A poop can tell you a lot about who made it. Like, was it a big animal or a small one? Were they an herbivore or a carnivore? And were they feeling okay when they... dropped it off? At the most basic level, the poops of terrestrial mammals are pretty round, be they tubes, patties, or pellets, which isn’t really surprising, considering the geometry of the place they come from. But what does it say about an animal when it poops cubes? How does that even work? And WHY?

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 Say hello to the only cube-pooper around: the wombat. These chubby, adorable Australian marsupials live in complex underground burrow systems and sleep for most of the day. They come out in the evenings, and spend their nights snacking on grasses and other tough plants. And, I went into this episode thinking that the back end of the wombat digestive tract was the more interesting end because, you know, cubic poops. BUT, it turns out, the rest of their digestive tract is also full of super-cool adaptations to their environment.

 Like, they’re the only marsupial with teeth that never stop growing. This adaptation also shows up in other groups, especially ones that gnaw on hard stuff. For instance, rodents have ever-growing incisors and the weirdest lemur around, the aye-aye, does too! In wombats, it helps them counteract the tooth wear caused by the tiny particles of silica found inside many plants that are part of their support structures. And, because wombats are grazers, they also eat a lot of soil, especially when conditions are dry, which doesn’t help their teeth. Having continuously growing teeth means their teeth never wear down so much that the wombat can’t eat. For a lot of other animals, teeth are basically a ticking time bomb in their mouths, counting down the years until they stop being able to chew. But not for wombats!

They also eat less, have lower metabolic rates, and use less water than scientists would expect for a mammal of their size, which allows them to survive when resources are scarce. And, okay, this isn’t about their digestive tract, but part of the way they conserve water is by not having any sweat glands, which, unfortunately, has the side effect of making them more susceptible to heat stress. The three living species of wombats also have different digestive strategies, because they live in different environments. Now, that doesn’t mean they choose how they digest food, it just means they each have adaptations that better fit their particular habitats.

The common wombat goes for quantity over quality. They live in wetter environments where plants are more abundant, and can pack away a lot of food that isn’t very nutritious or calorie-dense. In contrast, the two hairy-nosed wombats do more with less. They live in drier environments with generally fewer resources. They eat less, but have a long digestive tract that food moves through slowly, giving them more time to extract energy. And when the wombats are done digesting, they mark their feeding territory with their scent and with little piles of poop blocks. The common wombat can poop up to 100 cubes per day!

And both the why and the how of the cubic poops have been the subject of a lot of speculation. One popular idea about why was that it made the poops more stack-able and less likely to roll away. This would’ve made them better for marking their territory. But a lot of other animals mark their territory and none of them have cubic poop. Another vague hypothesis was that it had something to do with the shape of some part of their digestive tract. But, weirdly enough, a number of researchers working in the first half of the 1900s pretty thoroughly described the stomach and intestines of wombats, and nothing in there seems to  have stuck out to them as an obvious source of the cubes. Now, they weren’t necessarily looking for that, but I’d like to think they definitely would’ve noticed if the wombats had a cube-shaped colon or something.

 One factor that does seem to play a role is moisture. Wombats in zoos tend to have more access to water than most wild wombats do and with greater hydration comes less cubic poop And that might be a clue that the cubes just happen to be a byproduct of the wombat’s  intestines squeezing its poop to retain as much water as possible. As for how the cubes get made, well, a team of scientists have been working on this since 2018 by dissecting roadkill wombats and doing some fancy mathematical modeling. Isn’t science awesome?

It turns out wombat intestines have stretchier parts and stiffer parts, and the more elastic parts form two grooves. As the poop is squeezed along, the soft and stiff parts of the intestines contract at different rates, which is what shapes the corners of the cubes. In other animals, all parts of the intestines are equally stretchy, so poops turn out round instead! So, okay, does knowing how the cubes get made help us solve the mystery of why? Well, here’s the thing: wombats aren’t the only animals that live in dry environments, but their cube poop seem to be unique. And figuring out why unique traits appear in animals can be REALLY HARD. When a bunch of animals all converge on a single adaptation, we can look at the things they have in common to figure out why that thing might’ve evolved. With a sample size of one cube pooper we just can’t, at least, not right now. In some ways, the wombat is a lot like us. We’ve got our own set of unique adaptations, like, walking on two legs, that we’re still trying to explain the why of. And the really exciting thing is, that just means there’s a lot more work to be done to explain the science behind being a bizarre beast.

 The Bizarre Beasts pin club subscription window is open NOW through the end of June 6th. Check out how cute this wombat is! With its little cubes! Sign up now to get this pin in the middle of the month and the pins after that around the time each new video goes up. And if you like learning about animals, check out our friends over at Crash Course Zoology for deeper dives on topics like: what makes an animal an animal, all the different weird ways they eat, and how they perceive the world around them!

 As always, profits from the pin club go to support our community’s efforts  to decrease maternal mortality in Sierra Leone.

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