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Koalas are pretty darn cute, but...they aren’t the brightest bulb on the continent. They will only eat eucalyptus leaves off the branch. On a plate, a koala won’t recognize their one and only food. Yet, somehow, koalas are still alive. And it's all thanks to some pretty clever adaptations.

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Sources:

Brains:
https://www.researchgate.net/profile/Maciej-Henneberg/publication/233726395_1998_koala_brain/links/09e4150acb6041e673000000/1998-koala-brain.pdf
https://www.sciencedirect.com/science/article/pii/S0944200620301045
https://www.researchgate.net/publication/228885254_MR_imaging_of_brain_morphology_vascularisation_and_encephalization_in_the_koala
https://www.sciencedirect.com/science/article/pii/S0166223613000180
https://books.google.co.nz/books?id=KqtlPZJ9y8EC&pg=PA234&redir_esc=y#v=onepage&q&f=false

Food:
https://www.researchgate.net/publication/251779177_A_review_of_feeding_and_diet_selection_in_koalas_Phascolarctos_Cinereus
https://research.monash.edu/en/publications/digestion-and-nutrition-in-the-koala-a-review
https://link.springer.com/article/10.1007/BF00689622
https://www.sciencemag.org/news/2018/07/koala-s-diet-would-kill-most-mammals-their-genome-reveals-how-they-survive
https://www.publish.csiro.au/WR/WR00111

Disease:
https://www.sciencedirect.com/science/article/pii/S0378113513001405#bib0205
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6472425/
https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1751-0813.1987.tb06064.x?sid=nlm%3Apubmed
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0236758

Other:
https://www.environment.gov.au/system/files/pages/a117ced5-9a94-4586-afdb-1f333618e1e3/files/31-ind.pdf

Images:
https://www.storyblocks.com/video/stock/koala-awake-on-a-tree-hg8hhasrejwqcq3n7
https://www.istockphoto.com/photo/lone-koala-hanging-on-the-branches-of-a-tree-gm172129804-2354773
https://www.istockphoto.com/photo/organic-dried-eucalyptus-leaves-text-space-gm1170675305-324031293
https://www.storyblocks.com/video/stock/brain-1007-a-human-brain-rotates-on-a-grid-background-loop-shrpmbxyrwj9ivo8e1
https://www.storyblocks.com/video/stock/koala-eating-eucalyptus-leaves-marsupial-native-to-australia-hkk62n30ej1wywyws
https://www.storyblocks.com/video/stock/koala-eating-eucalyptus-leaves-marsupial-native-to-australia-hkk62n30ej1wywyws
https://www.istockphoto.com/photo/koala-baby-gm1272019097-374424410
https://www.istockphoto.com/photo/close-of-a-koala-yawning-gm1089852816-292371790
https://www.istockphoto.com/photo/karri-forest-western-australia-gm180883608-27065633
https://www.storyblocks.com/video/stock/a-koala-mother-carries-her-infant-on-her-back-in-a-eucalyptus-tree-swn75ciliqcct5ji
https://www.storyblocks.com/video/stock/ws-cows-and-sheep-grazing-in-countryside--cornwall-england-uk-b810kchqeiwmvlx3a
https://www.istockphoto.com/photo/koala-gm499656300-80357259
https://www.storyblocks.com/video/stock/koala-native-australian-marsupial-sleeping-in-tree-r6nuexjy-j203n77a
[♪ INTRO].

Koalas are pretty darn cute, but...they aren’t the brightest bulb on the continent. Like, if you take eucalyptus leaves from a branch and plop them on a plate in front of them, they won’t eat them.

Not to mention the fact that those leaves are poisonous. And yet, koalas are somehow still alive. Thanks to some pretty clever adaptations.

We all know that koalas are picky eaters:. They mostly eat eucalyptus leaves. But it turns out that they will also only eat leaves off the branch.

On a plate or in a bowl, a koala won’t recognize their one and only food. And it turns out that Koalas' inability to recognize leaves without a branch might have to do with how their brains are organized. If you looked at an image of a koala brain, you’d probably notice how smooth it looks compared to the wrinkly, pink mass we’re used to seeing.

There’s a term for this: gyrification, or how folded a brain is. Generally speaking, those folds mean a bigger cerebral cortex, the part of the brain that associates things like leaves on a branch versus a plate. Plus, having a brain with more folds means that brain regions are close together, making it quicker to send signals back and forth.

More brain cells, with more connections and speedier communication, add up to a smarter animal. Which leaves the smooth-brained koala fresh outta luck because their sleek brain probably means they have fewer neurons in the cortex. The thing is though, koalas might not need to be all that smart.

Big, hard-working brains use a lot of energy, something that koalas don’t have much of because of their nutritionally poor diet. So they’re better off sitting in a tree with their brains kind of on autopilot, eating the leaves dangling in front of them. But regardless of how you dish it up, plate or tree, the koala’s diet really sucks.

Not only are eucalyptus leaves low in nutrients, they’re also toxic to most mammals. Those toxins include high levels of cyanogenic glycosides:. Compounds that give off cyanide, a deadly chemical that causes cell death.

But, turns out, once again these smooth-brained little cuties have evolved a couple of clever ways of dealing with this. As babies, they’re fed poop, or pap, from their parents to fill their intestines with bacteria that help them digest the toxic leaves. Yum.

To extract what nutrients there are in eucalyptus, koalas have flat and wide molar teeth that grind and slide against one another to break down the leaves. The chewed bits then make their way to their extra-long caecum, a fiber pulverizing pouch that sits between the small and large intestine. Then it’s on to the hindgut, where the poop bacteria spring into action to break down some of the toxins from the leaves.

Some toxins also enter the koala’s bloodstream, so detoxifying enzymes in the liver can eliminate them. It might sound easier to eat something, you know, not toxic and actually nutritious. But that’s the thing: Koalas live in the eucalyptus forests of eastern and southeastern Australia where there isn’t much else.

In those forests, dry climate and history of erosion mean its soils are low nutrition too, so plants that grow there have to be hardy. And koalas have adapted to make the best of what they’ve got. Plus, nobody else is exactly excited about eating eucalyptus, so there’s not much competition for it.

But the glamorous life of a koala doesn’t stop with having to eat poop in order to not be poisoned by their food. They somehow make time in their busy schedules to pass chlamydia onto one another. It’s caused by the bacteria Chlamydia pecorum or Chlamydia pneumoniae, and it’s estimated that virtually all of Australia’s koala populations are infected with this bacterial disease.

Although koalas on some remote islands seem to have dodged it. And the koala version of chlamydia affects the animals’ ability to breed, see, urinate, and even breathe. Basically anything you need to stay alive!

But unlike their unwrinkled brains and digestive prowess, chlamydia isn’t something they’ve developed because of their unique environment. Scientists think that the disease was passed on from sheep and cattle when they were introduced to the country. The reason koalas are still clinging to life now is through treatment and vaccination efforts against koala chlamydia.

But it’s not easy. For example, when researchers treated koala chlamydia the same way as human chlamydia, they disrupted the koala’s gut microbiome, which koalas need to digest that tricky food of theirs. Biologists have had some success with one type of antibiotic, doxycycline, which according to a 2020 study, cured 97% of animals when it was injected.

They’re also developing vaccines to stop the animals’ infections from getting worse. But there’s still a lot of work to be done before those vaccines stop the disease from spreading in the wild in the first place. So while koalas have done pretty well at keeping themselves alive thanks to some clever adaptations, they haven’t evolved to fight off viral infections from farm animals we introduced.

So scientists continue to work at protecting these adorable, awesome animals. Thanks for watching this episode of SciShow! We’ve made thousands of educational videos over the years, and we’ve been able to offer them for free because of our patrons on Patreon.

So, to all our patrons, thank you for what you do to make SciShow happen. If you’re not a patron but want to learn more about what that means, you can go to Patreon.com/SciShow. [♪ OUTRO].