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General Characteristics: Koalas are relatively small to medium size marsupials. Several theories relating to the adaptive size of koalas have been proposed by researchers. First, they suggest that because koalas rely on fermentation of eucalyptus leaves to obtain energy, their body size must remain relatively small in order to increase the relative volume of fermentation chambers with respect to the rest of their body. This ensures that koalas are able to meet their energy requirements. Secondly, because koalas are arboreal, there must be a consideration for the energetic costs required to shift a body against the forces of gravity. If koalas grew too large, it would increase the necessary metabolism and energy required for climbing. Under the influence of these considerations, koalas have reached a relatively optimal size for a herbivorous, arboreal mammal (Degabriele and Dawson, 1979).

Several different factors have had influencial pressure on the morphology of koalas. In particular, the koala's poorly balanced diet and its arboreal lifestyle have played key roles in the evolution of its specialized morphological characteristics. The koala's arboreal lifestyle, low metabolic rate, and low energy requirements can all be partially attributed to their nutrient poor diet of eucalyptus leaves. Koalas evolved with gum trees as their primary source of nourishment, and their bodies began to adapt and specialize to cope with the diet (Australian Koala Foundation). As this happened, evolution selected for koalas to remain in the tree tops where they didn't have to expend large amounts of energy trying to escape from predators on the ground. By being arboreal, koalas can afford to have low metabolic rates and low energy requirements. As a result, the morphology of the koala is very reflective of these different evolutionary pressures. The general body shape of the koala, its paws, and its fur have all become highly specialized to support "lazy" life in the trees.

A study performed by Degabriele and Dawson examined the pelt of the koala in comparison to other marsupials. Their results were very interesting and help explain how the koala prevents possible thermoregulatory problems that could arise from their high exposure and low metabolic rates. They found that the insulation of the koala's fur was higher than that of any other marsupial. By closely examining the fur, they found that the dorsal surface of the koala was twice as dense as the ventral surface and much less reflective of solar radiation. The ventral body surface is noteably lighter in colour and reflects 52.3% of solar radiation while the darker dorsal, or back, surface was found to have 38.3% reflectance. They also found that the wind had little influence of the on insulation ofthe koala's fur. At wind speeds up to 4 m/s, the insulation of the dorsal fur only decreased by 14.2%. The density of the fur was variable across different sections of the body, but the more thinly furred surfaces occupied a much smaller proportion of the total body area. By changing their body posture, as in the picture above, koalas can protect or expose the thinly furred surfaces of their body to help achieve ideal temperatures. It was also discovered that koalas maintain slightly depressed body temperatures around 35.7°C while most mammals regulate their temperature around 37.0°C. This makes it a little easier for koalas to maintain constant temperatures, especially with a low metabolic rate. Different postures can drammatically enhance or reduce the surface area of the koala exposing desired areas of the body. When they ball up to keep warm, koalas reduce their surface area and expose their densely furred, poorly-reflective, and wind resistanct dorsal regions. To cool off, they allow their arms and legs to dangle and increase the exposure of more thinly furred surfaces. Panting is also a means of cooling for koalas (1979). So, koalas rely heavily on their fur to protect them in various types of environmental conditions. The koala's fur is highly adapt and specialized to prevent excessive heat loss or gain and support its low energy, arboreal lifestyle.

Fur: Phascolarctos cinereus is one of the few arboreal species that does not utilize at least some form of shelter for protection from the elements (Australian Koala Hospital Foundation). Because of the koala's poor diet, low metabolism, and low energy requirements, building a nest would be a time consuming and difficult task. Although the koala doesn't use shelter, it manages to be widely dispersed across the varying range of environmental conditions that characterize the Eastern coast of Austrialia. Life in the exposed tree tops could potentially present the koala with a number of thermoregulatory problems, but the fur of the koala has become highly specialized and efficient at supporting this highly exposed, low energy, arboreal lifestyle.

Paws: Due to their arboreal lifestyle, koalas must be excellent climbers. The paws of the koala have adapted and specialized to facilitate locomotion through the trees. Koalas have two digits on on their front paws that function as thumbs and are opposable to the other three digits. This adaptation allows them to maintain an excellent grip while climbing or eating. The hind paw has an opposable digit as well. The koala's paws are also equipped with long claws and rough pads that are well adapt for climbing (Sea World).

The size of a mature koalas is slightly variable, and males are typically larger than females. Full grown males range between 20 & 30 pounds while females generally range between 15 & 22 pounds. Males usually grow to be between 29 & 33 inches, and females grow to be between about 26 & 29 inches (Sea World). Koalas have stocky, pear-shaped bodies. Their wide base is advantageous for balance and wedging themselves into the forks of trees. Koalas also have a reduced tail and relatively long limbs which benefit in sitting and climbing (Australian Koala Hospital Foundation).