Why do wombats poop cubes?

February 1, 2021 • 11:00 am

There are three species of wombat, and all of them produce cubic poop. No other mammal can do this.  Voilà:

Now when you ask yourself “Why do wombats poop cubes?”, there are two ways of getting an answer. The first is the proximal explanation: what physiological and mechanical aspects of wombat digestion and excretion turn the ingested vegetation (they’re herbivores) into cubes? That’s the question the new paper below addresses (click on screenshot). The authors, a consortium of American and Aussie scientists, don’t really answer that question definitively, but at least suggest some approaches.

But the second way of answering it, which I find more interesting, is the ultimate explanation—the evolutionary one. Given that these species are the world’s only mammals with cubic poop, what is the evolutionary advantage of doing that? Of course, cubic turds may not be a directly adaptive trait; it could be an epiphenomenon or a byproduct of some other trait. Maybe wombats have square buttholes! The authors do in fact address that question (see below). But let’s see if there’s anything about having cubic scats that might be useful to a wombat. More on that in a short while.

The paper is at the first screenshot below, the pdf is here, and the full reference is at the bottom of the post.

If you have the mentality of a teenager, you’ll find the journal where this was published amusing:

Before we look at the model, we can dispose of one hypothesis: wombats have cubic poop because they have square buttholes. This is the “pasta theory”, which is that the soft, pliable feces would assume the shape of the extrusion opening. The authors reject this hypothesis in one of the more amusing parts of the paper:

If wombats were to make cubes similar to the way we make noodles, we would expect a square anal sphincter. In 2019, we obtain a CT scan of a live adult female wombat (Video S1, ESI). The scan shows that the wombat’s anus is round, a feature consistent with all other animals. Also, the pelvic bones, which the feces were once proposed to glide past, are nowhere in the vicinity of the colon. We thus conclude that wombats do not change their feces shape through extrusion.

Note that there are videos!

But I don’t understand why the authors couldn’t just lift the creature’s tail to see if it has a square anus. I guess that would be unsophisticated.

Here’s a picture of the species they studied, the bare-nosed  or common wombat (Vombatus ursinus). It produces about 100 cubic turds per day, many of them deposited on “latrines”: logs, rocks or rises that harbor the scats of five or more individuals, giving a clue to why the feces may be cubic (more later). A) shows the adorable creatures, and b) a latrine. Photos c) and d) show the production of the separated cubic turds in the long intestine, with the anal (distal) end to the lower right in d). Look at all the poop lined up, like planes queueing to land at O’Hare!

(From paper): Fig. 1 Wombats form cubic feces. All scalebars represent 5 cm. (a) A female wombat with her joey. (b) A typical wombat latrine consisting of feces placed on a low rock or stump. (c) A 2019 dissection of a wombat shows the cubic feces fully formed within the mid-distal colon, (d) the excised 3 m of wombat intestine shows feces transforming from a yellow yogurt-like slurry near the stomach to darkened dry cubes near the anus.

At any rate, let’s proceed to the intestinal dynamics.

The authors dissected three euthanized wombats that had been badly injured by cars, and found that the herbivores have long intestines that alternate between stiff sections and softer sections of muscle.  They then made a model in which poop would traverse areas of soft versus hard intestine, and be squeezed more in the latter bits. They also made a number of assumptions about the elastic properties of wombat turds, including “Reynolds numbers”, strength of contractions, and so on, producing a series of Fecal Equations. They also had to construct an index of “turd squareness” to see how the various squeezings of the feces, which would also extract water, making them hard, would result in cubes.

The upshot is that they could get approximately square shapes with just 10 rounds of intestinal squeezing, but didn’t try to model the estimated 100,000 bouts of squeezing that each turd actually experiences in the gut. But the result depend critically on the assumptions, like the “Reynold’s number” (a flow parameter) of the feces. Not knowing these parameters, all they can say is that in some simulations under some assumptions, they can get squarish turds, but not necessarily ones as square as the wombats actually produce.

But the more interesting issue is “why do wombats want to produce cubic turds?”, remembering that I’m using “want” here as shorthand for “what is the adaptive advantage (if any) to a wombat that poops cubes instead of the normally-shaped turds of its relatives (and ancestors)?”  The authors mention two hypotheses, which aren’t mutually exclusive:

1.) Cubic turds don’t roll off of the latrines. Latrines presumably are either territorial markers or ways of informing groups of “who’s in the group?” If you poop on a latrine, you want your poop to stay put, and cubes will do that better than spheres or log-shaped turds. The authors even did experiments with dough balls, showing that if dropped on an incline, cubical doughballs roll significantly shorter distances than do spherical ones. I like this hypothesis. Once wombats evolve the habit of putting their poop in given spots as a territorial or olfactory marker, then selection might act to favor those individuals whose turds tend to stay put when deposited.

2.) A cubic shape produces more surface area for olfactory communication. As the authors hypothesize:

It is possible that the feces’ cubic shape increases the surface area so that it can facilitate olfactory communication. Elevated scent-marking is a common behavior in many mammals and is hypothesized to increase scent dispersal and visibility. The purpose of scent-marking is typically territorial, however there is evidence that feces are also used in social communication or communicating reproductive status.

And it is true that cubes do have a higher surface area to volume ratio than do spheres (for a unit volume, a cube has the surface area of 6 compared to 4.83 for a sphere. (Tetrahedrons are even better, but presumably would be hard to form!).  But you’d have an even higher ratio if you pooped out a ton of very small turds, like a rabbit, or laid down a ribbon of feces, with a large surface area and small volume. On the other hand, a greater surface area means that you dry out faster, and with it the olfactory cues diminish. The authors don’t discuss the possibility of such a tradeoff.

I don’t think that this second explanation is on the mark, as I can’t imagine the marginal advantage of having more squarish turds can give you that much of an “olfactory advantage.” (But of course evolution works on very small margins, so I may be wrong.)

At any rate, the evolutionary hypothesis seems easier to investigate than the one that requires dissection and modeling. You could, for example, change the shape of wombat poop after it’s produced and see what, if any, changes occur in social interaction, odor profile and strength, and so on.

As the papers say, “There is much work to be done,” though I don’t think I want to be the one to do it!

h/t: Neil

________________

Yang, P. J., A. B. Lee, M. Chan, M. Kowalski, K. Qiu, C. Waid, G. Cervantes, B. Magondu, M. Biagioni, L. Vogelnest, A. Martin, A. Edwards, S. Carver, and D. L. Hu. 2021. Intestines of non-uniform stiffness mold the corners of wombat feces. Soft Matter 17:475-488.

32 thoughts on “Why do wombats poop cubes?

    1. Yep; this research won the Ig Nobel in physics in 2019.
      (Why physics instead of biology? I’m not sure, but could have something to do with the authors presenting their results in an APS Fluid Dynamics division meeting.)

  1. AFTER READING THIS…..I have to admit I looked at all the names of the people who wrote the paper and I was expecting to see at the end- the name Erno Rubik.

  2. This is amazing!

    Do wombats have underground tunnels with “bathrooms” like squirrels? If so, they might pack more efficiently in the tunnels – depending on the shape of the tunnel.

    I also wonder about how seeds would travel through the wombat, as I can’t recall any seed that is not oblate to spherical.

    Oh and how about the gut flora – bacteria might be involved with the shape … and are there objects on either side of the cube that is not observed after it is exported.

  3. This is truly interesting.

    And important for my peculiar interest:

    “Maybe wombats have square buttholes!”

    is a likely “sentence that has never been uttered before.”

    Much better than “square anal sphincter.”

  4. I don’t really have anything to add here, but, given my online/email name of Uommibatto, I am honor-bound to comment on anything wombat-related.

    In the interest of puerility, note also that the main newspaper covering this story said that scientists had “finally gotten to the bottom of the matter.”

    1. Omg omg omg! Authors on this team have Ignoble Prizes already in this area, dating back to 2019:
      PHYSICS PRIZE [USA, TAIWAN, AUSTRALIA, NEW ZEALAND, SWEDEN, UK]
      Patricia Yang, Alexander Lee, Miles Chan, Alynn Martin, Ashley Edwards, Scott Carver, and David Hu, for studying how, and why, wombats make cube-shaped poo.
      REFERENCE: “How Do Wombats Make Cubed Poo?” Patricia J. Yang, Miles Chan, Scott Carver, and David L. Hu, paper presented at the 71st Annual Meeting of the APS Division of Fluid Dynamics, Abstract: E19.0000, November 18–20, 2018
      REFERENCE: “Intestines of Non-Uniform Stiffness Mold the Corners of Wombat Feces,” Patricia J. Yang, Alexander B. Lee, Miles Chan, Michael Kowalski, Kelly Qiu, Christopher Waid, Gabriel Cervantes Benjamin Magondu, Morgan Biagioni, Larry Vogelnest, Alynn Martin, Ashley Edwards, Scott Carver, and David L. Hu, Soft Matter, vol. 3, 2021
      WHO ATTENDED THE CEREMONY: Patricia Yang, David Hu, Miles Chan, Alexander Lee, Scott Carver, Ashley Edwards
      NOTE: This the SECOND Ig Nobel Prize awarded to Patricia Yang and David Hu. They and two other colleagues shared the 2015 Ig Nobel Physics Prize, for testing the biological principle that nearly all mammals empty their bladders in about 21 seconds (plus or minus 13 seconds)

  5. Building materials of course. Cubes stack better. Wombats are embarked on a long term plan of global domination and stockpiling of cubic turds for building materials is a vital part of the plan.

    1. Even now, large feco-adobe structures are appearing in the outback, from which the wombats will launch their war…

  6. Just remember like all Australian wildlife a wombat when angry will attack you. But they are magnificent creatures to observe in the wild

  7. Wombats have more than cubic (well not really cubic: length and breath are about equal, but the hight is much less) poop: they dig with one forepaw at a time, they are said to crush would-be predators to the ceiling of their burrows with their (ample) butts, and their pouches face backwards. The latter is easy: they are burrowing animals, and you don’t want all the earth and grit in your pouch.
    The cubic(-ish) feces remain the most elusive for an ultimate cause.
    I don’t buy the ‘incline’ hypothesis, since they do not particularly live on inclined surfaces, the smell thing sounds better, but (as pointed out above) a lot of smaller spheres would do better (when I jog, even I, a smell-debilitated human compared to other mammals, can smell if there are rabbits around).
    I think, as with anything we can’t easily explain in these things, we may have to look into sexual selection. I’ll take the guy with the cutest cubes?
    Once we’ll found the ultimate cause (if ever) we’ll probably say (thanks TH Huxley): “So obvious, how stupid not to have thought of that!”

  8. As Mark Twain said:

    “There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.” (Life on the Mississippi)

    Of course, there is perhaps a bit more than a “trifling investment” here.

  9. Since god is well known to be a pure mathematician, this shows that there are 4 interesting families of species which are either undiscovered, or at least their solid bodily waste is (says the always polite man who likes the efficient wording of the Yorkshireman’s “shitters”, AKA “public conveniences”). An example of one would be the one whose turds are tetrahedral in shape. And 3 more, only 3, regular solids: those for which all faces are surrounded by the same number of edges, and all vertices are the endpoint of the same number of edges.

    Now topologically it gets more interesting, with the inner tube shape and the pretzel shape, and infinitely many others. Those are perhaps a product of wombat indigestion.

    I’m unsure about the 3-dimensional analogue of Moebius band. Perhaps the ‘musicians’, who surely by now have stolen that name, have retooled their toilet training. As long as they don’t fling the stuff around on the stage.

    The cube is the boring one, now also perhaps the malodorous one as well.

    1. “..pretzel shape..”

      As many times said to me: “Don’t get your shit in a knot about this trivial matter!”

  10. OH – if they use/eat soil or fine silt or sand, or clay, the minerals could coordinate organic material in specific geometries – wouldn’t that be amazing if there was such a nucleation that explains such a clear result.

  11. I wonder whether the cubic feces are a byproduct of the wombat’s famously low metabolism. Or perhaps the result of maximizing the amount of water that can be retrieved from the feces before they are excreted. Wombats live in a dry climate iircc. Of course, if that were the explanation we’d expect to see cubes from other dry climate species, and wombats are unique in this regard.

  12. I was half hoping that a wombat’s response to the question would be to fix the researcher with a steely gaze, and hiss menacingly “because we bloody well *can*!”
    🙂

  13. I rather like the first hypothesis that Jerry advanced here. Cubes, would certainly be less likely to roll off an elevated spot such as a boulder. But I suppose that doesn’t help answer the question of how such an adaptation evolved.

    I am an ornithologist (retired), but when teaching at my college’s summer field school in Ontario it was one of my assignments to collect wolf scat for our mammalogist when I was in the field. He was studying dietary preferences by hair analysis. Up on the shield boulders of granite regularly protrude from the ground. And that was quite evidently the preferred location for wolves to leave their scat. In my eight summers there, I picked up a fair amount of scat. And a number of times it was still warm, but I never saw a wolf.

    This past week I was surveying for locations to capture and band wintering Long-eared Owls at The Wilds, a large mammal conservation facility in southeastern Ohio, where I live. And each small wooden foot bridge I came to on the trails was heavily decorated with coyote scat. Those aren’t hard to see.

    But, alas, neither has cubic poop.

    1. “Cubes, would certainly be less likely to roll…”

      But then a tetrahedron would be even better–I’m being facetious!
      It’s 4 perfect triangles of the same size, glued together along edges, to form a closed off ‘container’—here you fill in the interior—but don’t dwell too much on the material used!

      The only other of the 5 that’s easy to describe is to take 2 identical pyramids–they’re not regular solids because of 4 triangles but one square on the bottom. But turn one upside down, glue the two squares together and erase their innards, and you get a Platonic solid with 8 triangles as its faces. IIRC, called the icosahedron.

      The other 2 are pretty complicated, but very symmetrical.

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