Few people would have given much thought to the physics involved in defecation and urination, although faeces and urine in themselves have long been studied as indicators of health or illness. Few people, that is, except a team of scientists who have investigated both processes in mammals, defecation more recently.
In a study of 34 species at Zoo Atlanta, they have come up with the finding that most mammals take, on an average, 12 seconds to defecate. This struck them as remarkable: Why must the defecation time be constant when the length of the rectum varies so much from mammal to mammal, from four centimetres for a cat to 40 cm for an elephant? This they answer with physics, the essence of their study boiling down to two broad findings. One, the longer the faeces and the higher its quantity, the greater the speed of defecation. Two, the longer the large intestine, the higher the quantity of mucus lining it, which is what drives the defecation speed.
The study appears in, quite appropriately, the journal Soft Matter. A few years earlier, two members of the same team had conducted a study on urination, going on to find that all mammals empty their bladders in 21 seconds. That work had fetched them the 2015 Ig Nobel prize, parody of the Nobel, besides leading them to the defecation study.
“We accidentally filmed an elephant defecating on our previous urination study,” Patricia Yang, a research student at Georgia Institute of Technology, told The Indian Express. Georgia Tech associate professor David Hu was the other researcher involved in both studies. The video of the defecating elephant led to a debate on whether or not faeces were deformed during defecation. “We realised that it was an important question but no one has ever asked,” Yang said.
So the team set to work at Zoo Atlanta, filming defecation and handpicking faeces from the 34 species. They measured the density and viscosity of the collected faeces, classified these as “floaters” or “sinkers”, and ranked faeces from most to least smelly, starting with tiger and rhino and going all the way to panda. The ranking was something they did inadvertently, Hu and Yang write in an article on the news portal The Conversation. They found that an elephant, whose faeces have a volume of 20 litres, defecates at six centimetres per second. This is nearly six times as fast as a dog, whose faeces have a volume of 10 ml. The speed of defecation for humans is in between: two centimetres per second.
“Together,” Hu and Yang write on the portal, “this meant that defecation duration is constant across many animal species – around 12 seconds (plus or minus 7 seconds) – even though the volume varies greatly.” The next question that presented itself was what allows the larger animals to defecate at higher speeds. The team found the answer in an ultra-thin layer of mucus lining the walls of the large intestine. This provides lubrication for objects passing through.
“The thickness of this layer determines the speed of faeces,” Yang said. “Larger animals have longer rectums and thicker mucus layers, thus they defecate at higher speed and empty their rectums as fast as small animals.” The study, in fact, neglects the flow of faeces during defecation; based on a finding that mucus is consistently one-third as viscous as faeces, it focuses on the flow of mucus. “In other words, faeces act like a solid plug, and the only flowing liquid is mucus,” the research paper says.
The research, for all its appearance of triviality, did find a practical application. Yang, who describes the Ig Nobel as an honour, is listed as a semifinalist at the NASA Poop Challenge earlier this year for a designer diaper for astronauts in space. The design, which aims to segregate faeces away from contact with the skin, is based on data and measurements from the defecation study. It just shows, as Hu and Yang point out in their article, that physics and mathematics can be used everywhere, even in your toilet bowl.