Why Do Wombats Poop Cubes? It Comes Down to Intestinal Fluid Mechanics

"When I first heard about wombat poop, I was very, very skeptical."

If all goes as planned, human feces should emerge in a cylindrical, sausage-like shape. But for wombats, “normal” is much, much different. In humans, lumpy poop in a similar shape means you need more fiber, hard lumps make for constipation, and goops mean you have diarrhea. Whatever mess your body is going through, however, you won’t see square-shaped poops floating in your bowl. That’s because only one animal on our good, green Earth has dumps that look a Rubix cube: the wombat.

While humans have long known that the feces of the Australian marsupial looks like Wendy’s hamburger patty, it hasn’t been understood exactly why. That’s where Patricia Yang, Ph.D., comes in: On Sunday at the American Physical Society’s Division of Fluid Dynamics 71st Annual Meeting in Atlantia, Georgia, Yang and colleagues presented the first study designed to investigate how the wombat creates cube-shaped poops. They discovered that it all stems from the wombat’s digestive process and the unique shape of its intestinal walls.

“When I first heard about wombat poop, I was very, very skeptical because I didn’t think that poop could be square,” Yang, a postdoctoral fellow at the Georgia Institute of Technology, tells Inverse. Squares, she elaborates, are very rare in the natural world. Humans manufacture cubes through molds and cuts, but the only naturally occurring squares Yang has encountered her research are wombat feces and the tail of a seahorse.

wombat poop
Cubed wombat poop.

To solve the mystery of this cube poop, Yang and her colleagues studied the digestive tracts of two wombats who were euthanized after being hit by cars in Tasmania, Australia. They compared wombat intestines to pig intestines by inserting a balloon into the animals’ digestive tracts and examining the balloon’s circumferential direction and how the intestines stretched to fit the balloon. Additionally, they designed an experiment using pantyhose that mimics the design of wombat intestines.

The experiments revealed that, when it comes to wombats, their feces change from a liquid-like state to a solid state in the last 25 percent of the intestine. Meanwhile, the very last 8 percent of the intestine is where the feces take on sharp corners and turn into cubes.

Yang explains that this shape change is due to the varying elastic properties of the intestinal wall tissue. The shape of wombats’ poop also has to do with the amount of water that’s within the feces.

“The feces of a wombat is only about 50 percent water, while normal human feces is about 70 to 80 percent water,” Yang says. “Rabbits, deer, and goats have pellets, and these droppings are about 55 to 60 percent water — as for those pellet animals, they have an extra long colon compared to other animals of a similar size.”

wombat
The common wombat ... but it's anything but common.

Scientists have known that wombats have a lower water content in their feces, but it’s Yang and her team’s research — which is not yet published — that provide the physical reason why their scat is square-shaped. Having cubed feces, meanwhile, helps the squat, pouched animal out: It’s generally accepted — although, Yang and her team note, poorly studied — that wombat poop is cubed because they build poop walls. These poop piles mark prominent places like burrows and work as a form of olfactory communication.

“Wombats have very poor eyesight so they pretty much communicate by the scent of poop,” Yang explains.

It’s a beautiful quirk of evolution that, if we needed to build poop walls to survive, perhaps we would share as well. Until then, we’ll have to stick with our (hopefully) regular, wet ovals.