Why is wombat scat (feces) shaped like cubes?

Question

I've heard that wombat scat is cube shaped, but I don't understand how that can happen. Has anyone studied the phenomenon? What would the evolutionary pressure have been to cause this?

Answer 1

I'm almost certain that your question is based on the press that Patricia J Yang's research is receiving (e.g., here and here).

Yang and her co-authors examined the structure and mechanics of some dead wombats to investigate this question further. They found that varying degrees of pressure in the latter portion of the wombat's intestines (in conjunction with a dehydrating of the fecal matter) led to the characteristic cube shape:

From their abstract:

In the final 8 percent of the intestine, feces changed from a liquid-like state into a solid state composed of separated cubes of length 2 cm. This shape change was due to the azimuthally varying elastic properties of the intestinal wall. By emptying the intestine and inflating it with a long balloon, we found that the local strain varies from 20 percent at the cube's corners to 75 percent at its edges. Thus, the intestine stretches preferentially at the walls to facilitate cube formation.

Why do this?

There seems to be two major reasons:

1. Mark territory
2. Attract mates

For example, see Wells 1989¹:

The rubbing of posts, logs and overhanging branches with their backs and rumps and deposition of faeces along trails may be a means of olfactory communication used in the maintenance of territories

One hypothesis is that by being square, the droppings don't roll as easily and therefore stay in the place that the wombat intended to mark.

I've also seen hypotheses about wombats stacking their square feces as some sort of signaling, but I couldn't find any reputable literature sources that make this suggestion.

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^{1: Wells, R.T., 1989. Vombatidae. Fauna of Australia, 1, pp.755-768.}

Answer 2

On a more serious note than my comment, and as a supplement to theforestecologist's answer, it's worth pointing out that a cube with rounded corners and edges has larger surface area to volume ratio than a spherical dropping, making it more efficient for the reabsorbtion of moisture, which would be an evolutionary advantage in a place where water is in short supply (as it is in large parts of Australia).

Additionally, in respect of the claim made here that droppings are used for marking, it may be advantageous to have a dropping that doesn't roll, if it's important some subsequent visitor can identify the precise spot the droppings were dropped, especially if one inhabits sloped terrain (which a wombat does - mountainous areas of Australia) where a round dropping might roll and mark an ambiguous spot.

Answer 3

Although the OP might no longer be active, I'm comming back to this question, because there is a new study on this released in "Soft Matter". With simulations Yang et al., 2021, describe how corners of these almost perfect cubes might be formed. From the abstract:

Using histology and tensile testing, we discover that the cross-section of the intestine exhibits regions with a two-fold increase in thickness and a four-fold increase in stiffness, which we hypothesize facilitates the formation of corners by contractions of the intestine. Using a mathematical model, we simulate a series of azimuthal contractions of a damped elastic ring composed of alternating stiff and soft regions. Increased stiffness ratio and higher Reynolds number yield shapes that are more square. The corners arise from faster contraction in the stiff regions and relatively slower movement in the center of the soft regions.

As described by the previous answer, there are apperently two stiffer regions opposite of each other in the last intestinal section, seen in the longitudinal direction. These press inward more strongly during intestinal peristalsis, i.e., repeated contraction, thus flattening the material further and further on opposite sides, so to speak.

Similarly, we form a cube from a spherical piece of plasticine by pressing inward with two parallel fingers on each side, doing so sequentially from different sides. In the wombat intestine, however, there is no change in the direction of indentation, so the researchers assume that the more elastic intestinal strips are pulled inward in the middle and in this way flatten the missing sides.

As @anudder already pointed out, the last author commented on this in "Science": How do wombats poop cubes? Scientists get to the bottom of the mystery

The stiffer portions are “like a stiff rubber band—[they’re] going to contract faster than the soft regions,” says David Hu, a biomechanics researcher at the Georgia Institute of Technology and author on the study. Softer intestinal regions squeeze slowly and mold the final corners of the cube, the team found. In other mammals, the wavelike peristalsis of the intestinal muscles are consistent in all directions. But in the wombat, the grooved tissue and the irregular contractions over many cycles shape firm, flat-sided cubes.

Why?
It is not yet proven, but they suspect that it has a purely practical reason. Like many other animals, wombats mark their territory with excrement. They often choose stones as a place to lay their excrement, from which cube-shaped piles roll down and away less easily than round specimens would.