The koala has one of the smallest brains

The koala has one of the smallest brains in proportion to body weight of any mammal,[40] being 60% smaller than that of a typical diprotodont, weighing only 19.2 g (0.68 oz) on average.[41] The brain’s surface is fairly smooth, typical for a “primitive” animal.[42] It occupies onl

y 61% of the cranial cavity[40] and is pressed against the inside surface by cerebrospinal fluid. The function of this relatively large amount of fluid is not known, although one possibility is that it acts as a shock absorber, cushioning the brain if the animal falls from a tree.

[42] The koala’s small brain size may be an adaptation to the energy restrictions imposed by its diet, which is insufficient to sustain a larger brain.[40] Because of its small brain, the koala has a limited ability to perform complex, unfamiliar behaviours. For example, when presented with plucked leaves on a flat surface, the animal cannot adapt to the change in its normal feeding rou

tine and will not eat the leaves.[43] The koala’s olfactory senses are normal, and it is known to sniff the oils of individual branchlets to assess their edibility.[40] Its nose is fairly large and covered in leathery skin. Its round ears provide it with good hearing,[35] and it

has a well-developed middle ear.[15] A koala’s vision is not well developed,[35] and its relatively small eyes are unusual among marsupials in that the pupils have vertical slits.[31] Koalas make use of a novel vocal organ to produce low-pitched sounds (see social spacing, below). Unlike typical mammalian vocal cords, which are folds in the larynx, these organs are placed in the velum (soft palate) and are called velar vocal cords.[44]