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Lots of people think some animal must hold the title for smelliest poop. But science has no agreed-upon ranking. How bad something smells depends on whose nose it is. And there's no single ruler to measure it with. There's no champion, but there is a pattern: the more meat an animal eats, the worse its poop usually smells.
Why? Meat is full of protein. Protein that isn't fully digested travels to the last stretch of the gut. Lots of bacteria live there. As bacteria break the protein apart, they give off some very smelly gases. One is hydrogen sulfide, which smells like rotten eggs. Another is called skatole. It's part of what gives poop its signature stink.
Our noses are extremely sensitive to it: we can smell it at less than one part per billion in the air. Scientists actually tested this: the same five men: ten days eating vegetarian, against ten days eating 600 grams of meat a day. On the meat diet, the stuff in their poop that gives off that rotten-egg smell went up about fifteenfold. Plant-eaters like cows, horses, and rabbits mostly have gut bacteria breaking down plants. What comes out is mostly not those smelly gases, so their poop usually isn't as harsh.
Meat-eaters' droppings carry more of those sulfur smells. Voles that sniff fresh cat poop get scared and hide. Mice can even tell whether a cat has been eating meat or a vegetarian diet. These smells are hard to erase, too. Take fossilized poop from thousands of years ago, even more than ten thousand. Test the smell molecules inside, and scientists can tell whether it came from a meat-eater, a plant-eater, or a person. But there are exceptions. Otters eat mostly fish, yet many people who track them in the wild say their droppings smell like jasmine tea.
Stranger still is skatole. In tiny amounts, it actually smells a little sweet. Orange blossom and jasmine contain a trace of it, and perfumers use it too. A lot stinks; a little smells lovely. So which animal has the smelliest poop? There's no agreed champion. But what it eats largely decides how bad it smells. Next time you have a big barbecue, the smell the next day is the bacteria in your gut, cleaning up the protein you didn't finish digesting.
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In one sentence
There's no agreed champion. But there is a pattern: the more meat an animal eats, the worse its poop usually smells.
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What you might think vs. reality
YOU MIGHT THINK
Some animal holds the “smelliest” title.
The internet is full of “smelliest poop” top-ten lists.
ACTUALLY
Science has no agreed ranking.
How bad a smell is depends on whose nose it is, and there's no single ruler to measure it. One explainer on farm odors put it plainly: there's no single answer to which animal smells worst.
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Where the stink comes from
Eat meatMeat is full of protein.
Reach the end of the gutProtein that isn't fully digested ends up there. Lots of bacteria live there.
Bacteria break it apartAs they do, they give off very smelly gases.
Two big stinksHydrogen sulfide smells like rotten eggs. Skatole is part of poop's signature smell.
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Meat-eaters, plant-eaters
About 15× moreSame five men: ten days eating 600 g of meat a day versus ten vegetarian days. The rotten-egg stuff in their poop went up about fifteenfold.
Plant-eaters are milderGut bacteria in cows, horses, and rabbits mostly break down plants, which mostly doesn't make those smelly gases.
Prey keep awayVoles that sniff fresh cat poop get scared and hide. Mice can even tell whether a cat ate meat or a vegetarian diet.
Even fossils tellTest the smell molecules in fossil poop thousands of years old, some over 10,000, and you can tell meat-eater, plant-eater, or human.
Our noses are extremely sensitive to skatole: we can smell it at less than one part per billion in the air.
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Two twists
OTTERS
They eat mostly fish, yet their poop smells like jasmine tea
That's how many people who track otters describe it: sweet, a bit musky, and fishy when fresh. The pattern is “usually,” not “always.”
SKATOLE
A lot stinks, a little smells lovely
In tiny amounts it actually smells a little sweet. Orange blossom and jasmine contain a trace of it, and perfumers use it too.
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A common mix-up
“Smelliest” isn't one animal's titleThe same animal can smell very different depending on what it eats. Instead of asking “who stinks most?”, ask “what did it eat?”
Next time you have a big barbecue
the smell the next day is the bacteria in your gut, cleaning up the protein you didn't finish digesting.
