Why Do Most Mammals Take Twelve Seconds to Do Their Business?

A team of mechanical engineers reveal all in a journal article.

We admit that it may appear a touch eccentric to start a post about the speed at which mammals poop with a few reflections on the extraordinary proposed transportation system, Hyperloop.

But quite honestly, there’s something about the thought of cylindrical objects racing down narrow tubes that seems to connect one concept to the other, at least to our poop-addled brains.

Of course, the Hyperloop system, originally proposed by Elon Musk, promises to whizz passengers from Los Angeles to San Francisco in a mere 35 minutes, at speeds of up to 760 mph.

As we’ll explore in a moment or two, poop passes through different mammals’ rectums at remarkably constant rates, so we wondered (as one does) exactly how long it might take a human bowel motion to travel from LA to SF at a human bowel motion’s usual speed.

Our calculations suggest it would take slightly over ten months.

Rather slower than the Hyperloop, and even a poor showing when compared to walking from the City of Angels to the City by the Bay, which — at twenty miles a day — would take just under three weeks.

Down to business, however.

We have scientists from the School of Mechanical Engineering at Georgia Institute of Technology to thank for a riveting recent paper that explores the mechanical side of defecation.

Led by David Hu, Associate Professor of Mechanical Engineering and Biology, and Adjunct Associate Professor of Physics (and don’t you just love that mash-up of disciplines?), the researchers correctly acknowledge that Americans spend billions of dollars a year to treat conditions such as gastrointestinal infections, inflammatory bowel disease, irritable bowel syndrome, and other gastrointestinal disorders — all of which can cause severe changes in bowel habits — yet, “despite years of veterinary and medical studies, defecation still does not have an agreed-upon physical basis.”

It was clearly time for some serious investigation to begin, and early evidence was gathered by filming elephants, giant pandas, and warthogs defecating at Zoo Atlanta.

The scientists also filmed dogs going about their business in a local park.

Then, proving that you can find anything on the internet, even YouTube delivered, with nineteen videos of defecating animals.

Such filmed evidence, of course, enabled the act of defecation to be timed.

Then, measurement of the lengths of the “product,” as well as physiological knowledge, enabled some fascinating numbers to be crunched.

The first was that, despite the length of a mammal’s rectum (ranging from four to 40 centimetres), animals of all shapes and sizes — from cats to elephants — defecate with a nearly constant duration of twelve seconds, plus or minus seven seconds.

The scientists propose that rapid expulsion of feces makes sound sense from the point of view of an animal’s survival.

Since the smell of body waste is likely to attract predators, it’s a good idea for wild animals not to be caught with their pants down, as it were.

Thanks to the fascinating research, we also now know that feces actually slides along the intestine on a layer of slippery mucus, described by the scientists as being like a sled sliding down a chute.

Or, in our minds, like a Hyperloop capsule racing through its tube, supported on all sides by a cushion of air.

Larger animals produce greater volumes of poop but also have thicker layers of mucus, enabling an elephant, which dumps around twenty litres of feces per session, to do so in approximately the same amount of time as a dog, whose excremental volume is around one-thousandth of an elephant’s, at just ten ml.

The Georgia researchers helpfully explain that whether you’re a hippo or a human, normal healthy pooping should require minimal pressure.

Cylindrical feces aren’t squeezed through a nozzle like a toothpaste tube – they’re more like those sleds slipping down a chute in the Winter Olympics.

In fact, they found that poop generally emerges coated with an ultra-thin layer of mucus on its surface, although this mucus evaporates rapidly.

Constipation can occur when this mucus is absorbed by the feces, removing the lubrication that generally enables relatively effortless evacuation.

Actually, the paper goes on to suggest that the study may support the idea of clinicians using defecation time in the diagnoses of ailments of the digestive system, although they don’t go as far as to specify how individuals might keep track of performance in this area.

Perhaps the makers of the Fitbit might try their hand at that?

Anyone for a Sh!tbit?

The mind boggles.

The research also explains that on average, animals defecate two pieces of feces, each the length of their rectum, and that an animal’s poop tends to be five times as long as it is wide, an aspect ratio somewhat similar to a school bus.

But smaller, of course, and hopefully not bright yellow.

In terms of precise pooping speeds, elephants defecate at a rate of six centimetres per second, which is almost six times as fast as a dog.

For us humans, it’s around two centimetres per second, hence our earlier ten-months-from-LA-to-SF calculation.

Drawing a broad conclusion, Patricia Yang, another of the paper’s co-authors, said if a human is taking far longer to defecate than twelve seconds, “I’d say you should go see someone about it. But you can’t count the newspaper time.”


Finally, we simply must draw attention to the name of the journal that published this fabulous paper.

Produced by the Royal Society of Chemistry, it’s devoted to studies that occur where “physics meets chemistry meets biology.”

Although defecation research seems to be covered only rarely by the journal, its title might suggest otherwise.

Grateful acknowledgements, therefore, to the April 2017 edition of Soft Matter.

More reading

Elephant pooping

Hydrodynamics of defecation


Intestinal mucus barrier in normal and inflamed colon

Most mammals big or small take about 12 seconds to defecate

Panda pooping

Why doing a poo takes almost all animals roughly 12 seconds