How wombats make their unique cube-shaped poop | Science News for Students

How wombats make their unique cube-shaped poop

The elasticity of the their intestines helps to shape those fecal cubes
Dec 10, 2018 — 6:45 am EST
a photo of a wombat facing towards the viewer

Wombats are stout marsupials native to Australia. They use their cubelike droppings to mark their territory.

mlharing/iStockphoto

Of all the poops in the world, only those of Australia’s wombats come out shaped like cubes.

Like many animals, wombats mark their territories with small piles of scat. Other mammals poop round pellets, messy piles or tubular coils. But wombats somehow sculpt their scat into cube-shaped nuggets. These may stack better than rounder pellets. They also don’t roll away as easily.

a photo of a cubelike wombat poop
Wombats’ cubelike droppings don't roll off of rocks as easily as would more cylindrical scat.
Bjørn Christian Tørrissen/Wikimedia Commons (CC BY-SA 3.0)

Cubic shapes in nature are very unusual, observes David Hu. He’s a mechanical engineer at Georgia Institute of Technology in Atlanta. An Australian colleague sent him and colleague Patricia Yang the intestines from two roadkill wombats. These had been collecting frost in the guy’s freezer. “We opened those intestines up like it was Christmas,” Hu says.

The intestines were packed with poop, Yang adds. In people, a poop-filled bit of intestine stretches out slightly. In wombats, the intestine stretches to two or three times its normal width to accommodate the feces.

Making and maintaining flat facets and sharp corners takes energy. So it’s surprising that the wombat’s intestines would create that shape. In fact, those intestines don’t look much different from those of other mammals. But their elasticity does vary, the researchers reported on November 18. They explained the potential importance of this at a meeting in Atlanta, Ga., of the American Physical Society’s Division of Fluid Dynamics.

Ballooning gut segments appear to be key

Yang used skinny balloons — the type that gets sculpted into animals at carnivals — to inflate the intestines. She then measured their stretchiness in different places. Some regions were more stretchy. Others were stiffer. The stiffer places probably help create the distinct edges on the wombat poop as the waste moves along, Yang proposes.

Sculpting the poop into cubes appears to be a finishing touch for the wombat gut. A typical wombat intestine is about 6 meters (almost 20 feet) long. Over that span, the poop takes on distinct edges only in the last half meter (1.6 feet) or so, Hu found. Up to that point, the waste is gradually solidifying as gets squeezed through the gut.

The finished turds are especially dry and fibrous. That may help them retain their signature shape as they’re released, Yang suggests. They can be stacked or rolled like dice, standing up on any of their faces. (She knows. She tried it.)

In the wild, wombats deposit their droppings on top of rocks or logs to mark their territory. Sometimes they even form small piles of their scat. The animals seem to prefer to poop in elevated spots, Hu says. Their stubby legs, though, limit this ability.

Yang and Hu are looking to confirm that the wombat gut’s varying elasticity really does create the cubes. To investigate, they have begun modeling the animal’s digestive tract — with pantyhose.

Power Words

(more about Power Words)

coil     Concentric rings or spirals formed by winding wire or some other fiber around and around a core; or the shape that such a wire would make.

colleague     Someone who works with another; a co-worker or team member.

digestive tract     The tissues and organs through which foods enter and move through the body. In people, these organs include the esophagus, stomach, intestines, rectum and anus. Foods are digested — broken down — and absorbed along the way. Any materials not used will exit as wastes (feces and urine).

dynamic     An adjective that signifies something is active, changing or moving. (noun) The change or range of variability seen or measured within something.

engineer     A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.

feces     A body's solid waste, made up of undigested food, bacteria and water. The feces of larger animals are sometimes also called dung.

fluid dynamics     The study of liquids and gases in motion.

gut     An informal term for the gastrointestinal tract, especially the intestines.

mammal     A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding their young, and (typically) the bearing of live young.

marsupial     A type of mammal that carries its young for a period after birth in external pouches. There the developing babies have access to their mother’s nipples — and milk. Most of these species evolved in Australian and have especially long hind-legs. Examples of marsupials include kangaroos, opossums and koalas.

mechanical engineer     Someone trained in a research field that uses physics to study motion and the properties of materials to design, build and/or test devices.

model     A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes. Or an individual that is meant to display how something would work in or look on others.

physical     (adj.) A term for things that exist in the real world, as opposed to in memories or the imagination. It can also refer to properties of materials that are due to their size and non-chemical interactions (such as when one block slams with force into another).

roadkill     A term for animals killed by traffic.

scat     The feces shed by a wild animal, usually a mammal.

society     An integrated group of people or animals that generally cooperate and support one another for the greater good of them all.

technology     The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts.

tract     A particular, well-defined area. It can be a patch of land, such as the area on which a house is located. Or it can be a bit of real estate in the body. For instance, important parts of an animal’s body will include its respiratory tract (lungs and airways), reproductive tract (gonads and hormone systems important to reproduction) and gastro-intestinal tract (the stomach and intestines — or organs responsible for moving food, digesting it, absorbing it and eliminating wastes).

waste     Any materials that are left over from biological or other systems that have no value, so they can be disposed of as trash or recycled for some new use.

Citation

Meeting: P. Yang et al. How do wombats make cubed poo? American Physical Society Division of Fluid Dynamics meeting, Atlanta, November 18, 2018.