The koala (Phascolarctos cinereus) is an easily recognizable Australian animal, but did you know it has something odd going on inside its head? Bizarrely, its brain is highly reduced and only occupies about 60% of the cranial cavity, with the rest of the space being cushioned by cerebrospinal fluid. (Image credit.)
The exact reason for this is still a mystery, although there are a couple of possible explanations. The fluid might act as a natural shock absorber to protect the brain if the koala falls out of a tree, or it could be an adaptation to their extremely low-energy foliage diet. Brain tissue is very resource-intensive, using a large amount of energy in proportion to its volume — and the koala’s evolution may have dealt with this in a rather novel way.