Why do most defecation animals take about 12 seconds?

Dogs, humans and elephants take the same amount of time to defecate, although they vary in size. .

Ancient Chinese people had a diagnosis of feces, a diagnosis of health based on the shape, size and texture of feces. So did the Egyptians, the Greeks, and nearly every ancient culture. Even today, your doctor may ask you about the last time you defecated and describe it in great detail.

Certainly it would be embarrassing to say this. But that's where science comes in, because what we don't want to talk about can still be harmful. Irritable bowel syndrome, inflammatory bowel disease, gastrointestinal infections and inflammatory bowel diseases cost Americans billions of dollars each year.

But trying to prevent these problems is not our main motivation to try to find the physics of defecation. It is something else, much more dangerous.

Elephants defecate at a speed of 6cm / sec, almost six times the speed of dogs.(Image copyright David Hosking / Alamy Stock Photo Image caption).

Years of changing diapers and then dumping turned me from a fledgling analytical person into a knowledgeable person. My life passed with a series of images: pea-hard feces to long stools like snakes, to a brown puddle.

Unlike the ancients, we do not believe that we can predict the future from children's faeces. But we do think it's worth trying to figure out where all the shapes come from. Having a laboratory to answer questions about the world daily is one of the distinct pleasures of a scientist.

As a fluid activist, we teamed up with colorectal surgeon Daniel Chu and two energetic students, Candice Kaminski and Morgan LaMarca, who filmed defecation and manual stool collection of 34 mammals in the Atlanta zoo to measure their density and viscosity.

We are told that most elephants and other herbivores produce "floating feces" while most tigers and other predators produce "sink feces" . We also categorize dung from the most rotten to the lowest, starting from tigers and rhinos and going all the way to pandas. The zoo's diverse animal species provide us with a wide range of stool sizes and shapes that are independent evidence to validate our mathematical model of defecation time.

We also put the stool in a device called a "finometer" , an accurate mixer that measures the properties of liquids and solids like chocolate and shampoo. Our lab shares two thermometers with physicist Alberto Fernandez-Nieves of Georgia Tech. We have classified "clean mollusks" and "Ddavid Hu thermometers" which have witnessed the clear division of the frog's saliva, mucus and droppings.

What else did we learn?Larger animals have longer stools. And bigger animals also defecate faster. For example, elephants defecate at a rate of 6cm / sec, almost six times the speed of dogs. The speed of defecation of human about 2cm / sec.

Combined, this means that the defecation time is constant for many animals, about 12 seconds (plus or minus 7 seconds), although the volume varies greatly . According to the bell curve distribution, 66% of animals took between 5 and 19 seconds to defecate. That's a surprisingly small amount because the dung of an elephant has a volume of 20 liters, nearly a thousand times bigger than dog feces, which is 10ml. How can large animals defecate at such a high speed?

The answer, we found, lies in the properties of an extremely thin layer of mucus covering the intestinal wall. The mucus is as thin as a human hair, so thin that we can only measure it by weighing the stool as the mucus evaporates. Although thin, the mucus is very slippery, it is 100 times more viscous than feces.

During defecation, the stool moves like a solid button. Therefore, under ideal conditions, the length and diameter of stool is determined simply by the shape of the rectum and large intestine. One of the major findings of our study is that the stool widens in the middle of the length of the large intestine from the rectum.

Considering the stool length along with the properties of mucus, we now have a cohesive physical story of how defecation occurs.Larger animals have longer stools, but also thicker slime , allowing them to achieve high speeds with the same pressure. Without this mucus, it is impossible to defecate. Changes in mucus can contribute to a number of diseases, including chronic constipation and even bacterial infections like C. difficile in the digestive tract.

In addition to simply following scientific curiosity, our stool measurement also has some practical applications. Our defecation data has helped us design astronaut diapers. Astronauts want to wear space suits for 7 days but are limited by their diapers. Taking advantage of the viscosity of stool, we designed a stool separator so that it does not come into direct contact with the skin. It was in the semi-finals of the "NASA Space Poop Challenge" earlier this year on handling manure dung.

This shows that physics and math can be used everywhere, even in your toilet.