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Why Does Poop Stink?

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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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This Just In
Criminal Gangs Are Smuggling Illegal Rhino Horns as Jewelry
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Valuable jewelry isn't always made from precious metals or gems. Wildlife smugglers in Africa are increasingly evading the law by disguising illegally harvested rhinoceros horns as wearable baubles and trinkets, according to a new study conducted by wildlife trade monitoring network TRAFFIC.

As BBC News reports, TRAFFIC analyzed 456 wildlife seizure records—recorded between 2010 and June 2017—to trace illegal rhino horn trade routes and identify smuggling methods. In a report, the organization noted that criminals have disguised rhino horns in the past using all kinds of creative methods, including covering the parts with aluminum foil, coating them in wax, or smearing them with toothpaste or shampoo to mask the scent of decay. But as recent seizures in South Africa suggest, Chinese trafficking networks within the nation are now concealing the coveted product by shaping horns into beads, disks, bangles, necklaces, and other objects, like bowls and cups. The protrusions are also ground into powder and stored in bags along with horn bits and shavings.

"It's very worrying," Julian Rademeyer, a project leader with TRAFFIC, told BBC News. "Because if someone's walking through the airport wearing a necklace made of rhino horn, who is going to stop them? Police are looking for a piece of horn and whole horns."

Rhino horn is a hot commodity in Asia. The keratin parts have traditionally been ground up and used to make medicines for illnesses like rheumatism or cancer, although there's no scientific evidence that these treatments work. And in recent years, horn objects have become status symbols among wealthy men in countries like Vietnam.

"A large number of people prefer the powder, but there are those who use it for lucky charms,” Melville Saayman, a professor at South Africa's North-West University who studies the rhino horn trade, told ABC News. “So they would like a piece of the horn."

According to TRAFFIC, at least 1249 rhino horns—together weighing more than five tons—were seized globally between 2010 and June 2017. The majority of these rhino horn shipments originated in southern Africa, with the greatest demand coming from Vietnam and China. The product is mostly smuggled by air, but routes change and shift depending on border controls and law enforcement resources.

Conservationists warn that this booming illegal trade has led to a precipitous decline in Africa's rhinoceros population: At least 7100 of the nation's rhinos have been killed over the past decade, according to one estimate, and only around 25,000 remain today. Meanwhile, Save the Rhino International, a UK-based conservation charity, told BBC News that if current poaching trends continue, rhinos could go extinct in the wild within the next 10 years.

[h/t BBC News]

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Big Questions
Do Cats Fart?
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Certain philosophical questions can invade even the most disciplined of minds. Do aliens exist? Can a soul ever be measured? Do cats fart?

While the latter may not have weighed heavily on some of history’s great brains, it’s certainly no less deserving of an answer. And in contrast to existential queries, there’s a pretty definitive response: Yes, they do. We just don’t really hear it.

According to veterinarians who have realized their job sometimes involves answering inane questions about animals passing gas, cats have all the biological hardware necessary for a fart: a gastrointestinal system and an anus. When excess air builds up as a result of gulping breaths or gut bacteria, a pungent cloud will be released from their rear ends. Smell a kitten’s butt sometime and you’ll walk away convinced that cats fart.

The discretion, or lack of audible farts, is probably due to the fact that cats don’t gulp their food like dogs do, leading to less air accumulating in their digestive tract.

So, yes, cats do fart. But they do it with the same grace and stealth they use to approach everything else. Think about that the next time you blame the dog.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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