Why Does Poop Stink?


Reader Bonnie wrote in to ask why feces smells so bad.

Between fall 2002 and spring 2003, researchers led by Val Curtis from the London School of Hygiene and Tropical Medicine (LSHTM) showed some 40,000 people from all over the world a series of photographs and asked them to rate how disgusting the image was, on a scale of one to five. 

In that set of 20 photos were seven pairs. The pictures in each pair were closely matched but one implied disease and the other didn’t. One pair, for example, showed the same plate of viscous liquid with two different color treatments. In one photo, the liquid was blue, like glass cleaner. In the other, it was yellowish with some darker spots, like mucus. Another pair showed the same person, but one photo had been manipulated to make him look wet, feverish, and spotty.

“More than 98 precent of people found the disease-relevant pictures equally, or more disgusting, than their pairs,” the researchers said, with the ick rating often doubling from one to the other.

Darwinian Disgust 

The easy answer to the question of poop’s smell is bacteria. The trillions of microorganisms that live in your gut (and the guts of other animals) produce a number of sulfurous compounds that pass out of the body along with the feces and give it its characteristic odor. This “which is grosser” study is part of a growing body of research that suggests an answer to a deeper question: Why do we think of that odor as particularly offensive?

Curtis’ work echoes a suggestion that goes back as far as Charles Darwin: that we think poop stinks for our own good. Our disgust towards certain sights and smells, Curtis says, is a “behavioral immune system”: an adaptation—biologically rooted, but tweaked by culture and social conditioning—that evolved to keep us from coming into contact with infection and disease. 

Curtis and other scientists have noted a number of things that almost universally elicit disgust among humans: bodily wastes and fluids, wounds, dead bodies, certain animals, spoiled food, and people with poor hygiene. As Philadelphia psychologists Paul Rozin and April Fallon summed it up, disgusting objects are those that “if they even briefly contact an acceptable food they tend to render that food unacceptable.”*

Poop, specifically, is gross all over the world. When Curtis and a colleague went looking for what disgusted people in the United Kingdom, the Netherlands, Burkina Faso, India and at the Athens International Airport, feces topped every list. Even some non-human animals try to avoid the stuff. Cattle, horses, sheep, cats, dogs, and some apes and monkeys have all been known to reject food and/or sleeping sites that had been contaminated with feces. 

Different Strokes for Different Folks

Disgust with poop isn’t monolithic, though. Some animals will consume feces (either their own, or someone else’s) to squeeze some more nutrients out of a meal or get a dose of beneficial gut bacteria. 

Among humans, disgust is partly shaped by local culture, environment, and personal experience. Bugs, for example, are usually deemed disgusting in the Western world, but in many other countries they're edible delicacies. Aversion to poo can also be lessened or overcome to accomplish other goals, like caring for family. When American and Australian researchers, for example, presented mothers with a series of dirty diapers, the moms consistently rated the smell of diapers that came from strangers’ babies as more disgusting than their own babies’, even when the sources of the diapers were incorrectly labeled or not identified.

*The human reaction to disgusting things is also almost identical and recognizable around the world. The face twists into a specific expression marked, pioneering facial expression researcher Paul Ekman notes, by a wrinkled nose, raised upper lip and cheeks and protruding lower lip.

Ted Cranford
Scientists Use a CT Scanner to Give Whales a Hearing Test
Ted Cranford
Ted Cranford

It's hard to study how whales hear. You can't just give the largest animals in the world a standard hearing test. But it's important to know, because noise pollution is a huge problem underwater. Loud sounds generated by human activity like shipping and drilling now permeate the ocean, subjecting animals like whales and dolphins to an unnatural din that interferes with their ability to sense and communicate.

New research presented at the 2018 Experimental Biology meeting in San Diego, California suggests that the answer lies in a CT scanner designed to image rockets. Scientists in San Diego recently used a CT scanner to scan an entire minke whale, allowing them to model how it and other whales hear.

Many whales rely on their hearing more than any other sense. Whales use sonar to detect the environment around them. Sound travels fast underwater and can carry across long distances, and it allows whales to sense both predators and potential prey over the vast territories these animals inhabit. It’s key to communicating with other whales, too.

A CT scan of two halves of a dead whale
Ted Cranford, San Diego State University

Human technology, meanwhile, has made the ocean a noisy place. The propellers and engines of commercial ships create chronic, low-frequency noise that’s within the hearing range of many marine species, including baleen whales like the minke. The oil and gas industry is a major contributor, not only because of offshore drilling, but due to seismic testing for potential drilling sites, which involves blasting air at the ocean floor and measuring the (loud) sound that comes back. Military sonar operations can also have a profound impact; so much so that several years ago, environmental groups filed lawsuits against the U.S. Navy over its sonar testing off the coasts of California and Hawaii. (The environmentalists won, but the new rules may not be much better.)

Using the CT scans and computer modeling, San Diego State University biologist Ted Cranford predicted the ranges of audible sounds for the fin whale and the minke. To do so, he and his team scanned the body of an 11-foot-long minke whale calf (euthanized after being stranded on a Maryland beach in 2012 and preserved) with a CT scanner built to detect flaws in solid-fuel rocket engines. Cranford and his colleague Peter Krysl had previously used the same technique to scan the heads of a Cuvier’s beaked whale and a sperm whale to generate computer simulations of their auditory systems [PDF].

To save time scanning the minke calf, Cranford and the team ended up cutting the whale in half and scanning both parts. Then they digitally reconstructed it for the purposes of the model.

The scans, which assessed tissue density and elasticity, helped them visualize how sound waves vibrate through the skull and soft tissue of a whale’s head. According to models created with that data, minke whales’ hearing is sensitive to a larger range of sound frequencies than previously thought. The whales are sensitive to higher frequencies beyond those of each other’s vocalizations, leading the researchers to believe that they may be trying to hear the higher-frequency sounds of orcas, one of their main predators. (Toothed whales and dolphins communicate at higher frequencies than baleen whales do.)

Knowing the exact frequencies whales can hear is an important part of figuring out just how much human-created noise pollution affects them. By some estimates, according to Cranford, the low-frequency noise underwater created by human activity has doubled every 10 years for the past half-century. "Understanding how various marine vertebrates receive and process low-frequency sound is crucial for assessing the potential impacts" of that noise, he said in a press statement.

Scientific Reports, Fernando Ramirez Rozzi
Stones, Bones, and Wrecks
Humans Might Have Practiced Brain Surgery on Cows 5000 Years Ago
Scientific Reports, Fernando Ramirez Rozzi
Scientific Reports, Fernando Ramirez Rozzi

In the 1970s, archaeologists discovered a site in France containing hundreds of cow skeletons dating back 5000 to 5400 years. The sheer number wasn't surprising—human agriculture in that part of the world was booming by 3000 BCE. What perplexed scientists was something uncovered there a few decades later: a cow skull bearing a thoughtfully drilled hole. Now, a team of researchers has released evidence that suggests the hole is an early example of animal brain surgery.

Fernando Ramírez Rozzi, a paleontologist with the French National Center for Scientific Research, and Alain Froment, an anthropologist at the Museum of Mankind in Paris, published their findings in the journal Nature Scientific Reports. After comparing the opening to the holes chiseled into the skulls of humans from the same era, they found the bones bore some striking similarities. They didn't show any signs of fracturing from blunt force trauma; rather, the hole in the cow skull, like those in the human skulls, seemed to have been carved out carefully using a tool made for exactly that purpose. That suggests that the hole is evidence of the earliest known veterinary surgery performed by humans.

Trepanation, or the practice of boring holes into human skulls, is one of the oldest forms of surgery. Experts are still unsure why ancient humans did this, but the level of care that went into the procedures suggests that the surgery was likely used to treat sick patients while they were still alive. Why a person would perform this same surgery on a cow, however, is harder to explain.

The authors present a few theories, the first being that these ancient brain surgeons were treating a sick cow the same way they might treat a sick human. If a cow was suffering from a neural disease like epilepsy, perhaps they though that cutting a hole in its head would relieve whatever was agitating the brain. The cow would have needed to be pretty special to warrant such an effort when there were hundreds of healthy cows living on the same plot of land, as evidenced by the skeletons it was found with.

Another possible explanation was that whoever operated on the cow did so as practice to prepare them for drilling into the heads of live humans one day. "Cranial surgery requires great manual dexterity and a complete knowledge of the anatomy of the brain and vessel distribution," the authors write in the study. "It is possible that the mastery of techniques in cranial surgery shown in the Mesolithic and Neolithic periods was acquired through experimentation on animals."

Either way, the bovine patient didn't live to see the results of the procedure: The bone around the hole hadn't healed at all, which suggests the cow either died during surgery or wasn't alive to begin with.


More from mental floss studios