Koala
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? Koala Conservation status: Lower
risk
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 A Koala climbing its new tree in
Otway National Park, Victoria,
Australia
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| Phascolarctos
cinereus ( Goldfuss, 1817) |
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The Koala (Phascolarctos cinereus) is a thickset arboreal marsupial herbivore native to Australia, and the only extant representative of the family, Phascolarctidae.
The koala's scientific name comes from the Greek: phaskolos meaning "pouch" and; arktos meaning "bear". The cinereus part is Latin and means "ash-coloured". Some people refer to the koala as a koala bear—this is technically incorrect, since koalas are not part of the bear family. The word "koala" comes from an Aboriginal word meaning "does not drink."
Koalas are found all along the eastern coast of Australia from near Adelaide to the base of the Cape York Peninsula, and as far into the hinterland as there is enough rainfall to support suitable forests. The koalas of South Australia were exterminated during the early part of the 20th century, but the state has since been repopulated with Victorian stock. Koalas are absent from Tasmania and Western Australia.
Taxonomy and evolution
Although three subspecies have been described, these are arbitrary selections from a cline and are not generally accepted as valid. Following Bergmann's Rule, southern individuals from the cooler climates are larger. A typical Victorian koala (formerly P. cinereus victor, see illustrations) has longer, thicker fur, is a darker, softer grey, often with chocolate-brown highlights on the back and forearms, and has a more prominently light-coloured ventral side and fluffy white ear tufts. Typical weights are 12 kg for males and 8.5 kg for females. In tropical and sub-tropical Queensland, however, Koalas are smaller (at around 6.5 kg for an average male and just over 5 kg for an average female), a lighter, often rather scruffy grey in colour, and have shorter, thinner fur. Queensland koalas were previously classified as the subspecies P. cinereus adustus, and the intermediate forms in New South Wales as P. cinereus cinereus. The variation from one form to another is continuous, and there are substantial differences between individual koalas in any given region such as hair colour.
Koala fossils are quite rare, but some have been found in northern Australia dating back 20 million years ago. During this time, the northern half of Australia was rainforest. Koalas did not specialise in a diet of eucalyptus until the climate cooled and eucalyptus forests grew in the place of rainforests. Fossils prove that before 50,000 years ago, giant koalas inhabited the southern regions of Australia. The koala fills the same ecological role as the sloth of South America. Its origins are unclear. However, since its pouch opens backwards instead of forwards like most other marsupials, it is theorised that it may have evolved from a burrowing marsupial.
Physical description
Koalas are broadly similar in appearance to the wombat (their closest living relative), but have a thicker, softer coat, much larger ears, and longer limbs, which are equipped with large, sharp claws to assist with climbing. Weight varies from about 14 kg for a large, southern male, to about 5 kg for a small northern female. They are generally silent, but male koalas have a very loud advertising call (a nasal snort that human children delight in imitating) that can be heard from almost a kilometre away during the breeding season. There is little reliable information about the lifespan of koalas. However, in captivity they have been observed to reach the age of 15 years.
Because of the low nutritional value of the koalas' diet, they have saved energy by evolving unusually small brains. 40% of the cranial cavity is filled with fluid, while the brain itself is;
"like a pair of shrivelled walnut halves on top of the brain stem, in contact neither with each other nor the bones of the skull. It is the only animal on Earth with such a strangely reduced brain". (Flannery, p. 86)
Ecology and behaviour
Koalas live almost entirely on eucalyptus leaves. This is likely to be an evolutionary adaptation that takes advantage of an otherwise unfilled ecological niche, since eucalyptus leaves are low in protein, high in indigestible substances, and contain phenolic and terpene compounds that are toxic to most species. Like wombats and sloths, koalas have a very low metabolic rate for a mammal (which conserves energy) and rest motionless for about 20 hours a day, sleeping most of that time. They feed at any time of day, but usually at night. An average koala eats 500 grams of eucalyptus leaves each day, chewing them in their powerful jaws to a very fine paste before swallowing. The liver deactivates the toxic components ready for excretion, and the hind gut (especially the caecum) is greatly enlarged to extract the maximum amount of nutrient from the poor quality diet. Much of this is done through bacterial fermentation: when young koalas are being weaned, the mother passes unusually soft faeces, called pap, which is rich in these bacteria, thus passing these essential digestive aids onto her offspring.
Koalas will eat the leaves of a wide range of eucalyptus, and occasionally even some exotic species, but they have firm preferences for particular varieties, which vary from one region to another: in the south Manna Gum, Blue Gum and Swamp Gum are favoured; Grey Gum and Tallowwood are important in the north, and the ubiquitous River Red Gum of the isolated seasonal swamps and watercourses that meander across the dry inland plains allows koalas to exist in surprisingly arid areas. Many factors determine which of the 800 species of eucalyptus trees koalas eat. Among trees of their favourite species, however, the major factor that determines which individual trees koalas choose is the concentration of a group of phenolic toxins called formylated phloroglucinol compounds.
Female koalas are solitary and occupy distinct home ranges that they rarely leave. In the more fertile areas, these ranges overlap; in areas where suitable food trees are scarce they tend to be larger and more exclusive. Males are not territorial, but do not tolerate one another, particularly not during the breeding season: dominant individuals attack subordinate ones, and most adult males carry scars on their face, ears and forearms as a result.
Koalas are almost entirely arboreal. They do not make nests, but sleep in a tree fork or on a branch. They climb using their powerful claws for grip, usually moving quite slowly but can climb rapidly when needed. Koalas will leap confidently from one tree to another if they are reasonably close together. Longer distances are traversed on the ground in a slow but effective waddle. If threatened, koalas break into a surprisingly athletic gallop, heading for the nearest tree and bounding up it to a safe height. There the Koala waits for the intruder to go away with the patience of a creature that routinely sleeps for 18 hours a day. If the intruder attempts to climb the tree to pursue the Koala, it is not unknown for the koala to urinate as a defense mechanism. Koalas are also rather adept at swimming, but this has rarely been witnessed.
Reproduction
Females reach sexual maturity at 2 to 3 years of age, males at 3 to 4 years. If healthy, a female koala can produce one young each year for about 12 years. Gestation is 35 days; twins are very rare. Mating normally occurs between December and March, the Southern Hemisphere's summer.
A baby koala is referred to as a joey and is hairless, blind, and earless. At birth the joey, only the size of a jelly bean, crawls into the downward-facing pouch on the mother's belly (which is closed by a drawstring-like muscle that the mother can tighten at will) and attaches itself to one of the two teats. Young remain hidden in the pouch for about six months, only feeding on milk. During this time they grow ears, eyes, and fur. The joey then begins to explore outside of the pouch. At about 30 weeks it has begun to eat the semi-liquid form of the mother’s excrement called "pap". The baby koala will remain with the mother for another six months or so, riding on her back, and feeding on both milk and gum leaves until weaning is complete at about 12 months of age. Young females disperse to nearby areas at that time; young males often stay in the mother's home range until they are two or three years old.
Conservation status
The koala was hunted almost to extinction in the early 20th century, largely for its fur. In recent years, some colonies have been hard hit by disease, especially chlamydia. Koalas require large areas of healthy, connected forest and will travel long distances along tree corridors in search of new territory and mates. The ever-increasing human population of the continent continues to cut these corridors for agricultural and residential development, forestry and road-building, marooning koala colonies in decreasing areas of bush. The Australian Koala Foundation has mapped 40,000 km² of land for koala habitat and claims it has strong evidence to suggest wild koala populations are in serious decline throughout the koala's natural range. Although the species covers a massive area, only 'pieces' of koala habitat remain. These pieces need to be managed, protected and restored in a coordinated way. Presently, many are being lost to weeds, cleared for agriculture, or carved up by developers. Other threats come from logging, poor management, attacks from feral and domestic animals, disease and roads.
In contrast to the situation on much of the mainland, where populations are declining, the koalas of many island and isolated populations have reached what some have described as "plague" proportions. On Kangaroo Island, in South Australia, koalas introduced some 90 years ago have thrived in the absence of predators and competition. Combined with an inability to migrate to new areas, this has caused the koala populations to become unsustainable and threaten the Island's unique ecology. In particular, species of Manna Gum, native to the island, are being stripped by Koalas at a rate faster than they can regenerate, endangering local birds and invertebrates that rely on them, and causing the extinction of at least one isolated population of manna. Koala numbers are estimated at over 30,000, with ecologists suggesting that the Island can sustain 10,000 at most. Although culling has been suggested as a means to reduce Koala numbers, with the South Australian Government seriously considering such in 1996, this has met with fierce opposition both domestically and internationally, and the species remains protected. The popularity of the Koala has made the possibility of a cull politically improbable, with any negative perception likely to impact on tourism and a Government's electability. In place of a cull, sterilisation and translocation programmes have had only limited success in reducing numbers thus far, and remain expensive. There is evidence that Koalas relocated to the mainland have difficulty establishing themselves in the different circumstances. A mooted alternative to the complex sterilisation method, wherein the animal must first be captured, are hormonal implants that can be injected via darts.
Koalas occur in four Australian states. Under state legislation, the species is listed as Vulnerable in the South East Queensland Bioregion, Vulnerable in New South Wales and Rare in South Australia. The species' national status is under review.