The Universe of Discourse

Sat, 26 Nov 2022

Wombat coprolites

I was delighted to learn some time ago that there used to be giant wombats, six feet high at the shoulders, unfortunately long extinct.

It's also well known (and a minor mystery of Nature) that wombats have cubical poop.

Today I wondered, did the megafauna wombat produce cubical megaturds? And if so, would they fossilize (as turds often do) and leave ten-thousand-year-old mineral cubescat littering Australia? And if so, how big are these and where can I see them?

A look at Intestines of non-uniform stiffness mold the corners of wombat feces (Yang et al, Soft Matter, 2021, 17, 475–488) reveals a nice scatter plot of the dimensions of typical wombat scat, informing us that for (I think) the smooth-nosed (common) wombat:

  • Length: 4.0 ± 0.6 cm
  • Height: 2.3 ± 0.3 cm
  • Width: 2.5 ± 0.3 cm

Notice though, not cubical! Clearly longer than they are thick. And I wonder how one distinguishes the width from the height of a wombat turd. Probably the paper explains, but the shitheads at Soft Matter want £42.50 plus tax to look at the paper. (I checked, and Alexandra was not able to give me a copy.)

Anyway the common wombat is about 40 cm long and 20 cm high, while the extinct giant wombats were nine or ten times as big: 400 cm long and 180 cm high, let's call it ten times. Then a propportional giant wombat scat would be a cuboid approximately 24 cm (9 in) wide and tall, and 40 cm (16 in) long. A giant wombat poop would be as long as… a wombat!

But not the imposing monoliths I had been hoping for.

Yang also wrote an article Duration of urination does not change with body size, something I have wondered about for a long time. I expected bladder size (and so urine quantity) to scale with the body volume, the cube of the body length. But the rate of urine flow should be proportional to the cross-sectional area of the urethra, only the square of the body length. So urination time should be roughly proportional to body size. Yang and her coauthors are decisive that this is not correct:

we discover that all mammals above 3 kg in weight empty their bladders over nearly constant duration of 21 ± 13 s.

What is wrong with my analysis above? It's complex and interesting:

This feat is possible, because larger animals have longer urethras and thus, higher gravitational force and higher flow speed. Smaller mammals are challenged during urination by high viscous and capillary forces that limit their urine to single drops. Our findings reveal that the urethra is a flow-enhancing device, enabling the urinary system to be scaled up by a factor of 3,600 in volume without compromising its function.

Wow. As Leslie Orgel said, evolution is cleverer than you are.

However, I disagree with the conclusion: 21±13 is not “nearly constant duration”. This is a range of 8–34s, with some mammals taking four times as long as others.

The appearance of the fibonacci numbers here is surely coincidental, but wouldn't it be awesome if it wasn't?

[ Addendum: I wondered if this was the only page on the web to contain the bigram “wombat coprolites”, but Google search produced this example from 2018:

Have wombats been around for enough eons that there might be wombat coprolites to make into jewelry? I have a small dinosaur coprolite that is kind of neat but I wouldn't make that turd into a necklace, it looks just like a piece of poop.


[ Addendum 20230209: I read the paper, but it does not explain what the difference is between the width of a wombat scat and the height. I wrote to Dr. Yang asking for an explantion, but she did not reply. ]

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