The bare-nosed wombat, Vombatus ursinus, waddles out of its burrow in the Tasmanian dusk and leaves behind something no other animal on Earth produces: a neat pile of cube-shaped droppings, stacked deliberately on top of a rock or fallen log. The pellets have flat sides and sharp-ish edges, and they hold their shape long enough for the wombat to arrange them like tiny message tiles above ground level. A round dropping would roll off the log. A cube stays put.
This is not a folk claim or a zookeeper’s exaggeration. Wombats are the only known animal in the world that produce faeces with flat sides, a fact confirmed in 2018 when a team led by Georgia Institute of Technology engineer Patricia Yang dissected the intestines of wombats killed on Australian roads and traced exactly how the cubes form.
The animal behind the cube
Wombats are stocky marsupials, closely related to koalas, with short legs, coarse fur, and a rump built like a shield. Adults are substantial animals that dig extensive burrow systems underground. There are three living species — the bare-nosed wombat and two hairy-nosed species — and all of them, so far as anyone has checked, produce cubes.
A single wombat deposits large numbers of cubes regularly. That is a lot of geometry.
The droppings are dry, fibrous, and dark, and they come out in strings of four to eight at a time, each one distinct from the next. Rangers in Australia can identify a wombat’s presence in a paddock just by finding the pellets balanced on a stump.
How the cube gets made
For a long time the working theory was that wombats had a square anus. They do not. The anus is round, like every other mammal’s. The shaping happens further upstream, inside the last stretch of the intestine, and it took a fluid-dynamics lab to work out how.
Yang’s team found that the final section of a wombat’s intestine has walls of uneven stiffness. Two regions are stretchy and thin. Two opposing regions are thick and rigid. As the fibrous faecal matter dries out — wombats are efficient at pulling water back from their gut, an adaptation for arid conditions — the intestinal walls contract unevenly, compressing the mass along the stiff edges and leaving the softer edges to bulge slightly.
Over the final stretch of the digestive tract, the mass is squeezed into a rough prism. By the time it reaches the exit, it has flat faces.
The discovery has been described as the first known example of a soft-tissue organ manufacturing a shape with flat sides. Every other cube-like object in nature — salt crystals, pyrite, bismuth — is a mineral. This one is biological, and it is squeezed into geometry by muscle and time.
Why cubes and not spheres
The evolutionary answer sits with what wombats do with the droppings once they are out.
Wombats are territorial and largely solitary, and they communicate mainly through smell. A wombat that wants to advertise its presence climbs onto something raised — a rock, a log, a tussock, sometimes a mushroom — and deposits its cubes there. Height matters, because scent carries further from elevation. Flat sides matter, because a scent-marker that rolls off into the grass is a scent-marker that has failed.
Fieldwork has documented wombats returning to the same rocks night after night, adding fresh cubes to the pile. Recent work on wombat latrines suggests these communal deposit sites function almost like a chemical noticeboard, telling passing wombats who has been through, when, and possibly whether they are ready to breed.
Round pellets would undermine the whole system. On a sloped rock in Tasmania, a spherical dropping is gone by morning. A cube sits.
An intestine unlike any other
To measure the shaping, Yang’s team inflated wombat intestines with long balloons and watched how they stretched. In a pig, the intestinal wall stretched roughly evenly around the circumference. In a wombat, the balloon bulged strongly in two places and barely at all in the two places between them. That uneven elasticity is what does the shaping.
Digestion in a wombat is also unusually slow. Food takes days to pass through, giving the gut time to extract as much water and nutrition as it can from the tough native grasses and roots the wombat lives on. By the final stretch, the material is dense, dry, and pliable in exactly the way modelling clay is pliable — firm enough to hold an edge, soft enough to be moulded.
Push wet slurry through the same intestine and you would get sausage-shaped droppings, as every other herbivore produces. Push a nearly solid brick of dried fibre through it, with uneven wall pressure, and cubes are what fall out.
What engineers want from a wombat
The reason a fluid-dynamics lab was dissecting wombat guts in the first place is that industrial manufacturing has essentially two ways of producing cubes: cutting them out of a larger block, or moulding them in a die. Both are expensive, both waste material. A wombat makes cubes by peristalsis alone, using nothing but soft tissue and differential stiffness. Yang has said publicly that the mechanism could inform new approaches to manufacturing and to clinical diagnostics — the way a gut shapes its contents may be a clue to how tumours and other soft-tissue masses take on their forms.
No factory has yet copied the wombat. But the paper has been cited widely, and it won an Ig Nobel Prize, awarded for research that first makes people laugh and then makes them think.
The territorial logic
Wombats are not gregarious. A single bare-nosed wombat may defend a substantial range, and confrontations at burrow entrances can be violent — the animal’s reinforced rump is a defensive weapon, used to crush attackers against the tunnel roof. Direct contact is costly. Scent-marking from a distance is cheap.
That is why the cubes end up where they do. Surveys of wombat latrines have found deposits concentrated at the edges of home ranges, at trail intersections, and on any raised object with a view. A wombat coming across a fresh cube on a familiar log gets a full biographical read from the smell alone: species, sex, individual identity, reproductive status, roughly how long ago the depositor passed.
The cube is the envelope. The scent is the letter.
A behaviour older than the shape
Scent-marking on raised objects is common across mammals. Lions rub their faces on trees and spray urine at specific heights. Rhinos build dung middens. Bears scratch and rub. What makes wombats unusual is not the behaviour but the packaging — the fact that their guts have, over evolutionary time, tuned themselves to produce a shape that stays where it is put.
The when of that evolution is not yet pinned down. Wombats and koalas diverged many millions of years ago, and koalas do not produce cubes. So the intestinal geometry likely developed as wombats shifted into drier, more open Australian habitats where water-conserving digestion and long-range scent communication both mattered more.
Watching for cubes
In wombat country — Tasmania, Victoria, parts of New South Wales and South Australia — the cubes are easier to find than the animal. Wombats are largely nocturnal and spend much of the day underground. But their signatures sit out in the open, on granite boulders and fallen eucalyptus trunks, in neat stacks that look, at first glance, like someone has been dropping dice.
Pick one up. It is light, dry, almost odourless to a human nose, and unmistakably six-sided. It has been through a complex stretch of asymmetric intestine over days to reach that shape, and it is sitting on the rock because a round one would not have.
Somewhere below, in a chamber at the end of a tunnel, the wombat that made it is asleep, and will come back tomorrow night to add more.