Biology

Koalas are arboreal (tree-dwelling) marsupials, with a distribution over most of the eastern part of Australia, although the populations are largely fragmented. They are found in sclerophyll forests and woodland areas ranging from the Eyre Peninsula and Kangaroo Island in South Australia, to the southern-most parts of Victoria (including populations on French, Snake and Raymond Islands) up to the southern edge of the Atherton tablelands in Northern Queensland.

 

Diet

Koalas are folivores, feeding on a diet that is nearly entirely made up of leaves from the Eucalyptus genus, although they will occasionally eat non-Eucalypt species such as Brush Box (Lophostemon confertus), Allocasuarina sp. and Melaleuca sp. Despite there being over eight hundred species of Eucalyptus in Australia, koalas as a species eat only around 40 to 50 of those. In most areas, koalas will have access to around seven species of eucalyptus. Soil type and seasons have a major impact on whether leaf in a certain area is palatable, so whilst they may favour a species of eucalypt from one area, they may not eat it if it comes from a different area.  There are also some species of eucalypt that koalas favour only in a specific season.

Some of the main browse species in South-east Queensland include:

  • Blue Gum (Forest Red Gum) (Eucalyptus tereticornis)
  • Tallowwood (E. microcorys)
  • Grey Gum (E. propinqua, E. punctata and E. major)
  • River Red Gum (E. camuldulensis)
  • Swamp mahogany (E. robusta)

Secondary browse species include:

  • Spotted Gum (E. maculata)
  • Gum-top Box (E. molucana) 
  • Red stringybark (E. resinifera)
  • Narrow-leafed Ironbark (E. crebra)
  • Narrow-leafed Redgum (E. seana)
  • Flooded gum (E. grandis)
  • Poplar Box (E. populnea)
  • Scribbly Gum (E. signata)
  • Sydney Blue Gum (E. saligna)

 

Home Range

Koalas are generally solitary animals and live in relatively well-defined home ranges. In breeding aggregations, these home ranges border or partially overlap adjacent home ranges of other individuals. Home ranges will most often consist of an alpha male whose home range encompasses a number of females’ home ranges. Each female will have her defined area within this range, with most overlapping that of another female. Often a non-dominant or younger male’s home range will slightly overlap one corner or section of the alpha male’s territory.

The size of a koala’s home range is highly variable and depends upon factors such as the koala’s age and sex, forest type, soil quality, reproductive status, area available and the presence of other koalas.   In more arid habitat that has less dense forest areas or woodland, the home ranges tend to be larger than those of koalas living in coastal Eucalypt woodlands.

Koalas in some areas can have a very small home range if the koala density in the area is heavy, particularly with dominant males. Fences, roads, abundance and access to food trees also play a part in the size of a koala’s home range.

Koalas tend to be faithful to their home range and will attempt to return if moved elsewhere (relocated).  The relocation of mature individuals that are healthy and who are coping well within their home range is discouraged unless there has been a recent and significant loss of habitat.   In Queensland, legislation prohibits the relocation of koalas so when it is considered the only option available, several key factors need to be taken into consideration and permission granted by the Department of Environment and Heritage Protection (EHP).

Reproduction and Development

The koala breeding season in South-east Queensland extends from around August through to January with most young born over the summer months.

Usually a single joey (twins are rare) is born after a 34 to 36-day gestation. The jelly-bean sized joey makes its way through its mother’s fur from the birth canal to the pouch, where it immediately attaches itself to one of the mother’s two teats. The joey will remain in the pouch, drinking its mothers milk from that same teat, until it is approximately 5.5 to 6 months of age. At this point the joey will become more adventurous and start to climb onto its mother’s belly.  The joey will fully emerge from the pouch at around 6 to 6.5 months of age. Once the joey reaches 8 to 8.5 months of age it will be too big to fit in the pouch and will become what is termed a ‘back young’ until it is around 12 months of age. Koalas are usually excellent mothers and are highly tolerant of their energetic young. Complete independence from the mother occurs between 12 to 18 months of age.

Sadly, the highest incidence of mortality in koala joeys occurs during the pouch emergence stage.

Between 18 months to 2 years of age juveniles generally move away from their maternal home range to establish their own territory.  Females will often choose to set up a home range that will overlap their mother’s or will settle not too far away.

During this dispersal time, juveniles may travel a great distance and are at a much higher risk of being attacked by a dog, hit by a car, or other misadventure.

Sexual maturity is from around 2 years onwards.

Koalas have few natural predators other than man but young can fall victim to birds of prey, pythons and dogs, both wild and domestic.  Adult koalas mostly come to grief because of motor vehicles, domestic dogs and disease.

The male koala has a prominent sternal gland (on the chest), which results in the fur in this area being stained an orange/brown colour. This gland becomes noticeable as the male gets older and is increasingly prominent in breeding season when both the gland and the emulating odour are present.

Male koalas emit a very loud bellow vocalization, which can sound quite eerie if you happen to be out in the dark. Young males, however, sound like a squeaky frog as they develop their bellowing skills.  Koalas have a variety of vocalizations to indicate a variety of emotions and situations.

 

Natural History

The Queensland female koala has a weight range of between 4.1kg and 7.3kg with an average of 5.1kg. Although the occasional 8kg to 9kg individual has been recorded, female koalas of this size are not common.

The Queensland male koala has a weight range of between 4.2kg and 9.1kg with an average of 6.5kg. Koalas in southern New South Wales and Victoria have a much heavier average weight than Queensland koalas with Victorian koalas being the heaviest of all the states.

The dorsal (back) fur of the koala is the most insulated of any marsupial found to date, and in the past, was heavily sought after for the fur industry. The ventral (belly) surface has half the density of the dorsal surface, and is more reflective of solar radiation. In cold weather, koalas will curl into a ball with the dorsal fur providing effective insulation from the cold and rain. In warmer months, they will often sit sprawled out with their ventral surface exposed to the sun, which has a cooling effect. Panting is the main means of evaporative cooling in high temperatures. Although some sweat glands exist on the hand and foot pads this is not an outlet for overheating. In extremely hot conditions, koalas have been known to seek refuge in caves and fallen tree hollows.

Capture and Handling

Koalas have strong, razor-sharp claws that can cause severe injuries.  They can also bite tremendously hard. Although they may appear docile, they are capable of lashing out very quickly when threatened. Even very sick koalas can react aggressively when handled. We do not recommend that anyone attempt to capture or handle a koala unless they have been specially trained to so do.

If you do find a sick or injured koala on the ground, we recommend that you secure the koala by placing an overturned laundry basket or large box over the koala to prevent it from wandering off or climbing a tree.  This makes it quicker and easier for a wildlife volunteer to collect the koala.  Never encourage a sick or injured koala to climb a tree as this makes it much harder for us to retrieve it.  Once captured, sick or injured koalas must be housed and transported in a large, secure wire mesh cage.

Reports of sick and injured koalas can be made to Wildcare or to your local wildlife care organisation so that a specially trained volunteer can assess and capture the koala if required.

 

How can I tell if a koala is sick or injured?

If you live in an area where you frequently see koalas, it makes good practice to keep an eye out for any signs of injury or disease. The sooner a sick koala receives veterinary treatment, the better its prognosis.

The following table provides a guide of the differences between a healthy and sick koala.

Healthy Koala Sick or Injured Koals

Coat

  • Should be dense, grey and uniform
  • Female koalas with back young or recently weaned young may show patchy fur loss and dark grey or brown patches around the shoulder region.

Coat

  • Brown, sparse, coarse or tufted in appearance
  • Hair loss
  • Scaly, encrusted skin
  • Any signs of wet fur which might indicate saliva from a dog

Body Condition

  • should be well nourished and slightly pot-bellied
  • pelvic bones and spine should not be discernible
  • body condition can hard to assess distantly

Body Condition

  • hollowness between the ribcage and pelvis
  • pelvic bones and spin easily discernible

Demeanour

  • Bright, alert and responsive to disturbance
  • Ears should become erect when disturbed

Demeanour

  • Unresponsive
  • Able to approach closely

Eyes

  • Should be clear, bright and free of discharge
  • Should have no fur loss around the eyes

Eyes

  • Crust formed over eyes
  • Inflammation around the eye
  • Pus forming in the eye

Vent and Rump

  • Bottom should be clean, white, dry and free of dirt or dark brown stains

Vent and Rump

  • Wet bottom
  • Dark stained bottom (brown colour)

Position

  • Healthy koalas should be up a tree during the day.
  • If on the ground, they should move away when approached.

Position

  • Sleeping on the ground
  • Sitting at the base of a tree for a period of time
  • Does not move away when approached

Mouth and Nose

  • Should be clean and free of discharges or drooling

Mouth and Nose

  • Discharge
  • Drooling

Breathing

  • Breathing should be barely discernable

Breathing

  • Open mouth breathing
  • Panting
  • Labored or noisy breathing

Koalas, like most wild animals, mask their signs of illness well. If you are in doubt about whether a koala is sick, check up on it a number of times over a day or so. Note however that some koalas will return to the same tree to sleep each day so checking during the night is also important.

Healthy koalas tend to move around within an area from night to night, whereas sick koalas may stay in the same tree for prolonged periods.

Some koalas will look extremely healthy but may be very ill. The onset of some diseases or injuries from accidents such as car hits and dog attacks can be sudden.

 

Common Diseases and Injuries of Koalas

Unfortunately koalas are prone to a number of diseases and injuries.

Some diseases of wild koalas are easily recognizable while others are only be detectable through a thorough veterinary assessment by an experienced koala veterinarian.

Koalas mask their illnesses very well – sick koalas are very sick koalas.

CHLAMYDIOSIS
A serious, debilitating and sometimes fatal disease caused by two different strains of the bacterium Chlamydia. It commonly causes ocular and urogenital disease manifested by keratoconjunctivitis (pink-eye), infertility and urine staining of the rump caused by cystitis (dirty tail).

Chlamydial disease is often complicated by secondary infections with other bacteria and fungi and is exacerbated by a number of factors including chronic stress, poor nutrition and immunosuppressive diseases.

Spread of the disease is almost certainly by the venereal route, and possibly also by close contact and flies.

Koalas suffering from Chlamydia have a good chance of recovery if they are treated in the early stages of the disease.

STARVATION/METABOLIC FAILURE
Manifested by weight loss, depression, dehydration and sometimes, concurrent disease. This condition is common in koalas, where the loss of habitat is having a tremendous impact on their ability to survive.

BONE MARROW DISORDER, CANCER AND IMMUNOSUPPRESSION
There is good evidence for the existence of a pathogenic retrovirus that infects koalas. It may cause bone marrow suppression leading to anaemia and immunosuppression, lymphoid neoplasia and other cancers. It may also cause an AIDS-like syndrome. With the exception of lymphoma, which causes enlarged lymph nodes, most of these diseases can only be detected by careful interpretation of blood tests by an experienced wildlife veterinarian.

TRAUMA
Mostly caused by road trauma and dog attacks, but can also be from tree felling, cattle injuries or misadventure. All koalas that are suspected of trauma MUST come into care and receive immediate veterinary attention, which includes appropriate pain medication.

Often the injuries associated with such trauma are not clearly visible. Even if the koala has managed to climb a tree, these animals must be reported promptly so that they can be safely retrieved and provided with urgent veterinary attention.

Trauma from car hits often results in fractures and internal bleeding.

Trauma from dog attacks can result in severe damage to muscle and underlying tissues, this type of damage can often go undiagnosed as it can appear superficial on the surface. Aggressive treatment is required in these circumstances, antibiotics and fluid therapy is required as soon as possible to help prevent the risk of infection. Even if a dog has only “nipped” a koala, it still requires immediate veterinary attention.

Rescuing and Caring for Koalas

Koalas are classified as a ‘specialised species’ under the Nature Conservation Act 1999 and as such, you need to hold a special Rehabilitation Permit issued by EHP to care for sick, injured or orphaned koalas.

Rescuing koalas requires the use of specialised rescue equipment including flagging poles, nets, koala traps and large transport cages.

If you are interested in learning how to rescue koalas, enrol to complete a koala workshop through WILDCARE.

Caring for injured and orphaned koalas is very time consuming and is generally undertaken by wildlife rehabilitators that have several years’ experience with caring for wildlife.

For more information on koalas, check out the following brochures and information sheets:-