Which animals can and cannot fart?

Who among us hasn’t wondered if a snake can fart?

Dani Rabiotti, a quantitative ecologist, asked that question on Twitter in 2017. Her research into flatulence in animals went viralNaturally. From there, #doesitfart spread quickly (just like a, well, you know). Rabiotti and her co-author Nick Caruso turned the subject into a book that reached 2018 New York Times Bestseller list.

Simply put, a fart is “any gas emitted from the end of an animal opposite its mouth,” Rabaiotti and Caruso explain in their groundbreaking book. Does It Fart?: The Definitive Field Guide to Animal Flatulence.

An animal’s fart can come through the anus, if it has one, or through the cloaca – a combined opening for urination and defecation. The release of gas causes the opening to vibrate against the sphincter muscles of an organism, creating a sound.

Which animals can fart?

Many animals can and do fart. People, of course, but hyenas, manatees, dogs and bobcats all do that too. Quite a few creatures you might not expect also contribute to the horn chorus. As mentioned above, there are plenty of flatulent animals for this fill a book.

Consider, for example, the herring. Not only can the fish fart, it does so on purpose to communicate. Herring fart at a frequency too high for predatory fish to hear, so their flatulence acts as a secret fishing code.

Farts are not all created equal, and they can serve different purposes. Passing gas can relieve intestinal discomfort or be used to communicate, as with the herring above. Farts can also be used to scare potential predators. When threatened, a Sonoran coral snake draws air into its cloaca (the single-opening snakes use to urinate and defecate) and exhales it with a popping sound. Rabiotti and Caruso write that this “cloacal popping” sounds like a “higher-pitched, shorter version of a human fart.” Keep that in mind the next time you hike in and around southern Arizona.

The Bolson Pupfish, which lives only in Mexico’s Cuatro Cienegas Reserve, has the illustrious distinction of being an animal that must fart to live. The fish feast on algae, which produce gas. As the gas builds up inside the fish, it begins to float to the surface, where hungry predators await. By farting, the fish sink back to the sediment in which they normally reside.

In addition to being able to survive absolutely anywhere, American cockroaches can also fart.

Which animals can’t fart?

Some animals can fart, but don’t do it, while others really can’t.

Bats, although they should be able to fart, don’t seem to. This may be because they digest their food so quickly, which prevents internal gas buildup. Most animals that digest their food theoretically have the ability to fart, although they may not do so audibly or often.

Animals that really can’t fart don’t have a digestive system that breaks down food in a way that creates gas in an enclosed space (like an intestine) and then expel it. For example, a Portuguese warship is incapable of farting. Technically it is a colony of specialized organisms; the warrior liquefies the prey he catches in his tentacles.

Birds generally don’t fart, Rabiotti and Caruso explain, because the food they eat passes through their digestive systems very quickly (no time for gas to build up) and their gut microbes are different (and produce less gas) than those of mammals. The jury is out on whether amphibians tour. Frogs’ sphincter muscles are not very strong, making the passage of gas less likely to cause strong vibrations to be audible.

An outlier among mammals: sloths don’t fart. They digest their simple food incredibly slowly and while their gut microbiome produces methane, that gas is absorbed and exhaled, not released as flatulence.

This story is part of Popular Science’s Ask Us Anything series, where we answer your most bizarre, mind-burning questions, from the common to the unusual. Do you have something you’ve always wanted to know? Ask us.