A new study led by researchers in the George W. Woodruff School of Mechanical Engineering finds that all mammals, from humans to elephants to cats, defecate in the same amount of time: about 12 seconds. That’s despite the fact that the length of their rectums can vary widely. For instance, an elephant’s is 10 times the length of a cat’s (40 centimeters vs. four).

The study suggests that the time is consistent because of mucus. The substance covering the the large intestine is very thin for small animals and much thicker for larger ones. According to the paper, mucus allows feces to move through the intestine “like a sled sliding through a chute.”

The extra fluid allows larger animals to defecate at higher speeds than smaller animals, even though both use the same amount of pressure to relieve themselves. In other words, defecation might not be possible without this previously unknown mucus layer.

The research also found that the length of feces is double that of the rectum, which means the rectum and the colon both store feces.

The study, “Hydrodynamics of defecation,” was published in the journal Soft Matter. It covers a topic that hasn’t been heavily researched within the scientific community. Professor David Hu (also in the School of Biological Sciences) and his mechanical engineering student Patricia Yang are the lead authors.

If mucus plays a role in normal physiology of defecation, which this study shows, then abnormalities in mucus may play a role in abnormal physiology. This possibility is intriguing and could expand our current understanding of how gastrointestinal disorders, like constipation or infectious colitis, may occur.

One line of thinking for why constipation occurs is because the nervous system of the colon is out of sync. That messes up the propulsion process. We often have to treat it with medications, like laxatives and other pro–motility agents, to essentially force the column of stool out of the body. Perhaps people who are constipated don’t have enough mucus for whatever reason. If we think that mucus is playing a role, then could we develop new treatment strategies based on medications, including enemas or oral agents, that more closely resemble mucus? These possibilities would be novel.