water quality testing. Raw meat must also be carefully washed and cooked before it can be safely eaten. Drinking water directly from a puddle on the roadside? Catching prey and eating it? This is absolutely unacceptable!

Data released by the World Health Organization show that more than 500,000 people die each year from diarrhea caused by drinking unclean water, and more than 200 million people become ill from eating undercooked meat or contaminated fresh food.

According to statistics in the early days of liberation, there were more than 10 million patients and 100 million people were threatened by schistosomiasis infection. The picture shows a schistosomiasis patient at the end of the last century.

But why do wild animals do this and nothing happens to them? Is it really okay?

Acute perception

First of all, it must be made clear that wild animals do not eat whatever they can get and drink water whenever they see it. They also eat selectively. In the Red Desert in southwestern Wyoming, the United States, lives the largest population of pronghorn antelope (Antilocapra americana). As we all know, water is the most precious and scarce resource in the desert, but interestingly, scientists have found that pronghorn antelopes are very principled and will not drink water without hesitation. They almost never visit water sources with a total dissolved solids (TDS) of more than 5000ppm, and when the pH of the water exceeds 9.2, they will leave collectively and look for new water sources. Although we are not sure how they do it, they can indeed sense the differences in water quality between different water bodies in some way, and they are very sensitive.

We need pH test paper to know things, but they seem to have superpowers. Image source: invasive.org

So the seemingly bad water these animals drink may not actually be as bad as imagined.

In the observation of tigers' feeding behavior, people also found that they are very sensitive to the smell of food, and can easily distinguish whether the meat in front of them is fresh by simply sniffing it. In this regard, our human sense of smell pales in comparison.

Tigers may also have had the experience of diarrhea. Source: wildlifetrip.org

Adaptive immunity

However, even the most powerful sensory system cannot be completely leak-proof. Compared with physical and chemical indicators such as pH and odor, various pathogenic microorganisms hidden in water and food are often more difficult to detect and more deadly. At this time, the animal's own immune system plays a key role.

In the study of the immune system of wild Western European house mice (Mus musculus domesticus), people found that populations of the same species living in different places showed great differences in serum antibody concentrations, the number of immune cells in the body, the state of immune response activation, etc. However, the immune levels of different individuals in the same region were relatively close. Studies on voles (Microtus agrestis) also confirmed that although they have the same immune system, they are affected by different external environmental factors, and individuals from different populations may have great differences in the expression of genes that generate key immune factors.

Top: Wild house mouse; bottom: Vole. Image source: naturephoto-cz.com; lookphotos.com

Back to the issue of drinking water and eating meat, these wild animals have lived in similar environments for generations, using water sources in the same area and hunting similar prey. They have developed basic immunity to common pathogenic microorganisms in the water/food in the area (individuals that cannot adapt will become sick and be eliminated). This resistance will be passed on to offspring through maternal immunity , and the cycle will continue, continuously consolidating their own and the group's immunity.

This is also why animals that have been raised in captivity for a long time are prone to acclimatization once they enter the wild environment. Although their equipment seems to be no different from that of their wild counterparts, because they have never been exposed to these wild pathogens, they cannot activate effective immune mechanisms to fight against them.

Strong digestive power

In addition to the immune system, the digestive system of wild animals also plays an important role in this adaptation process.

First of all, in addition to water, various nutrients, and digestive enzymes, the animal's mouth also contains a variety of immune components such as lysozyme, white blood cells, and immunoglobulins , which form their first line of defense against "disease from the mouth". In in vitro tests, canine saliva has shown excellent bactericidal power, and has a good killing effect on common bacteria in food such as Escherichia coli and Streptococcus. Think back again, do you often see animals licking their wounds with their tongues? This is actually them using the "small medicine box" in their mouths to help clean wounds and accelerate healing.

Hey, I can even eat human feces. Image source: doghealth.com

Secondly, wild carnivores are equipped with a more powerful gastrointestinal barrier . The average pH of the stomach environment of omnivorous animals is about 2.9, and the average pH of the stomach environment of humans is 1.5-3.0. The pH of the stomach of scavengers such as hyenas and vultures is as low as an astonishing 1.3. Under the protection of such strong acid, it is not easy for pathogens in food to stir up trouble. Some scholars even suggested that due to the reduction of pathogens in our food, the demand for using gastric acid to block harmful bacteria is gradually decreasing. In the future, human gastric acid may evolve in the direction of reduction or dilution? !

Carnivorous animals have a lower average pH, especially scavengers.

In addition to stomach acid, their shorter intestines are also an advantage. Our intestines are about 7-8 meters long, 4-5 times our height, while tigers' intestines are only 5.4 meters long, 3-4 times their body length, and wolves' intestines are only 3 times their body length. This can greatly reduce the time that harmful bacteria stay in the body through food, expel them from the body as soon as possible, and minimize the damage.

Intestinal length of dogs and cats Source: veteriankey.com

Finally, these animals also have intestinal microbial systems that are more suitable for eating raw meat in the wild. Scientists compared the intestinal flora of wild wolves and domestic dogs and found that the flora in wild wolves is very different from that in dogs. In dogs, there are more microorganisms related to carbohydrate and polysaccharide metabolism. In the intestines of wild wolves, the number of microorganisms related to immunity, high protein, and high fat intake and absorption is significantly higher, which fully reflects the perfect match of intestinal microorganisms for the host's living environment and food. Similar microbial differences have also been found in wild and captive Siberian tigers. So it may not be a good idea to simulate the diet of their wild ancestors and feed too much raw meat to your pet cats and dogs at home!

In fact, it is safest to feed your pet special pet food

The result of the above series of adaptive changes is that these wild animals have acquired stronger and more targeted disease resistance and better food absorption efficiency.

Survivorship bias

The so-called "survivor bias" refers to the fact that we pay too much attention to the "survivors" and ignore those individuals who did not survive or were not observed, thus drawing wrong conclusions. We see wild animals drinking water at the water hole and eating carrion on the grassland, but this does not mean that they will not get upset. It may just be that we did not notice or have no chance to observe those negative cases. Maybe the cases of wild animals getting sick and dying from drinking raw water and eating raw meat are far more common than we think. Therefore , keeping the water source in the habitat of wild animals clean is a very important and important part of wildlife protection that cannot be ignored.

In 2020, more than 330 elephants died in Botswana after consuming toxins produced by cyanobacteria in the water.

We don’t have to feel sorry for not being able to eat raw meat or drink water. Cooking water and meat before eating may be the best way to survive for us humans! Doing so can not only help us reduce the occurrence of pathogens in food, but also greatly reduce the investment in digesting food, allowing us to spend more time and energy on other meaningful things.

References

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[2]Marigorta, Urko M., Oscar Lao, Ferran Casals, Francesc Calafell, Carlos Morcillo-Suárez, Rui Faria, Elena Bosch et al. "Recent human evolution has shaped geographical differences in susceptibility to disease." BMC genomics 12 (2011): 1-14.

[3]Hart, Benjamin L., and Lynette A. Hart. "How mammals stay healthy in nature: the evolution of behaviors to avoid parasites and pathogens." Philosophical Transactions of the Royal Society B: Biological Sciences 373, no. 1751 (2018): 20170205.

[4]Ning, Yao, Jinzhe Qi, and Guangshun Jiang. "Comparative analysis of microbial community structure and function in the gut of wild and captive Amur tiger." Frontiers in Microbiology 11 (2020): 532171.

[5]Chen, Lei, Mengyao Sun, Di Xu, Zenghao Gao, Yuying Shi, Shen Wang, and Yiping Zhou. "Gut microbiome of captive wolves is more similar to domestic dogs than wild wolves indicated by metagenomics study." Frontiers in Microbiology 13 (2022): 1027188.

[6]Sundstrom, Charles. "Water consumption by pronghorn antelope and distribution related to water in Wyoming's Red Desert." In Proc. Antelope States Workshop, vol. 3, pp. 39-47. 1968.

[7]McKee, JE, and HW Wolf. 1963. “Water quality criteria.” Publication 3-4. State Water Quality Control, Sacramento, California, USA.

[8]Abolins, Stephen, Luke Lazarou, Laura Weldon, Louise Hughes, Elizabeth C. King, Paul Drescher, Michael JO Pocock, Julius CR Hafalla, Eleanor M. Riley, and Mark Viney. "The ecology of immune state in a wild mammal, Mus musculus domesticus." PLoS biology 16, no. 4 (2018): e2003538.

[9] Arriero, Elena, Klara M. Wanelik, Richard J. Birtles, Janette E. Bradley, Joseph A. Jackson, Steve Paterson, and Mike Begon. "From the animal house to the field: Are there consistent individual differences in immunological profile in wild populations of field voles (Microtus agrestis)?." PLoS One 12, no. 8 (2017): e0183450. Beasley, DeAnna E., Amanda M. Koltz, Joanna E. Lambert, Noah Fierer, and Rob R. Dunn. "The evolution of stomach acidity and its relevance to the human microbiome." PloS one 10, no. 7 (2015): e0134116.

[10]Fujimori, Shunji. "Gastric acid level of humans must decrease in the future." World journal of gastroenterology 26, no. 43 (2020): 6706.

[11]Pobiner, Briana. "Evidence for meat-eating by early humans." Nature Education Knowledge 4, no. 6 (2013): 1.

Planning and production

Source: Bowuzai (ID: bowuzai)

Author: Ge Zhiyuan

Editor: He Tong

Proofreading: Xu Lai, Lin Lin