I just learned how the koala is one of the dumbest mammals
The only thing it eats is the eucalyptus leaf, which is tough to chew, has little nutritional value, and IS POISONOUS
For some dumb reason the koala's evolutionary ancestors, instead of deciding to eat something else, decided to evolve a stomach that slowly ferments the leaves into something it can digest.
It's young, however, can't do that so instead the adult koala poops out partially digested food and feeds that to it's young,
Most herbivore marsupials have evolved ways to deal with teeth eventually wearing away. But the koala has not. When it's teeth wear down it just starves to death.
Also, It has the smallest brain to body ratio of all mammals. If it doesn't pick the leaves from the tree itself, it doesn't know what to do with them, for example