Koala brains are unusually small but I think scientists might be wrong on the cause. The general thinking is that koalas have such a low-energy diet that they must reduce brain size to conserve energy (the brain is an energy hog). This is definitely the case with lemming brains but I think in lemmings it is not the brain that got smaller but rather the stomach area and spine length that got bigger to support a slower digestive system. The unique thing about koala brains (other then being smaller then normal for body size) is the over-sized brain case is filled with fluid to fill in the gap. I have food allergies with fish being so bad it swells shut my eyes and wind pipe. The one time I tried a tiny piece of fish due to it not bothering me as much as normal just being in the same room... I had vision anomalies like those experienced by severe migraines. Later when trying to sleep off the painful swelling, I had fever-like dreams. I now recognize that these things are caused by pressure on the brain from the brain swelling and not having anywhere to expand to. With koala diet being rather poisonous leaves of several tree species, I would expect the trees to have more then just poisons to keep their leaves off the diets of animals. Using allergen-rich body parts (namely the blood) is the method of stripped grass mice to keep from being eaten (most animals will have their mouth swell up if they eat one). The eucalyptus trees might also employ such a strategy and the koalas are prepared for a bit of brain swelling without negative effects. The other idea is that koalas live in trees and have rather short limbs so the chance of hitting their heads during a fall is high. The cranial cavity can handle a bit more abuse without bruising brain tissues.