The Thing About Koalas

Koalas have a few noteworthy characteristics.

They’re fuzzy and cute, for one thing! The indigenous people of Australia have created stories and lore about koalas for tens of thousands of years, and they’ve become something of a symbol of Australia today.

Possibly adding to the cuteness, koalas are marsupials, meaning they carry their infants around in a little pouch. Put a pin in this for now; we’ll return to this detail in a bit.

They’re also mainly active at night, but “active” is relative: koalas prefer to sleep for most of the day, and even when they’re moving around, they’re not exactly going very fast. In fact, their diet consists of only one item: the leaf of the eucalyptus tree.

That’s because koalas face little competition in acquiring these leaves. Eucalyptus is lower in nutritional value than a lot of the other plants in the area, but it’s higher in fiber, so generally much tougher for an animal to digest.

Even worse, eucalyptus is poisonous to nearly all animals. Specifically, there are strong psychoactive compounds called terpenes and phloroglucinols that can affect an animal’s central nervous system.

If you’ve seen koalas lazing about while eating eucalyptus leaves, that’s because they’re sedated while they’re eating them. Maybe it’s no wonder they sleep for 20 hours a day, given that they’re quickly doped up during their waking (mostly nighttime) hours.

So, while other animals are deterred from eating eucalyptus, this leaves the big score to the koalas.

How do they do it, though? Why can koalas eat eucalyptus, while other animals would suffer nausea and vomiting, seizures, and long term liver damage, among other horrible symptoms? In other words, this would be a very, very bad trip for most other animals.

Not koalas, though. They’ve adapted to suit their incredible laziness, or maybe their laziness has come about simply because it’s possible. Either way, their digestive tract has become specialized to deal with the toxins in eucalyptus, and there’s even a special organ they use for this purpose called a cecum. This is located at the juncture of the small and large intestine, and most mammals have a vestigial one.

The koala’s cecum is a very purposeful pouch containing its own little world of microorganisms specifically designed to break down the eucalyptus cellulose through fermentation. While food might pass through you or me in about 24 hours, the koala might need as much as 100 hours to digest those tough leaves.

These bacteria, protozoa, and fungi need time to work their magic. Specialized bacteria start by producing an enzyme that breaks down the cellulose into a simpler type of sugar. That sugar is then fermented by those other microbes, creating a byproduct known as volatile fatty acids, or VFAs for short. VFAs are where koalas get most of their energy.

Meanwhile, other microbes help detoxify the psychoactive elements in the eucalyptus leaves, although it’s clear from koala behavior that this only limits how much of the toxin enters their bloodstream—it certainly does not eliminate it!

How do all those microbes get in there?

Koalas, like all marsupials, emerge from their mother’s womb as something called a neonate. Before they become the cute joeys (baby koalas) we all know and love, koalas start their lives without eyes, ears, or fur, and they’re just about 2 centimeters long (shorter than an inch). To keep them safe, these little pups live in their mother’s pouch for six or seven months, which seems appropriate given their one month gestation period.

While grownup koalas like to spend most of their waking time eating, so do the joeys…. only they can’t eat leaves. They can barely even chew, must less digest the complex and tough cellulose of their primary food source.

Naturally, their mothers come to the rescue, and milk is the primary food source for the first few months. However, in order to prepare the neonates for the harsh world outside, koala mothers change gears and start supplementing the diet of their joeys around six months in.

To do so, they take care of the toughest parts of digestion by first eating the leaves, then pooping them out. This not only aids in the digestion for the neonates, but it also implants these all-important microbes into their young guts, where they will grow and prosper.

Biologists call this practice coprophagy, which might win some kind of prize for being the most literal new word based on ancient Greek: copro- means poop, and phage means eat.

That’s right! You’ve stepped in it again today, and I’ve written yet another article about poop.

Koalas aren’t the only mammals with notable excrement, and you might well enjoy reading about why wombats should have poop that’s shaped like cubes next:

We’ve visited the place where many ancient Romans pooped, learned how pigeon poop helped us prove that the universe was expanding, and I’ve opined about how and why poop can be incredibly valuable.

We’ve also looked at the very human side of dookie. There was the Great Stink of 1858, when the smell of feces in the river became too much for Parliament. There’s a nearly-unmentionable incident from the 12th century that reminds us to be cautious when building structures. And, on a more somber note, I asked whether toilets or transistors would most help the developing world.

I don’t specifically seek out scatological concepts to share with you, but I also don’t shy away from the sorts of spots that might repulse another curious writer. I’ve learned that if something bothers or disgusts me, I want to understand why, and in that pursuit, I’ve found a lot of unturned stones, some of which have interesting excrement underneath them.