Anatomy of the Koala
The koala is classified with wombats, in the suborder Vombatiformes, which is the sister group to the clade containing macropods (kangaroo, wallabies) and possums (1). There are three different subspecies of Koala distinguished by their pelage colour, body size, and skull shape: Queensland koala, New South Wales koala, and Victorian koala. The smallest koala of these three species is the Queensland variety, and the largest is the Victorian koala (2). Studies have found that there is low genetic diversity, suggesting limited gene flow due to environmental barriers such as rivers and roads. More studies have found that populations of koala present quite significant levels of inbreeding (3).
This stocky animal boasts a rather broad head and non-distinguishable, vestigial tail. The koala is between 60-85cm long and weighing anywhere from 9 to over 30 pounds. There is sexual dimorphism such that males are usually 50% larger than the female, and also have more curved noses and hairless patches on their chest indicating the location of the chest glands. The male’s penis is bifurcated such that it is separated into two columns which interact with the female’s two lateral vaginas and uteri. The penis is also separate from the urinary system (4). The male’s prepuce, where they store the penis when it is not in use, also hosts a bacterial flora which researchers postulate has an important role in the preparation of semen and its storage (6).
The thick and long pelage of the koala grows shorter towards the ventral belly, and varies between light grey and dark brown. The koala is the most effective insulator of any marsupial and can resist wind, rain, and reflect solar radiation with its white belly fur (7). The koala also has sharp claws adapted for tree climbing, and opposable digits that permit them to grab branches. They also have long arms and friction ridges on their paws (8), combined with a muscular and short upper torso.
Interestingly, the koala’s brain is incredibly small in proportion to its body weight, one of the smallest of any mammal with regards to this ratio. The small brain is posited to be an adaptation that balances the energy restrictions imposed from its strict diet, and thus the animal cannot carry out complex behaviours and lives quite simply (4).
Also unusual as a marsupial, the koala has quite poorly-developed vision with vertical slits. It also possesses a vocal organ that emits low-pitched sounds, and these vocal cords are located in the soft palate (8).
Physiological Adaptations for Diet
Physiological adaptations in relation to its unique diet are as follows. The leaf is first grasped by the incisors, snipped by the premolar and shredded by use of molars and cheek teeth. The koala also has the option to store leaves in the cheek pouch before chewing it (9). Since eucalyptus is of low nutritional value, high toxicity and high dietary fibre (4), then the koala can regurgitate its food. As a hindgut fermenter the digestive retention of the koala may be up to 100 hours in the wild, and twice as long in captivity. This is due mainly to its 200 cm long cecum, the largest proportionally of any known animal’s. The large cecum allows koalas to select which particles of food they wish to retain for a longer fermentation period, typically allowing large particles to pass through faster than smaller particles. Only about 10% of the animal's energetics is derived from the fermentation of its food (10).