Cubed wombat poo puzzle completed by researchers with the help of physics, volcanic science


Puddle, log, pellet or cube? Mammal life is diverse, and so are the poos mammals produce.

But, to quote the father of evolutionary biology, Charles Darwin: What is behind “these endless forms most beautiful and most wonderful?”

A team of scientists from around the world — including associate professor Scott Carver from the University of Tasmania — have turned their minds to this question and found some fascinating faecal facts.

It is not the first time the disease ecologist’s poo research has made a splash.

He and his colleagues discovered that wombats’ famously square-shaped poo was created within the intestine and not at “point of exit”.

This nabbed them an Ignoble Prize in 2018 — a prestigious award for science that makes you laugh and then makes you think.

However, questions about the peculiar poo still remained.

“We understood how they produced the corners that give them the square shape in cross-section,” Dr Scott Carver told ABC Hobart’s Helen Shield.

“But not why they fragmented at such regular intervals along the length and came out as perfectly consistent little poos that are about the same length.”

Turning to volcano science and physics to uncover the answer, the researchers have not only discovered why wombats have “perfectly consistent little poos”, but also what determines the shape of mammal poo in general.

Why look to volcanoes?

It was the cooling process of lava beds that inspired Dr Carver and his collaborators.

After an eruption, lava beds cool and can solidify into specific and regular shapes.

One of the most famous places to see the results of this process is The Giant’s Causeway in Northern Ireland.

“They have these hexagonal structures that are really consistent,” he said.

The structures were formed because the lava they were made from cooled more slowly on the surface than internally, creating forces that resulted in regular cracking.

This got Dr Carver and his team wondering: “Does the same sort of principle apply to faeces and the sorts of different lengths and shapes that you get?”.

Except, unlike lava beds, the cracking of mammal poo occurs before expulsion, in the large intestine.

What do lava beds and intestines have in common?

As food travels through a digestive system, it gets broken down and digested, leaving a sloppy “slurry” of waste, Dr Carver says.

During the digestion process, the slurry enters the distal colon, where the body reclaims some of the moisture from the waste.

“For a while, the moisture of the faeces is so much that it doesn’t really matter … they are still the slurry … but eventually they start to get drier,” Dr Carver said.

The researchers had an inkling that this drying process created cracks through “water flux” in a similar way to how “heat flux” created the even hexagons of The Giants Causeway.

Testing their theory

In the lab, the team recreated the drying of waste in the intestine.

Corn starch was used to simulate poo and the stand-in for intestines were plastic troughs.

Heat lamps were used to dry the corn starch slurry at different rates and then the researchers analysed crack formation.

The greater the drying, the more closely spaced the cracks.

The water flux creates a “shearing process” that influences how regularly the breaks in the faeces occur — just as heat flux sheared the cooling lava of The Giants Causeway.

So, a wombat’s distinctive cubic poo is a result of the drying forces of its intestine being just right for the creation of poo that’s about as long as it is high.

Mystery solved.

What about other poo shapes?

The researchers also compared the surprisingly ample data available on poo shape and water content — so what is the scoop?

They found that when the water content of faeces dropped below 70 per cent, cracks formed and animals produced pellet-shaped poo.

“Whereas humans have about 75 per cent [water content in our stools]. So it is much wetter and we produce tubes most of the time,” Dr Carver said.

Cow pats and their sloppy ilk occur when poo reaches 85 to 90 per cent water: “Then it is just a puddle,” he said.

While the heady world of animal poo formation might seem distant from the doctor’s office, Dr Carver believes this research may one day help medical scientists.

“Colorectal cancer, stress and various other things can influence the kind of faeces that people produce,” he said.

While he says there will not be any direct medical breakthroughs from his work, it will add to “cumulative knowledge that contributes to helping understand human medical conditions”.

ABC News, 21 February 2023