In the Stuff to Blow Your Mind episode "The Fartonomicon," Joe and I have a lot of fun with humanity's more imaginative takes on flatulence: butt-trumpeting medieval demons, toot-based banishments and the fart competition paintings of Edo-period Japan. And indeed, fart-based humor seems to go back as far as 1900 B.C.E., if the world's oldest joke is any indication (more on that below).
Flatulence-related humor comes as naturally to humans as their actual toots — and indeed, it's difficult to imagine life without these noisy emissions. After all, they're largely a standard feature of mammalian biology. We're aided in our digestion by our gut flora, a complex legion of microorganisms that thrive in our digestive tract. They help break down our food, releasing gases and even venting their own waste in the process. It all contributes to the chemistry of flatus and produces that signature stink.
Human farts vary greatly from person to person due to differences in diet, microbiome and overall health. For instance, only half the human population boasts the right gut flora to produce flammable methane, writes author Mary Roach in her book "Packing for Mars." As you might imagine, such people have been of special interest to NASA. Over the years, the space agency's diligent focus on preserving orbital air quality even led them to keep Nobel prize-winning flatus researcher Michael Levitt on retainer as a consultant. Roach notes that another NASA researcher, Edwin Murphy, went so far as to identify the ideal astronaut on toots alone: a subject who produced essentially no flatus on a specialized diet of bean meal.
But you needn't scour the human population for a fart-free organism. Birds, for instance, have never been reliably observed to pass gas. They simply lack the diet-and-microbe combo responsible for the mammalian toot. And as explored in the book "Does It Fart?" by Nick Caruso and Dani Rabaiotti, we can point to at least one fart-exempt mammal: the noble sloth.
As the authors point out, sloths are famously sluggish and boast slow digestion. As many as five days can pass between poops. They depend on an incredibly leafy diet and boast simplified gut flora to deal with it. Their gut flora produces methane, but instead of exiting through farts, it enters the bloodstream and the sloth just breathes it out.
According to Caruso and Rabaiotti, it's also possible that bats don't fart, though dedicated research on the matter is (strangely?) lacking. They have the right mammalian gut bacteria, but their digestion is rather speedy to cut weight for flight. The authors note that even the largest species of bat, the flying fox, has a mouth-to-anus digestion time of 12-34 minutes.
Still, bats and sloths would prove rather useless on a prolonged space mission. As Joe and I discuss in the episode, the possibilities for ideal astronaut flatus depend on our growing understanding of the human microbiome. We've all read articles on the effectiveness of fecal transplants to treat various gastrointestinal diseases, but research indicates that the microbiome influences multiple areas of human health – including mental health.
So as much as we'd like to banish flatulence to the realm of jokes and insults, it's just part of the balance inside us, between our own complex, multicellular bodies and the diverse colonies of microorganisms living within.