Wombats poop in cubes. Scientists are figuring out how
These stocky, waddling marsupials have baffled researchers for years with their block-shaped turds—but a team of mechanical engineers is now sniffing out the science behind nature’s most deviant defecators.
For years, researchers have been baffled by wombat feces, which take the shape of cubes. A team of mechanical engineers and biologists is now starting to unravel the mysteries behind this prismatic poop. Photo Credit: Pascal Renet, Pexels
When it comes to poop, you may think you’ve seen it all. From fluffy to firm and liquidy to lumpy, feces come in all sorts of shapes, sizes, textures, colors, and scents. After peering down into that all-too-familiar toilet bowl one too many times, it’s easy to feel a bit jaded.
Well, wombats are here to drop a deuce on your excrement ennui. These stocky, waddling marsupials are nature’s most deviant defecators, and their rebellion is all about geometry.
Wombat crap in cubes—square-faced, sharp-edged, three-dimensional cubes. And, as scientists report today at the 71st Annual Meeting of the American Physical Society's Division of Fluid Dynamics, we’re finally a little closer to understanding how they manage this fecal feat.
“This work is brilliant,” says Louise Gentle, a behavioral ecologist at Nottingham Trent University in the United Kingdom who did not participate in the research. “Wombats are the only species we know of that produce cubic poo… and before this, it was all speculation as to how.”
Researchers have known for years that wombats produce distinct droppings. But cubes are rare in nature for a reason: Compared to spheres of the same volume, cubes have more surface area, and energy must be put in to create corners and edges, explains Birgitt Boschitsch, a mechanical engineer at Pennsylvania State University not affiliated with the work. “There must be some use to putting in the effort to make this shape,” she adds.
Though there’s no complete consensus on why wombats cube their feces, says study author Scott Carver, a wildlife ecologist at the University of Tasmania in Australia, the current thinking is this: Wombats purposefully poo in piles as a means of communication, and rectangular prisms make for good stacking—especially compared to more classic shapes like cylinders or pellets, which, when heaped together, might just roll away.
Wombats are the only known animals that poop in cubes. On the fronts and backs, the feces run about one inch by one inch, and are around an inch and a half long. Photo Credit: Patricia Yang and David Hu, Georgia Institute of Technology
It’s an odd strategy for sure, but perhaps less so when you consider the nocturnal wombat lifestyle. Adapted to the darkness, wombats don’t have terribly good eyesight, but their sense of smell is commendable. They extrude towers of turds in prominent places (atop rocks or logs, for instance) in the hopes that a fellow wombat wanderer will spy the elaborate assemblage and get keen on sniffing out the message within. What exactly these odiferous undertakings are saying isn’t entirely clear—but they sure do have structural integrity.
Theories on the physiological genesis of cubic feces were quick to surface. Some posited the poo was being compressed between the bones of the wombat pelvis; others thought parallel lines of muscles at the juncture of the small and large intestines might be exerting some structured shaping. Carver’s personal favorite involves the squeezing action of a square anal sphincter that shapes turds as they vacate the wombat bowel. None of these theories have panned out.
It wasn’t until a team of mechanical engineers led by Patricia Yang and David Hu of the Georgia Institute of Technology took on the project that the stalemate on stool was finally broken. Yang, who has dedicated her career to studying the fluid mechanics of bodily functions like urination and defecation, was immediately intrigued by the possibility of prismatic poop.
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Most feces, Yang explains, fall into one of three (s)categories: cylindrical, pellet-shaped, or watery. If there’s structure to the poop, it tends to conform to the typical shape of the colon: round. Wombats, like humans, have classic tube-like intestines. So a cubic product presents a baffling incongruity. “This kind of violates principles of poop,” Hu says.
To solve the mystery, Yang and Hu reached out to Carver, who, as a part of his research, collects the bodies of wombats who have been humanely euthanized by local veterinarians. “When we contacted him, he said, ‘I have 10 wombats in my freezer—how many do you need?’” Yang recalls.
Carver dissected the digestive tracts out of two common wombats (Vombatus ursinus) and shipped them overseas, and Yang and her team got to work. When the researchers split the wombat guts open, they found that the final fecal forms measured just shy of one inch by one inch front and back (the parts facing the mouth and anus), and were about an inch and a half long, though this varied a bit from chunk to chunk. While they resembled rectangular prisms more than perfect cubes, the feces were far more symmetrically shaped than typical cylindrical feces, which usually have a length around five times their diameter.
The common wombat (Vombatus ursinus) is one of three wombat species alive today. These marsupials are protected throughout most of Australia, but are vulnerable to concerning diseases like mange. Photo Credit: JJ Harrison, Wikimedia Commons
Close to the stomach, however, wombat feces were more commonplace. Within the small intestine, where most nutrients are absorbed, what would become wombat fecal matter was still pretty wet, runny, and shapeless. It was only further down the line—in the final five feet of the wombats’ intestines, deep into the colon—that the poop began to firm up into recognizable rectangles. The droppings then acquired sharp corners in the last foot and a half before the anus.
To suss out what made this last stretch of the colon so remarkable, the researchers measured its elasticity by inflating a long balloon inside empty intestinal tissue. They found that, compared to the intestine of a pig—one of many boring cylindrical poopers—the walls of the wombat intestine didn’t stretch uniformly: Two regions along its circumference were extra stretchy while two were stiffer. These differences in elasticity might create some areas where feces could bulge out into edges as they pass, Yang believes.
But this result felt a bit odd: four regions apiece, corresponding to four edges and four faces, would have made more sense. The researchers are currently conducting more experiments that might, as Carver puts it, square away the issue.
“This is a clever approach,” says Nicole Xu, a bioengineer at Stanford University who did not participate in the research. “One highlight of the work is they’re not cutting the tissue, which would destroy it. They’re just stretching it volumetrically.”
These observations could also present a bit of a chicken and egg issue, however. It’s possible that years of cubic feces passing through the colon have deformed the tissue. If that’s the case, Yang says, then we still may not know what’s responsible for the feces’ boxy silhouette.
A few other pieces of this pooptastic puzzle have yet to fall into place. Regardless of shape, feces typically segment out into discrete units at some point along the digestive tract. But it’s not entirely clear how wombats are dicing their droppings with such precision.
Carver thinks it may have something to do with the muscles that line animal intestines, which fall into a couple groups: some muscles squeeze inward like contracting rubber bands, while others push fecal matter down toward the anus like thumbs rolling toothpaste down a tube. As they dry out and sharpen up, the nascent cubes may start to pack together, Carver explains, until the compression flattens the two sides that don’t contact the intestinal walls.
Dissected wombat intestines, with feces numbered. The poop begins to take cubic shape in the last five feet of the intestines, deep in the colon. Photo Credit: Patricia Yang and David Hu, Georgia Institute of Technology
At long last, we come to a final butt-burning question: If the wombat anus isn’t square-shaped, how do feces make it out without deforming? Yang says the Internet has dubbed this paradox the “reverse pizza box problem.” But it’s not as complicated as it might seem.
Wombat intestines stretch out to about 20 or 30 feet, and these critters have particularly long colons for their size. It can take wombats up to two weeks to fully digest the vegetation on which they nosh. By the time their excrement exits the body, it’s almost completely devoid of water. This leaves the fibrous feces in pretty solid form—akin to the consistency of a wicker basket, Hu says. It would take a lot to crush a block this hard and dry back into a cylinder.
Now that the researchers have dropped this first scoop on wombat poop, they’re currently hard at work on the next set of questions. Hold tight: The rest of us may not have long to wait to see how it all comes out in the end.
