The pelage of the koala is denser on the back. Back fur colour varies from light grey to chocolate brown. The belly fur is whitish; on the rump it is mottled whitish and dark. The koala has the most effective insulating back fur of any marsupial and is resilient to wind and rain, while the belly fur can reflect solar radiation. The koala has curved, sharp claws well adapted for climbing trees. The large forepaws have two opposable digits (the first and second, which are opposable to the other three) that allow them to grip small branches. On the hind paws, the second and third digits are fused, a typical condition for members of the Diprotodontia, and the attached claws (which are still separate) function like a comb. The animal has a robust skeleton and a short, muscular upper body with relatively long upper limbs that contribute to its ability to climb. The thigh muscles are anchored further down the shinbone, increasing its climbing power. thumb|upright|left|Mounted skeleton For a mammal, the koala has a disproportionately small brain, 60% smaller than that of a typical diprotodont, weighing only on average. The brain's surface is fairly smooth and "primitive". It does not entirely fill the cranial cavity, unlike most mammals, and is lightened by large amounts of cerebrospinal fluid. It is possible that the fluid protects the brain should the animal fall from a tree. The koala's small brain may be an adaptation to the energy restrictions imposed by its diet, which is insufficient to sustain a larger brain. Its small brain limits its ability to perform complex behaviours. For example, it will not eat plucked eucalyptus leaves on a flat surface, which does not match its feeding routine.
The pelage of the koala is denser on the back. Back fur colour varies from light grey to chocolate brown. The belly fur is whitish; on the rump it is mottled whitish and dark. The koala has the most effective insulating back fur of any marsupial and is resilient to wind and rain, while the belly fur can reflect solar radiation. The koala has curved, sharp claws well adapted for climbing trees. The large forepaws have two opposable digits (the first and second which are opposable to the other three) that allow them to grip small branches. On the hind paws, the second and third digits are fused, a typical condition for members of the Diprotodontia, and the attached claws (which are still separate) function like a comb. The animal has a robust skeleton and a short, muscular upper body with relatively long upper limbs that contribute to its ability to climb. The thigh muscles are anchored further down the shinbone, increasing its climbing power. thumb|upright|left|Mounted skeleton For a mammal, the koala has a disproportionately small brain, 60% smaller than that of a typical diprotodont, weighing only on average. The brain's surface is fairly smooth and "primitive". It does not entirely fill the cranial cavity, unlike most mammals, and is lightened by large amounts of cerebrospinal fluid. It is possible that the fluid protects the brain should the animal fall from a tree. The koala's small brain may be an adaptation to the energy restrictions imposed by its diet, which is insufficient to sustain a larger brain. Its small brain limits its ability to perform complex behaviours. For example, it will not eat plucked eucalyptus leaves on a flat surface, which does not match its feeding routine.