The Koala, one of Australia’s icons is the only mammal, other than the Greater Glider and the Ringtail Possum, which can survive on a diet of eucalyptus leaves (gum leaves). Koalas are persnickety eaters with solid preferences for different types of gum leaves and will only eat from a few of the over six hundred types of eucalypti in Australia. Within the koalas’ home browsing range, as few as one, and typically no more than three species of eucalypti will be eaten. On rare occasions, koalas eat from other native Australian trees such as wattle, tea tree or paper bark but mostly use the non-eucalypts for sitting or sleeping.
Different species of eucalyptus grow in different parts of Australia, so a koala in Victoria would have a varied diet from one in Queensland. An adult koala eats between half a kilogram and one kilogram of leaves each night, depending on its size, gender and location. Eucalyptus leaves are especially fibrous, low in nutrition and poisonous to most animals. The koala’s digestive system is equipped to detoxify the poisonous chemicals in the leaves. Perhaps this is why koalas will eat only certain types of eucalyptus leaves, and only when they are growing on certain soils because the trees which grow on less fertile soils seem to have more toxins that those growing on good soils. A forest has a carrying capacity, meaning the available gum trees can only feed a certain number of koalas, making habitat preservation exceedingly important to the survival of the species.
Koala teeth are especially adapted for their gum leaf diet. Their sharp front teeth nip the leaves from the tree and their back molars are shaped for cutting and grinding the leaves to extract the most nourishment. There is a gap between the incisors and the molars that allows the tongue to move the leaves around in the mouth efficiently. Koalas increase the digestibility of their food by regurgitating and masticating it a second time. The marsupial also possesses a specialized gland in its stomach wall that secretes enzymes to neutralize the toxic compounds in the plant leaves. The koala has the distinction of having the largest fiber digesting organ called a caecum, (two hundred centimeters in length), for its size of any mammal. A slow metabolic rate allows koalas to retain food in this specialized fermentation chamber for seven to twelve days. This time allows the koala to extract and absorb between twenty-five to fifty percent of the plant energy into their system. This slow metabolic rate minimizes energy requirements, and is why koalas sleep somewhere between eighteen and twenty-two hours each day.
Koalas get all the moisture that they need from the gum leaves. However, they can drink if necessary, such as in times of drought when the leaves may not contain adequate moisture.
Koalas use a range of sounds to communicate with one another over large distances. There is a deep grunting bellow which the male uses to signify its social and physical position. Males save fighting energy by bellowing their dominance and to allow other koala to accurately locate them.
Females do not bellow as often as males but their calls are also used to express aggression as well to attract a mate. Mothers and babies make soft clicking, squeaking, tender humming or murmuring sounds to one another, as well as mild grunts to show displeasure or annoyance. All koalas share one common call which is brought on by fear. This call released when a koala is under stress sounds like a human baby screaming, and is often accompanied by shaking.
Koalas live in complex social groups and are territorial. Individual members of koala society maintain their own home range. A home range consists of a number of trees that will provide the koala with food, shelter and places for social contact for its whole life if the habitat is preserved. A home range varies in size depending on the habitat quality which can be measured by the density of key food trees. Home range trees define the area of an individual koala’s territory similar to a surveyor’s pegs marking the extent of a property boundary. These boundaries are not visible to the human eye, but koalas can tell whether a tree belongs to another koala or not by scent markings and scratches left by the current occupant.
Within a socially stable group, the home ranges of individual koalas overlap with those of their neighbors. It is in the shared, overlapping trees that most of the koala’s social interaction takes place making these trees extremely important to the koala. Even after a koala has died, other koalas typically will not move into the empty home range for about a year, which is the time it takes for the scent marking and scratches of the deceased inhabitant to disappear through weathering and peeling of the bark.
All the home range trees and the food trees are important to the welfare of each individual koala and the group in which it lives. Removal of any of these trees causes disruption in the community and extensive clearing can potentially destroy the group. Because of the intricate structure of the overlapping home ranges, one koala cannot just move into the next tree if it’s tree is removed as the next tree already belongs to another koala. If a road or house is placed between the trees that encompass the koala’s home range, it may be cut off from adequate food and shelter and risk injury from being hit by a car, or attacked by a dog in its quest for basic needs. It is understood that human beings need modern conveniences, but perhaps proper planning should be followed to ensure that these conveniences are not placed where koalas and other wildlife are already living.
Australia Koala Foundation, www.savethekoala.com
Koala Conservation at The Cape Otway Centre for Conservation Ecology