Why the pudgy marsupials might benefit from six-faced faeces is generally agreed upon: wombats mark their territorial borders with fragrant piles of poo and the larger the piles the better. With die-shaped dung, wombats boost the odds that their droppings, deposited near burrow entrances, prominent rocks, raised ground and logs, will not roll away. That, at least, is the thinking.

But quite how the animals produce the awkward-shaped blocks – and they can pass up to 100 per night, presumably with some trepidation – has proved a harder one to work out. Scientists who find themselves intrigued by the phenomena have made little progress beyond ruling out the nagging suspicion that the animals possessed square anuses.
...
Close inspection revealed that the wombat’s excrement solidified in the last 8% of the intestine, where the faeces built up as blocks the size of long and chunky sugar cubes. By emptying the intestines and inflating them with long modelling balloons, of the sort used to make balloon animals at children’s parties, the researchers measured how the tissue stretched in different places.

In work to be presented at a meeting of the American Physical Society’s fluid dynamics division in Georgia, the team explain how the last section of the wombat intestine does not stretch evenly, unlike the rest of the intestine. When measured around the circumference, some parts give more than others. This allows the intestine to deform in such a way that packs faeces into 2cm-wide cubes rather than the usual sausage shapes. The findings were buoyed up by tests on pig intestines which found no such irregularities in how those stretched.

https://www.theguardian.com/science/2018/nov/18/scientists-unravel-secret-of-cube-shaped-wombat-faeces