Faster faeces Finnbarr Webster / Alamy Stock Photo

Everyone poops, and it takes them about the same amount of time. A new study of the hydrodynamics of defecation finds that all mammals with faeces like ours take 12 seconds on average to relieve themselves, no matter how large or small the animal.

The research, published in Soft Matter, reveals that the soft matter coming out of the hind ends of elephants, pandas, warthogs and dogs slides out of the rectum on a layer of mucus that keeps toilet time to a minimum.

“The smell of body waste attracts predators, which is dangerous for animals. If they stay longer doing their thing, they’re exposing themselves and risking being discovered,” says Patricia Yang, a mechanical engineer at the Georgia Institute of Technology in Atlanta.


Yang and colleagues filmed elephants, pandas and warthogs at a local zoo, and one team member’s dog in a park, as they defecated. All these animals produce cylindrical faeces, like we do, and this is the most common kind among mammals. Though the animals’ body masses ranged from 4 to 4,000 kilograms, the duration of defecation remained constant.

Slimy Chute

That consistency across animals is down to a few things. First, the length of faecal pieces was 5 times as long as the diameter of the rectum in each of the animals.

Yang also found that the normal, low-level pressure animals apply to push through a bowel movement is constant, and unrelated to a creature’s body mass. This means that, whether it’s a human or a mouse, the pressure used on normal excrement is the same. This is similar to her previous finding that mammals take the same amount of time to empty their bladders.

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The final piece of this puzzle is the mucus layer in the colon, which plays a big role in the duration of evacuation. Creatures with cylindrical faeces aren’t squeezing matter through a nozzle like a toothpaste tube. “It’s more like a plug that just goes through a chute,” she says. Yang says larger animals have more rectal mucus, which facilitates quicker expulsion.

Constipation happens when that mucus is absorbed by the faeces. Without this slick layer, a human applying no pressure at all would take 500 days to void their bowels, Yang says. “It would be shortened to 6 hours if you apply maximum pressure, but I believe you’d still need to see a doctor,” she says.

All animals produce on average two pieces of faeces. Larger animals have longer faeces and a longer rectum, but they have thicker mucus, which makes the faeces accelerate faster – so they travel a longer distance in the same amount of time.

Floaters and sinkers

Yang’s team also used Youtube videos of animals relieving themselves to measure the average time of defecation among 23 different species. “There’s a surprising amount of poop videos online. They’re mostly from zoos where tourists film it and upload it,” she says.

They collected stool samples from 34 species and found that diet affects the density of faecal matter. Floaters – droppings that are lighter than water – are produced by pandas and other herbivores like elephants and kangaroos. They eat low-nutrition, high-fiber foods and defecate much of it in undigested form.

Sinkers are produced by large carnivores like bears, tigers and lions. They eat heavier, indigestible ingredients, including fur and bone.

Using a rheometer – a device that measures the way fluids flow under applied force – Yang found that faeces are shear-thinning, which means they have lower resistance the faster they’re deformed. That’s why dog poop feels slippery when you step on it.

Based on animals at the Atlanta Zoo, they found that on average, animals take in about 8 per cent of their body mass in food, and expel 1 per cent of body mass in faeces.

Their observations fed into a mathematical model that can predict defecation times for various problems within the digestive system. “If it’s taking far longer than 12 seconds, I’d say you should go see someone about it,” she says. “But you can’t count the newspaper time.”

Journal reference: Soft Matter, DOI: 10.1039/C6SM02795D

This article was amended to clarify that most mammals produce cylindrical faeces.