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This might not seem to bad, where people might say, “Oh, well, I’m sure there are other eucalyptus trees, so surely the koala can figure out how to find a new one.” No, unfortunately, they can’t, or at least they’re not very good at that sort of thinking. You see, human brains are shaped the way they are for a reason: those ridges and bumps let us make cognitive decisions and give us the ability to figure out jigsaw puzzles and let us know that a certain leaf comes from a certain tree. It let’s us figure out how to get from where we are to where we want to go. Koalas, on the other hand, have smooth brains: they can’t figure stuff out on their own. If you took a bunch of eucalyptus leaves and gave them to a koala, the koala wouldn’t be able to figure out that they were eucalyptus leaves.
They have a smaller brain which means they have limited intellectual scope to understand (Source & Source for above)