Evolution

The koala is classified with wombats (family Vombatidae) and several extinct families (including marsupial tapirs, marsupial lions and giant wombats) in the suborder Vombatiformes within the order Diprotodontia.^[10] The Vombatiformes are a sister group to a clade that includes macropods (kangaroos and wallabies) and possums.^[11] The koala's lineage possibly branched off around 40 million years ago during the Eocene.^[12]

Reconstructions of the ancient koalas Nimiokoala (larger), and Litokoala (smaller), from the Miocene Riversleigh Fauna

The modern koala is the only extant member of Phascolarctidae, a family that includes several extinct genera and species. During the Oligocene and Miocene, koalas lived in rainforests and had more generalised diets.^[13] Some species, such as the Riversleigh rainforest koala (Nimiokoala greystanesi) and some species of Perikoala, were around the same size as the modern koala, while others, such as species of Litokoala, were one-half to two-thirds its size.^[14] Like the modern species, prehistoric koalas had well developed ear structures which suggests that they also made long-distance vocalisations and had a relatively inactive lifestyle.^[13] During the Miocene, the Australian continent began drying out, leading to the decline of rainforests and the spread of open Eucalyptus woodlands. The genus Phascolarctos split from Litokoala in the late Miocene,^[13][15] and had several adaptations that allowed it to live on a specialised eucalyptus diet: a shifting of the palate towards the front of the skull; upper teeth lined by thicker bone, molars located relatively low compared the jaw joint and with more chewing surface; smaller pterygoid fossa;^[13] and a larger gap separating the incisor teeth and the molars.^[16]: 226

P. cinereus may have emerged as a dwarf form of the giant koala (P. stirtoni), following the disappearance of several giant animals in the late Pleistocene. A 2008 study questions this hypothesis, noting that P. cinereus and P. stirtoni were sympatric during the middle to late Pleistocene, and the major difference in the morphology of their teeth.^[17] The fossil record of the modern koala extends back at least to the middle Pleistocene.^[18]

Genetics and variations

Three subspecies are recognised: the Queensland koala (Phascolarctos cinereus adustus, Thomas 1923), the New South Wales koala (Phascolarctos cinereus cinereus, Goldfuss 1817), and the Victorian koala (Phascolarctos cinereus victor, Troughton 1935). These forms are distinguished by pelage colour and thickness, body size, and skull shape. The Queensland koala is the smallest of the three, with silver or grey short hairs and a shorter skull. The Victorian koala is the largest, with shaggier, brown fur and a wider skull.^{[21]: 7 [22]} The geographic limits of these variations are based on state borders, and their status as subspecies is disputed. A 1999 genetic study suggests koalas exist as a cline within a single evolutionarily significant unit with limited gene flow between local populations.^[22]

Other studies have found that koala populations have high levels of inbreeding and low genetic variation.^[23][24] Such low genetic diversity may have been caused by declines in the population during the late Pleistocene.^[25] Rivers and roads have been shown to limit gene flow and contribute to the isolation of southeast Queensland populations.^[26] In April 2013, scientists from the Australian Museum and Queensland University of Technology announced they had fully sequenced the koala genome.^[27]

Foraging and activities

Foraging

Koalas are herbivorous, and while most of their diet consists of eucalypt leaves, they can be found in trees of other genera, such as Acacia, Allocasuarina, Callitris, Leptospermum, and Melaleuca.^[9]: 73 Though the foliage of over 600 species of Eucalyptus is available, the koala shows a strong preference for around 30.^[42] They prefer plant matter with higher protein over fibre and lignin.^[16]: 231 The most favoured species are Eucalyptus microcorys, E. tereticornis, and E. camaldulensis, which, on average, make up more than 20% of their diet.^[43] Despite its reputation as a picky eater, the koala is more generalist than some other marsupial species, such as the greater glider. The koala does not need to drink often as it can get enough water in the eucalypt leaves,^{[9]: 73–74} though larger males may additionally drink water found on the ground or in tree hollows.^[16]: 231 When feeding, a koala reaches out to grab leaves with one forepaw while the other paws hang on to the branch. Depending on the size of the individual, a koala can walk to the end of a branch or must stay near the base.^[9]: 96 Each day, koalas eat up to 400 grams (14 oz) of leaves, spread over four to six feeding periods.^[2]: 187 Despite their adaptations to a low-energy lifestyle, they have meagre fat reserves and need to feed often.^[2]: 189

Due to their low-energy diet, koalas limit their activity and sleep 20 hours a day.^{[9]: 93 [44]} They are predominantly active at night and spend most of their waking hours foraging. They typically eat and sleep in the same tree, possibly for as long as a day.^[21]: 39 On warm days, a koala may rest with its back against a branch or lie down with its limbs dangling.^{[9]: 93–94} When it gets very hot, the koala rests lower in the canopy and near the trunk, where the surface is cooler than the surrounding air.^[45] It curls up when it gets cold and wet.^[21]: 39 A koala will find a lower, thicker branch on which to rest when it gets windy. While it spends most of the time in the tree, the animal descends to the ground to move to another tree, leaping along.^{[9]: 93–94} The koala usually grooms itself with its hind paws, with their double claws, but sometimes uses its forepaws or mouth.^{[9]: 97–98}

Social life

Resting

A bellowing male in the Lone Pine Koala Sanctuary

Koalas are asocial animals and spend just 15 minutes a day on social behaviours. Where there are more koalas and fewer trees, home ranges are smaller and more clumped while the reverse is true for areas with fewer animals and more trees.^[9]: 98 Koala society appears to consist of "residents" and "transients", the former being mostly adult females and the latter males. Resident males appear to be territorial and dominant.^[46] The territories of dominant males are found near breeding females, while younger males must wait until they reach full size to challenge for breeding rights.^[2]: 191 Adult males occasionally venture outside their home ranges; when they do so, dominant ones retain their status.^[9]: 99 As a male climbs a new tree, he rubs his chest against it and sometimes dribbles urine. This scent-marking behaviour probably serves as communication, and individuals are known to sniff the bottom of a newly found tree.^{[21]: 54–56 [47]} Chest gland secretions are complex chemical mixtures — about 40 compounds were identified in one analysis — that vary in composition and concentration with the season and the age of the individual.^[48]

Scent gland on the chest of an adult male. Lone Pine Koala Sanctuary

Adult males communicate with loud bellows — "a long series of deep, snoring inhalations and belching exhalations".^[49] Because of their low frequency, these bellows can travel far through the forest.^[21]: 56 Koalas may bellow at any time of the year, particularly during the breeding season, when it serves to attract females and possibly intimidate other males.^[50] They also bellow to advertise their presence to their neighbours when they climb a different tree.^[21]: 57 These sounds signal the male's actual body size, as well as exaggerate it;^[51] females pay more attention to bellows that originate from larger males.^[52] Female koalas bellow, though more softly, in addition to making snarls, wails, and screams. These calls are produced when in distress and when making defensive threats.^[49] Squeaking and sqawking are produced when distraught; the former is made by younger animals and the latter by older ones. When another individual climbs over it, a koala makes a low closed-mouth grunt.^{[9]: 102–03 [49]} Koalas also communicate with facial expressions. When snarling, wailing, or squawking, the animal curls the upper lip and points its ears forward. Screaming koalas pull their lips and ears back. Females form an oval shape with their lips when annoyed.^{[9]: 104–05}

Agonistic behaviour typically consists of quarrels between individuals that are trying to pass each other in the tree. This occasionally involves biting. Strangers may wrestle, chase, and bite each other.^{[9]: 102 [53]} In extreme situations, a male may try to displace a smaller rival from a tree, chasing, cornering and biting it. Once the individual is driven away, the victor bellows and marks the tree.^{[9]: 101–02} Pregnant and lactating females are particularly aggressive and attack individuals that come too close.^[53] In general, however, koalas tend to avoid fighting due to energy costs.^[2]: 191

Reproduction and development

A young joey, preserved at Port Macquarie Koala Hospital

Koalas are seasonal breeders, and give birth from October to May. Females in oestrus lean their heads back and shake their bodies. Despite these obvious signals, males will try to copulate with any female during this period, mounting them from behind. Because of his much larger size, a male can overpower a female. A female may scream and vigorously fight off her suitors but will accede to one that is dominant or familiar. The commotion can attract other males to the scene, obliging the incumbent to delay mating and fight off the intruders. A female may learn who is more dominant during these fights.^{[21]: 58–60} Older males usually have accumulated scratches, scars, and cuts on the exposed parts of their noses and their eyelids.^[2]: 192

Koalas are induced ovulators.^[54] The gestation period lasts 33–35 days,^[55] and a female gives birth to one joey (although twins do occur). As marsupials, the young are born tiny and barely formed, weighing no more than 0.5 g (0.02 oz). However, their lips, forelimbs, and shoulders are relatively advanced, and they can breathe, defecate and urinate. The joey crawls into its mother's pouch to continue the rest of its development.^[21]: 61 Female koalas do not clean their pouches, an unusual trait among marsupials.^[2]: 181

The joey latches on to one of the female's two teats and suckles it.^[21]: 61 The female lactates for as long as a year to make up for her low energy production. Unlike in other marsupials, koala milk becomes less fatty as the joey grows in the pouch.^[21]: 62 After seven weeks, the joey has a proportionally large head, clear edges around its face, more colouration, and a visible pouch (if female) or scrotum (male). At 13 weeks, the joey weighs around 50 g (1.8 oz) and its head is twice as big as before. The eyes begin to open and hair begins to appear. At 26 weeks, the fully furred animal resembles an adult and can look outside the pouch.^[21]: 63

Mother with joey on back

At six or seven months of age, the joey weighs 300–500 g (11–18 oz) and fully emerges from the pouch for the first time. It explores its new surroundings cautiously, clutching its mother for support.^[21]: 65 Around this time, the mother prepares it for a eucalyptus diet by producing a faecal pap that the joey eats from her cloaca. This pap comes from the cecum, is more liquid than regular faeces, and is filled with bacteria.^[56] A nine month old joey has its adult coat colour and weighs 1 kg (2.2 lb). Having permanently left the pouch, it rides on its mother's back for transportation, learning to climb by grasping branches.^{[21]: 65–66} Gradually, it becomes more independent from its mother, who becomes pregnant again after a year, and the young is now around 2.5 kg (5.5 lb). Her bond with her previous offspring is permanently severed and she no longer allows it to suckle, but it will stay nearby until it is one-and-a-half to two years old.^{[21]: 66–67}

Females become sexually mature at about three years of age and can then become pregnant; in comparison, males reach sexual maturity when they are about four years old,^[57] although they can experience spermatogenesis as early as two years.^[21]: 68 Males do not start marking their scent until they reach sexual maturity, though their chest glands become functional much earlier.^[48] Koalas can breed every year if environmental conditions are good, though the long dependence of the young usually leads to year-long gaps in births.^[16]: 236

Health and mortality

Koalas may live from 13 to 18 years in the wild. While female koalas usually live this long, males may die sooner because of their more risky lives.^[21]: 69 Koalas usually survive falls from trees and can climb back up, but they can get hurt and even die, particularly inexperienced young and fighting males.^{[21]: 72–73} Around six years of age, the koala's chewing teeth begin to wear down and their chewing efficiency decreases. Eventually, the cusps disappear completely and the animal will die of starvation.^[58] Koalas have few predators. Dingos and large pythons and some birds of prey may take them. Koalas are generally not subject to external parasites, other than ticks around the coast. The mite Sarcoptes scabiei gives koalas mange, while the bacterium Mycobacterium ulcerans skin ulcers, but even these are uncommon. Internal parasites are few and have little effect.^{[21]: 71–74} These include the tapeworm Bertiella obesa, commonly found in the intestine, and the nematodes Marsupostrongylus longilarvatus and Durikainema phascolarcti, which are infrequently found in the lungs.^[59] In a three-year study of almost 600 koalas taken to the Australia Zoo Wildlife Hospital in Queensland, 73.8% of the animals were infected with parasitic protozoal genus Trypanosoma, the most frequent of which was T. irwini.^[60]

Koalas can be subject to pathogens such as Chlamydiaceae bacteria,^{[21]: 74–75} which can cause keratoconjunctivitis, urinary tract infection, and reproductive tract infection.^{[9]: 229–30} Such infections are common on the mainland, but absent in some island populations.^[21]: 114 The koala retrovirus (KoRV) may cause koala immune deficiency syndrome (KIDS) which is similar to AIDS in humans. Prevalence of KoRV in koala populations suggests a trend spreading from north to south, where populations go from being completely infected to being partially uninfected.^[61]

The animals are vulnerable to bushfires due to their slow speed and the flammability of eucalypt trees.^[21]: 26 The koala instinctively seeks refuge in the higher branches, where it is vulnerable to intense heat and flames. Bushfires also break up the animal's habitat, which isolates them, decreases their numbers and creates genetic bottlenecks.^{[2]: 209–11} Dehydration and overheating can also prove fatal.^[9]: 80 Consequently, the koala is vulnerable to the effects of climate change. Models of climate change in Australia predict warmer and drier climates, suggesting that the koala's range will shrink in the east and south to more mesic habitats.^[62]

History

George Perry's illustration in his 1810 Arcana was the first published image of the koala.

The first written reference to the koala was recorded by John Price, servant of John Hunter, the Governor of New South Wales. Price encountered the "cullawine" on 26 January 1798, during an expedition to the Blue Mountains,^[63] but his remarks would first be published in Historical Records of Australia, nearly a century later.^[2]: 8 In 1802, French-born explorer Francis Louis Barrallier encountered the animal when his two Aboriginal guides, returning from a hunt, brought back two koala feet they were intending to eat. Barrallier preserved the appendages and sent them and his notes to Hunter's successor, Philip Gidley King, who forwarded them to Joseph Banks. Similar to Price, Barrallier's notes were not published until 1897.^{[2]: 9–10} Reports of the "Koolah" appeared in the Sydney Gazette in late 1803, and helped provide the impetus for King to send the artist John Lewin to paint watercolours of the animal. Lewin painted three pictures, one of which was used as a print in Georges Cuvier's Le Règne Animal (The Animal Kingdom) (1827).^{[2]: 12–13}

Botanist Robert Brown was the first to write a formal scientific description of the koala in 1803, based on a female specimen captured near what is now Mount Kembla in the Illawarra region of New South Wales. Austrian botanical illustrator Ferdinand Bauer drew the animal's skull, throat, feet, and paws. Brown's work remained unpublished and largely unnoticed, however, as his field books and notes remained in his possession until his death, when they were bequeathed to the British Museum (Natural History) in London. They were not identified until 1994, while Bauer's koala watercolours were not published until 1989.^{[2]: 16–28} William Paterson, who had befriended Brown and Bauer during their stay in New South Wales, wrote an eyewitness report of his encounters with the animals and this would be the basis for British surgeon Everard Home's anatomical writings on them.^{[2]: 33–36} Home, who in 1808 published his report in the journal Philosophical Transactions of the Royal Society,^[64] coined the scientific name Didelphis coola.^[2]: 36

George Perry would officially publish the first image of the koala in his 1810 natural history work Arcana.^[2]: 37 Perry called it the "New Holland Sloth", and his dislike for the koala, evident in his description of the animal, was reflected in the contemporary British attitudes towards Australian animals as strange and primitive:^[2]: 40

...  the eye is placed like that of the Sloth, very close to the mouth and nose, which gives it a clumsy awkward appearance, and void of elegance in the combination ... they have little either in their character or appearance to interest the Naturalist or Philosopher. As Nature however provides nothing in vain, we may suppose that even these torpid, senseless creatures are wisely intended to fill up one of the great links of the chain of animated nature ...^[65]

Natural history illustrator John Gould popularised the koala with his 1863 work The Mammals of Australia.

Naturalist and popular artist John Gould illustrated and described the koala in his three-volume work The Mammals of Australia (1845–1863) and introduced the species, as well as other members of Australia's little-known faunal community, to the public.^{[2]: 87–93} Comparative anatomist Richard Owen, in a series of publications on the physiology and anatomy of Australian mammals, presented a paper on the anatomy of the koala to the Zoological Society of London.^[66] In this widely cited publication, he provided an early description of its internal anatomy, and noted its general structural similarity to the wombat.^{[2]: 94–96} English naturalist George Robert Waterhouse, curator of the Zoological Society of London, was the first to correctly classify the koala as a marsupial in the 1840s, and compared it to fossil species Diprotodon and Nototherium, which had been discovered just recently.^{[2]: 46–48} Similarly, Gerard Krefft, curator of the Australian Museum in Sydney, noted evolutionary mechanisms at work when comparing the koala to fossil marsupials in his 1871 The Mammals of Australia.^{[2]: 103–105}

Britain finally received a living koala in 1881, which was obtained by the Zoological Society of London. As related by prosecutor to the society, William Alexander Forbes, the animal suffered an accidental demise when the heavy lid of a washstand fell on it and it was unable to free itself. Forbes dissected the fresh specimen and wrote about the female reproductive system, the brain, and the liver — parts not previously described by Owen, who had access only to preserved specimens.^{[2]: 105–06} Scottish embryologist William Caldwell — well known in scientific circles for determining the reproductive mechanism of the platypus — described the uterine development of the koala in 1884,^[67] and used this new information to convincingly map out the evolutionary timeline of the koala and the monotremes.^[2]: 111

Cultural significance

Koala souvenir soft toys are popular with tourists

Amy and Oliver the bronze koalas (by Glenys Lindsay)

The koala is well known worldwide and is a major draw for Australian zoos and wildlife parks. It has been featured in popular culture and as soft toys.^[9]: ix It benefited the Australian tourism industry by over $1 billion in 1998, and this has subsequently grown.^[2]: 201 Its international popularly rose after World War II, when tourism to Australia increased and the animals were exported to zoos overseas.^[9]: 156 In 1997, about 75% of European and Japanese tourists placed the koala at the top of their list of animals to see.^[2]: 216 According to biologist Stephen Jackson: "If you were to take a straw poll of the animal most closely associated with Australia, it's a fair bet that the koala would come out marginally in front of the kangaroo".^[9]: ix Factors that contribute to the koala's enduring popularity include its teddy bear-like appearance with childlike body proportions.^[21]: 3

The koala is featured in the Dreamtime stories and mythology of Indigenous Australians. The Tharawal people believed that the animal helped them get to the continent by rowing the boat.^[9]: 21 Another myth tells of how a tribe killed a koala and used its long intestines to create a bridge for people from other parts of the world.^[21]: 17 How the koala lost its tail has been the subject of many tales. In one, a kangaroo cuts it off to punish the koala for its uncouth behaviour.^[9]: 28 Tribes in both Queensland and Victoria regarded the koala as a wise animal which gave valuable guidance. Bidjara-speaking people credited the koala for making trees grow in their arid lands.^{[9]: 41–43} The animal is also depicted in rock carvings, though less so than some other species.^{[9]: 45–46}

Early European settlers in Australia considered the koala to be a creeping sloth-like animal with a "fierce and menacing look".^[9]: 143 At the turn of the 20th century, the koala's reputation took a more positive turn. It appears in Ethel Pedley's 1899 book Dot and the Kangaroo, as the "funny native bear".^[9]: 144 Artist Norman Lindsay depicted a more anthropomorphic koala in The Bulletin cartoons, starting in 1904. This character also appeared as Bunyip Bluegum in Lindsay's 1918 book The Magic Pudding.^[9]: 147 The most well known fictional koala is Blinky Bill. Created by Dorothy Wall in 1933, the character appeared in several books and has been the subject of films, TV series, merchandise, and a 1986 environmental song by John Williamson.^{[9]: 149–52} The koala first appeared on an Australian stamp in 1930.^[2]: 164

US President Barack Obama with a koala in Brisbane, Australia

The song "Ode to a Koala Bear" appears on the B-side of the 1983 Paul McCartney/Michael Jackson duet single Say Say Say.^[9]: 151 A koala is the main character in Hanna-Barbera's The Kwicky Koala Show and Nippon Animation's Noozles, both of which were animated cartoons of the early 1980s. Food products shaped like the koala include the Caramello Koala chocolate bar and the bite-sized cookie snack Koala's March. Dadswells Bridge in Victoria features a tourist complex shaped like a giant koala^{[9]: 155–58} and the Queensland Reds rugby team has a koala as its icon.^[9]: 160

Koala diplomacy

Several political leaders and members of royal families had their pictures taken with koalas, including Queen Elizabeth II, Prince Harry, Crown Prince Naruhito, Crown Princess Masako, Pope John Paul II, US President Bill Clinton, Soviet premier Mikhail Gorbachev and South African President Nelson Mandela^[9]: 156 At the 2014 G20 Brisbane summit, hosted by Prime Minister Tony Abbott, many world leaders, including Russian President Vladimir Putin and US President Barack Obama, were photographed holding koalas.^[68][69] The event gave rise to the term "koala diplomacy",^[70][71] which then became the Oxford Word of the Month for December 2016.^[72] The term also includes the loan of koalas by the Australian government to overseas zoos in countries such as Singapore and Japan, as a form of "soft power diplomacy", like the "panda diplomacy" practised by China.^[73][74]