Patricia Yang is a postdoctoral fellow in civil and environmental engineering from Stanford University in Taiwan, whose academic focus is on the fluid mechanics of flocks of birds, blood, and excreta. In her previous academic career, Yang Peiliang has studied the excretion behavior of many animals, such as elephants counting their own dung balls.
In 2015, when she was presenting her latest mathematical model of animal defecation at an academic lecture, a scientist present here asked her: "Does this model of yours apply to wombats?" ”
Yang Peiliang was still a little confused at the time, because she was not a biologist and really did not understand the feces of wombats. Immediately after the presentation, she searched for pictures of wombat droppings.
It was the strangest animal feces she had ever seen.
In our article "Animal Investigation "Chrysanthemum", the strange "Chrysanthemum" Department of the Animal Kingdom, which has opened the eyes of scientists", we have introduced the new things that various animals in nature have done in defecation, including the feces of wombats. We also say that if you don't suggest it, many people at first glance think that these are human-processed brown sugar cubes or chocolate.
To this day, this remains the only animal known to humans to excrete square feces. From this point of view, only wombats going to the toilet can be called "convenient"...
However, the wombat's intestines obviously do not have machines, there are no drawings, and its chrysanthemum is also round, so why can it excrete square poop?
Yang Peiliang was immediately intrigued, and she decided that she must study this animal and its feces carefully.
In 2018, Yang Peiliang finally obtained the intestines of a bare-nosed wombat. The wombat suffered a car accident in Australia and was later euthanized. Biologists from the University of Tasmania dissected it and then gave it to Yang Peiliang. Yang was so excited that she and her colleague David Hu, a lab director at Georgia Tech, studied the wombat intestine.
A wombat on Maria Island, Australia
The gut showed them the process of how digested food scraps turned little by little into square poop, and they found that these residues were liquid most of the time, until they entered the last 17% of the intestine (this is the latest data, the previous article was the old data, it was 8% at the time) the location was processed into angular squares. That is, wombats' droppings are square before they are excreted.
Subsequently, they performed CT scans of live wombats and found that the wombats' chrysanthemums, like other animals, were round, not square. Both of these efforts proved that the processing of square feces was only involved in the intestines and did not involve the chrysanthemums of the wombats.
It's a bit hard for its farts, how does a round outlet excrete square poop?
The first thing to note is that the intestines of wombats are very long, reaching 9 meters, while our human large intestines are only 1.5 meters. Such a long intestine allows wombats to absorb more water and nutrients from their feces. Of course, this also makes the process of converting from food to feces from wombats to feces much longer than in humans, and even up to 5 days.
This is very necessary, after all, wombats often face food and water shortages when living in the wild, so the efficient absorption process is crucial for their survival. Studies have shown that wombat droppings are at least twice as dry as humans. In this case, its feces are more likely to have a pronounced shape.
In the 9-meter-long wombat intestine, we can see different stages of poop —
The researchers found that the thickness and stiffness of the tissues and muscles in the wombats' intestines were not the same everywhere, and some parts showed differences from the average. Where muscle thickness varies, the degree of intestinal contraction varies. In the more densely muscled parts, the intestine contracts faster and can push the stool forward; other areas contract more slowly, and the stool is shaped into angles here.
By creating the model, the researchers found that in less than 10 contraction cycles, the feces formed edges and angles.
"This contraction occurs every few seconds, and over a five-day cycle, the feces experiences 100,000 contractions," they say in the paper. "It is during this 100,000 contractions that the feces become drier and drier, and in the final stages, blocks form.
Hulid explains to us: "It's like baking a cake, the batter is very wet and pully at first, and as it is constantly heated in the oven, it becomes drier and drier." These batters are initially spread over the edges of the cake mold, then angular and smooth planes appear, while the main solidification process takes place in the final stages. ”
In the intestines of wombats, food scraps probably go through such a process, and eventually form a block-shaped feces, although this process takes 5 days, but the amount of wombats excreted is not much at all, and an average of about 100 such "brownie square cakes" can be excreted every day.
This is the first time that humans have systematically described the production process of wombat feces, and it can be regarded as solving a puzzle that has been lingering in the minds of scientists for a long time.
Then there's a question to discuss: Why do wombats go to great lengths to excrete square feces? Is there a fancy contest in nature, and why is it unique?
For now, this remains a mystery, but the researchers have also come up with their own speculations. They pointed out that wombats' vision is not very good, so they may usually choose to use feces to convey information. They excrete feces on rocks, trees, or other relatively high locations to reinforce the signal.
In this case, the square poop has an advantage over the spherical poop, the spherical poop is easier to roll away, and the square poop can remain relatively stable in its place. Moreover, the poop of the square is also easier to pile, which may also help the wombats mark their territory, which is probably the so-called "dung" map "wall".
In addition, it has also been suggested that the poop of the square has a larger relative surface area than the spherical poop, which is also more conducive to the volatilization of odors, so that these odors better convey the information and reproductive state of the wombats.
Others believe that wombats' square poop may have no essential survival significance, but are just a natural consequence of dehydration in the intestines. Their basis is also very strong, that is, the wombats in the zoo that are not short of water have not pulled out such a square.
In short, the secret of wombats' distinctive poop remains to be further studied by scientists. Dare to study these objects that we usually respect, which shows that researchers have also paid a lot. Even Randy Ewoldt, the mechanical engineer who asked Mr. Yang in 2015, said that few people can show this unprecedented research and global collaboration like they did, which is also a pioneering work.
Related reading: Animal investigation "Chrysanthemum", the "Chrysanthemum" department of the animal kingdom, which has opened the eyes of scientists