It's a revolting job, but somebody has to figure out the physics of feces. Vertebrates have been pooping on this planet for hundreds of millions of years, yet our understanding of the hydrodynamics of defecation is alarmingly limited. Thankfully, an intrepid team of researchers is on the job.
A new study published in the journal Soft Matter (no, this is not fake news, this is real news) reports that no matter the animal's size, it takes every mammal an average of 12 seconds to produce a bowel movement. We're talking cats, elephants, kangaroos, moles, water buffaloes — you name it, the poop slips out of their individual rectums on a cushion of mucus at the same impressive speed.
But why? And what allows for this consistency in so many different types of animal bodies?
First, in the natural world, it's kind of a good idea to be an expedient pooper, regardless of size:
"The smell of body waste attracts predators, which is dangerous for animals," lead author Patricia Yang, a mechanical engineer at Georgia Tech told Chelsea Whyte at New Scientist. "If they stay longer doing their thing, they're exposing themselves and risking being discovered."
So, it's pretty obvious why a wild animal wouldn't want to dally on the potty, but the mechanism by which animals of all sizes are able to do this job relatively quickly has been more mysterious. The researchers got to the bottom of the issue by filming different mammals' toilet times at dog parks and the Atlanta, Georgia, zoo, and also by finding videos of pooping zoo animals that tourists have uploaded to YouTube:
"There's a surprising amount of poop videos online," said Yang, perhaps unfamiliar with the internet and human curiosity.
The body masses of the 23 species of animals studied ranged from about 9 pounds to almost a ton (from 4 to 4,000 kilograms). What the scientists found with animals whose poops are cylindrical — most mammals, in fact — is that their individual turds tended to be about five times the diameter of the animal's rectum, and the necessary pressure each animal needed to apply to the process of expelling their waste was about the same, no matter the species.
Another piece of this poop puzzle had to do with mucus. Although it seems the act of defecation is a process of squeezing something out of a tube, it's actually more like opening the gate of a chute. Mucus is necessary to lubricate the stool so it can move out of the colon to freedom — according to Yang, if humans didn't have this mucus, it would take 500 days for a poop to move completely through our bowels if no pressure were applied. (Fun Fact: Constipation happens when the stool actually absorbs the mucus that's needed to move everything along smoothly!) The researchers found large animals have much thicker mucus than smaller animals, which gets their longer fecal pieces through the rectum at about the same speed as the smaller ones.
Yang and her coauthors have some practical, scientifically backed advice for humans who count on their 15 minutes of newspaper time on the toilet:
"If it's taking far longer than 12 seconds, I'd say you should go see someone about it," she said.