Koala

The koala is the most specialised mammalian herbivore on Earth. It is not a bear, not a primate, and not closely related to any other living animal except the wombat. It is the only surviving member of the family Phascolarctidae, the last twig on a branch of the marsupial family tree that once included several larger, stranger relatives now extinct. Its entire life revolves around a single genus of plant -- Eucalyptus -- that is poisonous to almost every other mammal on the continent.

This guide covers every aspect of koala biology and ecology: size and physical description, the eucalyptus diet that shapes its entire anatomy, the famous 18-22 hour sleep cycle, reproduction, the downward-opening pouch inherited from a burrowing ancestor, human-like fingerprints, the chlamydia epidemic devastating wild populations, the 2019-2020 Black Summer bushfires, and the koala retrovirus that is rewriting the species' genome in real time. It is a reference entry for readers who want specifics: grams of leaves per day, percentages of populations infected, hectares of habitat burned, species of eucalyptus eaten.

Etymology and Classification

The scientific name Phascolarctos cinereus combines Greek phaskolos (pouch) and arktos (bear) with Latin cinereus (ash-coloured), literally 'ash-grey pouched bear'. The name was assigned by German naturalist Georg August Goldfuss in 1817, only a few decades after Europeans first recorded the animal. Early colonial naturalists called it the 'native bear' or 'tree bear' because of the superficial resemblance to a small bear, a nickname that persists in casual use. Koalas are not bears. They are marsupials, and their closest living relatives are wombats.

The common name 'koala' comes from the Dharug word gula, recorded by colonists around Sydney in the late 1700s. A popular claim that the word means 'no drink' in an Aboriginal language is widespread but disputed by linguists. Koalas do drink water, although rarely, and they gain most of their moisture from the eucalyptus leaves they eat.

Koalas sit in the order Diprotodontia, the largest and most diverse order of marsupials, which also contains kangaroos, wallabies, possums, and wombats. Within that order they form the family Phascolarctidae. Every other member of that family is extinct. Fossil relatives include Litokoala, Nimiokoala, and the enormous Phascolarctos stirtoni, which was roughly a third larger than the modern koala and shared its forests during the Pleistocene. Today only one genus and one species remain, although some biologists still debate whether the three regional populations -- Queensland, New South Wales, and Victoria -- should be treated as separate subspecies (P. c. adustus, P. c. cinereus, and P. c. victor respectively).

Size and Physical Description

Koalas are medium-sized marsupials built entirely for slow life in the treetops. Unlike most tree-dwelling mammals they have no tail, a feature they share with their nearest relatives the wombats.

Body dimensions:

• Head-body length: 60-85 cm
• Tail: vestigial, a small cartilage nub hidden under fur
• Standing weight bearing on all fours: 30-40 cm at the shoulder

Weight by region:

• Queensland (northern) koalas: males 6-8 kg, females 4-6 kg
• New South Wales koalas: intermediate, males 7-10 kg, females 5-7 kg
• Victorian and South Australian (southern) koalas: males 10-15 kg, females 8-11 kg

The size gradient from north to south follows Bergmann's rule -- mammals tend to be larger in colder climates because larger bodies conserve heat more efficiently. Southern koalas also have noticeably thicker, shaggier, darker fur, up to 3 cm deep on the back in winter. Queensland koalas are shorter-haired, greyer, and look almost sleek in comparison.

The koala's most distinctive external features are its rounded fluffy ears, broad leathery nose, and hands. Each hand has five digits arranged in an unusual pattern: two opposable 'thumbs' and three fingers, all tipped with strong curved claws. This double-thumbed grip lets the koala wrap its hand around a branch from both sides simultaneously, allowing it to hang by a single arm while feeding. The hindfeet are equally unusual -- the second and third toes are fused into a single digit used as a grooming comb, and the first toe is large and clawless for gripping bark.

Male koalas are roughly 50% heavier than females and carry a prominent dark scent gland on the centre of the chest. They rub this gland on tree trunks to mark territory, and a large mature male will scent-mark dozens of trees within his home range. Females lack this gland. Both sexes have fingerprints on the pads of their fingers.

Human-Like Fingerprints

Koala fingerprints are so structurally similar to human prints that a trained forensic examiner looking through a microscope typically cannot distinguish them. The whorls, loops, arches, and ridge spacing fall within the same size and density ranges seen in Homo sapiens. Koalas are the only non-primate mammal with this combination of features.

The evolutionary explanation is convergence. Humans and koalas last shared a common ancestor over 100 million years ago, and no other primate relative of humans has forgotten their fingerprints. Koalas independently evolved the same fine ridge pattern. The functional reason appears to be the same in both species -- repeated gripping of irregular surfaces with fine tactile feedback. Humans grip tools, food, and rocks. Koalas grip bark and pluck individual eucalyptus leaves one at a time. Both tasks benefit from increased friction, improved mechanoreceptor sensitivity, and the ability to feel fine differences in surface texture.

The implication is striking enough that forensic researchers have seriously noted the theoretical possibility of koala prints contaminating a crime scene in Australia. No such case has been confirmed, but the biology is real.

The Eucalyptus Diet

Koalas eat the leaves, shoots, and bark of eucalyptus trees and almost nothing else. This is one of the most extreme dietary specialisations documented in any mammal.

Australia has more than 700 species of Eucalyptus. Koalas regularly feed on roughly 30 of them, and any single population typically relies on a short list of about 5-10 preferred local species. Favourite species across the range include Eucalyptus camaldulensis (river red gum), E. microcorys (tallowwood), E. tereticornis (forest red gum), E. punctata (grey gum), E. viminalis (manna gum), and E. ovata (swamp gum). Koalas select individual trees, individual branches, and individual leaves based on nutrient content, moisture, and toxin levels -- a single koala will walk past many eucalyptus trees that other koalas eat from simply because the chemistry is wrong.

Daily intake:

Eucalyptus leaves are an extraordinarily difficult food. They are low in protein, low in calories, high in indigestible fibre, and contain a suite of defensive chemicals including terpenes, phenolics, tannins, and cyanogenic glycosides. Most of these compounds are toxic or lethal to mammals. Koalas survive on them through an impressive set of adaptations working in parallel.

First, a very long caecum filled with specialised bacteria ferments fibre and partially neutralises toxins. Second, a modified liver produces an unusual set of cytochrome P450 enzymes that metabolise terpenes and phenolics at far higher rates than most mammals. Third, the koala maintains an extremely low metabolic rate, so the small amount of energy extracted from a slow, sparse diet is enough. Fourth, koalas have a specialised soft palate and a cheek pouch arrangement that allows efficient chewing of tough leaves -- the molars have transverse crests that slice and grind.

The specialisation is so extreme that koalas will starve even in zoos if the wrong eucalyptus species are offered. Captive koalas in Asia and North America depend on continuous air-freighted shipments of approved leaves from Australia.

Why Koalas Sleep 18-22 Hours a Day

Koalas are famous for sleeping more than almost any other mammal. Most wild adults sleep between 18 and 22 hours out of every 24, split across multiple bouts wedged in tree forks. This is not laziness or a quirk of personality. It is the mathematical consequence of the eucalyptus diet.

Eucalyptus leaves provide perhaps 1,000-1,500 kilojoules of digestible energy per kilogram of leaf, compared to tens of thousands in fruits, grains, or meat. Extracting that energy requires hours of slow caecal fermentation, and processing the toxins simultaneously demands continuous liver work. Any unnecessary movement is energetically expensive. The koala has responded by evolving one of the lowest basal metabolic rates of any mammal its size -- roughly 70% of the expected rate -- and by turning extreme stillness into a survival strategy.

Sleeping koalas pick a comfortable tree fork, wedge their body in place, and let their long curved claws anchor them. They can sleep safely at heights of 20 metres or more. Body temperature drops slightly during sleep, further reducing energy demand. Active time is used almost entirely for feeding, with short periods for grooming, social interaction during the breeding season, and movement between trees.

Brain anatomy supports the low-energy life. Koalas have one of the smallest brains relative to body size of any mammal. The brain occupies only about 60% of the braincase -- the remaining volume is filled with cerebrospinal fluid. This reduced brain is not a sign of stupidity so much as another energy-saving adaptation: nervous tissue is extremely expensive to maintain, and the koala has evolved to spend as little energy on it as possible.

The Downward-Opening Pouch

Koalas are marsupials, which means their young complete most of their development attached to a nipple inside the mother's pouch rather than in a long-gestation placental womb. Koala pregnancy lasts only 33-35 days. A newborn joey emerges at about 2 cm long, weighing less than half a gram, blind, earless, with only its forelimbs developed enough to crawl.

What is unusual about the koala pouch is that it opens downward, toward the mother's hind legs. Upward-opening pouches are more familiar from kangaroos and wallabies, and they seem more intuitive for animals that spend time upright. A downward-opening pouch in a tree-climbing mammal looks like a design flaw.

The explanation is evolutionary. Koalas and wombats share a common ancestor within the order Diprotodontia, and that ancestor was almost certainly a burrower -- similar in lifestyle to modern wombats, which are the koala's closest living relatives. For a digging animal, a downward-opening pouch makes sense because it stays clear of soil, leaf litter, and debris kicked up during excavation. The koala's ancestors kept this anatomical layout even after moving into the trees, long enough ago that reshaping the pouch would have required major structural evolution.

To keep the joey safely inside a downward pouch, the koala has powerful sphincter muscles around the pouch opening that contract to form a seal. Mothers also adjust their climbing posture to keep the pouch opening pressed against the body. Once joeys are old enough to leave the pouch -- around six to seven months -- they cling to the mother's back or belly until about 12 months of age.

Pap: The Strangest Meal in Mammal Parenting

Mammalian milk contains a standard package of fats, sugars, protein, and antibodies. Koala milk does too, but milk alone cannot transfer the bacteria a joey needs to digest eucalyptus. Newborn koalas have sterile guts, and without the right microbes they cannot survive weaning.

The solution is pap. Around the time a joey first pokes its head out of the pouch and starts sampling leaves, the mother produces a special soft, moist form of her own faeces called pap. The joey eats it directly from the mother's cloaca over a period of several weeks. This inoculates the joey's caecum with the bacterial community needed to ferment and detoxify eucalyptus, including microbes that break down specific terpenes in the leaves.

Pap is not regular faeces. It is softer, richer in bacteria, and chemically different -- a specialised excretion produced only during the weaning period. The behaviour is shared by wombats, which face a similar though less extreme dietary challenge. The habit looks unappealing from the outside, but it is a clean solution to a real problem: passing gut bacteria vertically from mother to young.

Joeys continue to feed on both milk and pap during the transition phase, then on milk and leaves, and finally on leaves alone. Full independence from the mother typically occurs around 12-18 months of age, at which point young koalas disperse to find their own home ranges.

Life Cycle and Reproduction

Koalas breed seasonally, with most activity between September and March across the southern hemisphere spring and summer. Females reach sexual maturity at 2-3 years of age and males at 3-4 years, although males may not hold a territory large enough to breed until they are older. Females can breed every year but often skip a year, resulting in roughly one surviving joey every 1-2 years on average.

Reproductive milestones:

• Gestation: 33-35 days
• Birth size: ~2 cm, ~0.5 g
• Pouch life: 6-7 months
• Back-riding phase: 6-12 months
• Weaning: completed around 12 months
• First dispersal from mother: 12-18 months

Males compete for mates primarily through vocal displays rather than physical combat. A dominant male produces a long, deep, rattling bellow that can be heard a kilometre away through forest. The sound is startling to visitors expecting a cuddly noise -- it sounds more like a pig, a cow, or a drunk horse than anything small and furry. The call functions to advertise size (larger males produce lower frequencies) and to intimidate rivals.

Mating is brief, sometimes forceful, and takes place in a tree. Gestation is among the shortest in mammals at scarcely over a month. Newborn joeys crawl unassisted from the birth canal through the mother's fur into the pouch, where they latch onto one of two teats. The teat swells inside the joey's mouth, forming a near-permanent connection until the joey is old enough to release voluntarily.

Populations and Range

Koalas are restricted to eastern and southeastern Australia. The total wild population has been estimated between 95,000 and 238,000 animals as of the most recent surveys, a figure that carries considerable uncertainty because koalas are difficult to count in continuous forest.

Population distribution:

In February 2022, the Australian federal government formally upgraded koalas in Queensland, New South Wales, and the Australian Capital Territory from Vulnerable to Endangered under the Environment Protection and Biodiversity Conservation Act. Victorian and South Australian populations remain classified separately and in some locations are locally abundant -- or even overabundant on small islands, where koalas were introduced historically and now browse eucalypt stands to destruction.

Conservation Status and Threats

The IUCN Red List classifies koalas as Vulnerable globally with a decreasing trend. Australian state and federal classifications are more severe, reflecting recent declines and the unique pressures on northern populations.

Primary threats:

• Habitat loss. Land clearing for agriculture, grazing, urban development, and infrastructure is the single largest driver of long-term koala decline. Many local populations are now confined to fragments of forest separated by roads, farms, and suburbs.
• Vehicle strikes. Thousands of koalas are killed on Australian roads each year. Wildlife corridors, rope bridges, and underpasses help but are not widely implemented.
• Dog attacks. Domestic dogs in semi-rural and suburban areas kill large numbers of koalas that move between tree fragments on the ground.
• Bushfires. The 2019-2020 Black Summer fires killed or displaced an estimated 60,000 koalas. Some northern New South Wales populations lost more than 80% of their animals. Recovery is slow because koalas are site-faithful and rebuilding requires both surviving animals and intact feed trees.
• Chlamydia. Koala strains of Chlamydia pecorum and Chlamydia pneumoniae cause blindness, urinary tract disease, and infertility. Infection rates exceed 50% across much of the range and approach 100% in parts of Queensland.
• Koala retrovirus (KoRV). A retrovirus currently inserting itself into the koala germline, weakening immunity and interacting with chlamydia to accelerate disease progression.
• Climate change. Heatwaves cause direct mortality, and rising atmospheric CO2 reduces the protein content of eucalyptus leaves, worsening an already marginal diet.
• Inbreeding on small populations. Island refuges and isolated fragments show reduced genetic diversity and increased disease susceptibility.

Conservation responses include protected reserves, wildlife hospitals (notably the Port Macquarie Koala Hospital and Magnetic Island sanctuaries), captive breeding, habitat restoration, anti-chlamydia vaccine trials, and translocation of animals between fragmented populations. None of these individually addresses the scale of decline, and long-term survival of the species in the wild will depend on large, connected habitat networks that do not currently exist at the required scale.

The Chlamydia Pandemic

Chlamydia is the defining disease crisis in modern koala conservation. Unlike the human sexually transmitted infection caused by Chlamydia trachomatis, koala chlamydia is caused mainly by Chlamydia pecorum, with some contribution from Chlamydia pneumoniae. Both are bacterial pathogens adapted to koalas and spread through sexual contact, birth, and close contact during grooming or fighting.

Symptoms include chronic conjunctivitis that progresses to blindness, cystitis that soaks the rump with urine and produces the condition colloquially called 'dirty tail', pneumonia, and reproductive tract damage leading to infertility. In pregnant females infection can cause miscarriage or transmit directly to joeys during birth. Infection rates vary by population: Victorian populations are often under 20%, Queensland populations often over 70%, some hotspots approaching 100%.

Treatment is difficult. Antibiotics that kill chlamydia also destroy the gut flora koalas need to digest eucalyptus, and many treated animals die of starvation or metabolic failure even when the infection clears. Veterinary teams have developed protocols combining short antibiotic courses with faecal microbiome support, but these interventions are only feasible for animals in wildlife hospitals.

A vaccine developed by researchers at the University of the Sunshine Coast has been in field trials since the mid-2010s with encouraging early results -- reduced infection severity and improved reproductive outcomes in vaccinated animals. Scaling vaccination to wild populations is logistically expensive and politically contested, but it remains one of the few interventions with measurable effect on the disease burden.

The Koala Retrovirus

Koalas are currently host to a retrovirus known as KoRV -- the koala retrovirus -- that is in the process of integrating itself permanently into the species' genome. This is unusual to witness. Most endogenous retroviruses colonised their host genomes millions of years ago and are detected today only as fossilised DNA. KoRV is doing it now.

Retroviruses insert their RNA genome into host DNA as part of their reproductive cycle. When a retrovirus infects a germline cell -- a sperm or egg precursor -- the inserted sequence can be passed to the next generation and becomes part of every cell of that offspring. Enough germline insertions over enough generations and the virus transitions from an infectious pathogen to a permanent resident of the genome.

Northern koala populations in Queensland appear to be further along this transition than southern populations. Most northern koalas carry KoRV in every cell, with multiple insertion sites in their chromosomes. Southern populations show a mix of infected and uninfected animals. The virus also appears to weaken immune function, possibly contributing to the severity of chlamydia in the same populations.

Scientists studying KoRV have a rare opportunity to observe genome colonisation as it happens, offering direct insight into processes that shaped every mammal genome, including ours -- roughly 8% of human DNA is derived from ancient endogenous retroviruses.

Koalas and Humans

Human relationships with koalas span tens of thousands of years of Aboriginal custodianship and just a few centuries of European contact, with very different outcomes for the species.

Aboriginal peoples across the koala's range hunted and ate koalas in small numbers, wove stories around them, and respected their presence in the forests. Koala populations were stable for millennia under traditional management.

European colonisation was catastrophic. Between the 1870s and 1930s, commercial hunters killed millions of koalas for fur. A single Queensland season in 1919 produced one million pelts for export to the United States and Europe. Koalas were eradicated from South Australia, reduced almost to extinction in Victoria, and sharply reduced in New South Wales. Public outcry eventually led to protections, though not before many regional populations had been lost entirely.

Modern Australia generally regards koalas as a national icon. Tourism centred on wildlife parks, zoos, and sanctuaries generates meaningful income. Wildlife hospitals treat thousands of sick, injured, or burned koalas each year. Rehabilitation and release programmes return survivors to the wild where possible. At the same time, land clearing for urban expansion and agriculture continues to destroy koala habitat, and conflicts between development and conservation remain unresolved in courts, planning bodies, and state parliaments.

Koalas are also one of the most intensely studied marsupials, with dedicated research teams investigating chlamydia vaccines, KoRV biology, population genetics, habitat modelling, and fire recovery. The combination of public affection, scientific attention, and political leverage has kept koalas on the conservation agenda even as populations have declined.

Related Reading

• Marsupials: Australia's Extraordinary Pouched Mammals
• Kangaroo Pouch: How It Works
• Red Kangaroo: Australia's Giant Marsupial
• Tasmanian Devil: The Screaming Marsupial
• Quokka: The Happiest Animal Smile

References

Relevant peer-reviewed and governmental sources consulted for this entry include the IUCN Red List assessment for Phascolarctos cinereus (2020, 2024), the Australian Department of Climate Change, Energy, the Environment and Water EPBC Act listing documents (2022), the New South Wales Koala Strategy, published research in Australian Mammalogy, Conservation Genetics, Virology, Wildlife Research, and Proceedings of the Royal Society B, and technical reports from the University of the Sunshine Coast KoRV and chlamydia research programmes. Population estimates reflect the consolidated figures published in the 2020 Australian Koala Foundation assessment and subsequent state reviews following the 2019-2020 bushfire season.