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Why Does Poop Stink?

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ThinkStock

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.

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Mark Ralston, AFP/Getty Images
How a Hairdresser Found a Way to Fight Oil Spills With Hair Clippings
Mark Ralston, AFP/Getty Images
Mark Ralston, AFP/Getty Images

The Exxon Valdez oil tanker made global news in 1989 when it dumped millions of gallons of crude oil into the waters off Alaska's coast. As experts were figuring out the best ways to handle the ecological disaster, a hairdresser from Alabama named Phil McCroy was tinkering with ideas of his own. His solution, a stocking stuffed with hair clippings, was an early version of a clean-up method that's used at real oil spill sites today, according to Vox.

Hair booms are sock-like tubes stuffed with recycled hair, fur, and wool clippings. Hair naturally soaks up oil; most of the time it's sebum, an oil secreted from our sebaceous glands, but it will attract crude oil as well. When hair booms are dragged through waters slicked with oil, they sop up all of that pollution in a way that's gentle on the environment.

The same properties that make hair a great clean-up tool at spills are also what make animals vulnerable. Marine life that depends on clean fur to stay warm can die if their coats are stained with oil that's hard to wash off. Footage of an otter covered in oil was actually what inspired Phil McCroy to come up with his hair-based invention.

Check out the full story from Vox in the video below.

[h/t Vox]

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Bristly
A New Chew Toy Will Help Your Dog Brush Its Own Teeth
Bristly
Bristly

Few pet owners are willing to sit down and brush their pet's teeth on a regular basis. (Most of us can barely convince ourselves to floss our own teeth, after all.) Even fewer pets are willing to sit calmly and let it happen. But pet dental care matters: I’ve personally spent more than $1000 in the last few years dealing with the fact that my cat’s teeth are rotting out of her head.

For dog owners struggling to brush poor Fido’s teeth, there’s a slightly better option. Bristly, a product currently being funded on Kickstarter, is a chew toy that acts as a toothbrush. The rubber stick, which can be slathered with doggie toothpaste, is outfitted with bristles that brush your dog’s teeth as it plays.

A French bulldog chews on a Bristly toy.
Bristly

Designed so your dog can use it without you lifting a finger, it’s shaped like a little pogo stick, with a flattened base that allows dogs to stabilize it with their paws as they hack at the bristled stick with their teeth. The bristles are coated in a meat flavoring to encourage dogs to chew.

An estimated 80 percent of dogs over the age of 3 have some kind of dental disease, so the chances that your dog could use some extra dental attention is very high. In addition to staving off expensive vet bills, brushing your dog's teeth can improve their smelly breath.

Bristly comes in three sizes as well as in a heavy-duty version made for dogs who are prone to ripping through anything they can get their jaws around. A Bristly stick costs $29 and is scheduled to start shipping in October. Get it here.

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