How long can a shit be? It's twice the length of your rectum...

Expert of this article: Yang Chao, PhD in Chemistry, University of Chinese Academy of Sciences

Review of this article:

Zhang Zhijian, anesthesiologist at Harbin Medical University

Xie Weiqiang, PhD in Mechanics

Shit is one of the important daily life activities of normal people, and people cannot do without it.

Not only humans need to poop, but animals also need to poop. Although it is common, it is not simple.

Do you know how long a piece of feces is? The answer is: twice the length of your rectum. Someone has figured it out.

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There is a doctor named Patricia J. Yang (hereinafter referred to as Dr. Yang, not the author of this article) at Georgia Institute of Technology in the United States. She specializes in studying the pooping of various animals and has published many research papers on pooping in academic journals. After reading them, your imagination of the trivial matter of pooping will be completely overturned.

Important reminder: This article is very strong, you can eat, but don't eat too much!!!

Average defecation time for mammals

All about 12 seconds?

In order to observe animals pooping, Dr. Yang led a team to the Atlanta Zoo. After waiting in the zoo, she finally filmed the pooping process of three animals: an elephant, a panda, and a warthog. She also filmed the pooping process of a puppy in the park. However, she felt "not satisfied yet", so she studied the pooping videos of 19 animals on Youtube.

Dr. Yang photographed elephants, pandas, warthogs, and dogs pooping. Source: Yang PJ, LaMarca M, Kaminski C, et al. Hydrodynamics of defecation[J]. Soft Matter, 2017, 13(29): 4960-4970.

Feces must pass through the rectum before being excreted. The length of the rectum of different animals is different. The rectum of a cat is only 4 cm long, while the rectum of an elephant is 40 cm long. Although the length of the rectum of different species of animals can differ by 10 times, the duration of these animals' defecation is almost the same.

Dr. Yang starts timing from the moment the tip of the feces appears. This initial state of pooping usually lasts about 1 second, and then the animal will use about 10 seconds to poop all at once. The total pooping time is generally about 12 seconds on average. When you feel like pooping tomorrow, you can also measure whether your pooping time is also around 12 seconds.

The defecation time of different animals, source: Yang PJ, LaMarca M, Kaminski C, et al. Hydrodynamics of defecation[J]. Soft Matter, 2017, 13(29): 4960-4970.

How do different animals manage to keep their poop time to around 12 seconds? Except, of course, for humans who poop with their phones.

In order to answer this question, Dr. Yang, who is known for his strong sense of taste, conducted a more detailed study on animal feces.

The total length of a shit

Generally twice the length of the rectum?

Dr. Yang asked keepers at Zoo Atlanta to collect the animals' feces and measure various parameters of 87 pieces of feces, from the number of pieces to the length and diameter. These measured data were then statistically analyzed with the length and inner diameter of the animals' rectum.

The following four "important" conclusions were obtained:

1. Mammals poop 1-4 times, with an average of 2 poops (except for animals that poop in balls);

2. The total length of the feces is generally twice the length of the rectum;

3. The length of a single piece of feces is generally 5 times the diameter of the rectum;

4. The larger the animal, the faster it poops.

Schematic diagram of the size of the rectum and feces, source: Yang PJ, LaMarca M, Kaminski C, et al. Hydrodynamics of defecation[J]. Soft Matter, 2017, 13(29): 4960-4970.

Many studies have shown that the rectum is the only storage "warehouse" for feces, but if the total length of the feces is twice that of the rectum, it means that some feces still exist in the colon.

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Since different animals take about the same amount of time to poop, what "advantage" do larger animals have? They poop faster! Large animals poop much faster than smaller ones, and the speed is linearly related to the animal's body length. Most animals normally poop at a speed of 1-10 cm/second.

Shit is squeezed out

Or was it "pushed" out?

The process of feces being pulled out is a "sliding" process. It is "pushed" out, rather than squeezed out from a very small toothpaste tube like toothpaste. Therefore, the diameter of the feces is generally the same as the diameter of the rectum.

In order to reduce the friction between feces and rectum and make defecation smoother, there needs to be a layer of mucus between feces and rectum to act as a lubricant. Generally, the thickness of the mucus layer in the intestine is 30-100 microns.

Larger animals not only produce more feces, but also have a thicker layer of mucus, which helps to move feces out.

Although the amount of feces is proportional to body size, all mammals defecate in a constant amount of time due to the mucus coating the walls of the large intestine.

Why do some shit float on water?

Some can’t?

In order to explore whether the feces of different animals would float on the water, Dr. Yang measured the density of the feces of different types of animals and found that the density of the feces of different types of animals ranged from 0.2 to 1.5 grams per cubic centimeter.

The feces of these animals can be divided into two categories according to their density relative to water: feces that sink to the bottom of the water and feces that float on the water.

Civet cat feces and elephant feces, source: Yang PJ, LaMarca M, Kaminski C, et al. Hydrodynamics of defecation[J]. Soft Matter, 2017, 13(29): 4960-4970.

Most of the animals in Zoo Atlanta are large carnivores, such as lions, tigers, bears and civet cats. The average density of their feces is 1.4 grams per cubic centimeter, which is the type that can sink to the bottom of the water. This is because these carnivores eat difficult-to-digest foods such as fur and bones.

In contrast, giant pandas and some herbivores, such as ungulates, elephants and kangaroos, which eat low-nutrient foods and poop out in an undigested form, have low-density feces, with an average density of 0.83 g/cubic centimeter, so they can float on the water.

What does pooping have to do with fluid dynamics?

Unless it is diarrhea, shitting is not strictly speaking ordinary fluid mechanics, because there are cracks in shit, and ordinary fluids should not have such cracks visible to the naked eye.

Dr. Yang uses a rheometer to measure the rheological parameters of animal feces, but most of the feces collected are untestable. They are either too dry or contain too many rocks and plant roots, which are not suitable for testing using a rheometer.

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Finally, Dr. Yang used rabbit, cat, and dog feces for testing, obtained the rheological test results of feces, and derived a relationship between the viscosity of feces and the applied shear rate, characterized by the feces becoming thinner under the action of shear force, and the shear strength decreasing when faster deformation occurs.

The shear-thinning properties of shit can explain why dog ​​poop feels silky when you step on it.

Why is poop cube-shaped?

Feces do not only come in long strips or spherical shapes, wombats can also excrete cube-shaped feces. This is all due to the wombat's digestion process and the unique shape of its intestinal wall.

Wombat poop, source: Yang PJ, LaMarca M, Kaminski C, et al. Hydrodynamics of defecation[J]. Soft Matter, 2017, 13(29): 4960-4970.

The last quarter of the intestine is where the wombat's poop changes from liquid to solid. Meanwhile, the last tenth of the intestine is where the wombat's poop develops sharp corners and becomes cube-shaped.

Source: See watermark

This shape change is due to the different elastic properties of the wombat's intestinal wall tissue and the water content in the feces.

Has the above in-depth research on the feces of various animals solved your various confusions about poop?

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