Koala
The koala, sometimes inaccurately called the koala bear, is an arboreal herbivorous marsupial native to Australia. It is the only extant representative of the family Phascolarctidae. Its closest living relatives are the wombats. The koala is found in coastal areas of the continent's eastern and southern regions, inhabiting Queensland, New South Wales, Victoria, and South Australia. It is easily recognisable by its stout, tailless body and large head with round, fluffy ears and large, dark nose. The koala has a body length of and weighs. Its fur colour ranges from silver grey to chocolate brown. Koalas from the northern populations are typically smaller and lighter in colour than their counterparts further south. These populations are possibly separate subspecies, but not all researchers accept this.
Koalas typically inhabit open Eucalyptus woodland, as the leaves of these trees make up most of their diet. This eucalypt diet has low nutritional and caloric content and contains toxic compounds that deter most other mammals from feeding on them. Koalas are largely sedentary and sleep up to twenty hours a day. They are asocial; only mothers bond to dependent offspring. Adult males communicate with bellows that intimidate rivals and attract mates. Males mark their presence with secretions from scent glands located on their chests. Like other marsupials, koalas give birth to young known as joeys at a very early stage of development. They crawl into their mothers' pouches, where they live for their first six to seven months. They are fully weaned around a year old.
Koalas have few natural predators and parasites, but are threatened by pathogens such as Chlamydiaceae bacteria and koala retrovirus. Koalas are listed as a vulnerable species by the International Union for Conservation of Nature. Among the many threats to their existence are habitat destruction caused by agriculture, urbanisation, droughts, and associated bushfires, some related to climate change. In February 2022, the koala was officially listed as endangered in the Australian Capital Territory, New South Wales, and Queensland. They are however over-abundant in parts of South Australia and Victoria, with population growth at unsustainable levels.
Because of their distinctive appearance, koalas, along with kangaroos, are recognised worldwide as symbols of Australia. They were hunted by Aboriginal Australians and depicted in myths and cave art for millennia. The first recorded encounter between a European and a koala was in 1798, and an image of the animal was published in 1810 by naturalist George Perry. Botanist Robert Brown wrote the first detailed scientific description in 1814 although his work remained unpublished for 180 years. Artist John Gould illustrated and described the koala, thereby introducing the species to the British public. Further details about the animal's biology were revealed in the 19th century by English scientists.
Etymology
The word "koala" comes from the Dharug gula, meaning. Although the vowel "u" was originally written in the English orthography as "oo", the spelling for that sound later became "oa"; the word is now pronounced in three syllables possibly in error based on that new spelling. Another hypothesis is that "koala" was an aboriginal name from the Hawkesbury River district near Sydney.Adopted by white settlers, the word "koala" became one of hundreds of Aboriginal loan words in Australian English, where it was also commonly referred to as "native bear", later "koala bear", for its resemblance to a bear. It is one of several Aboriginal words that made it into International English alongside words like "didgeridoo" and "kangaroo".
The koala's generic name, Phascolarctos, is derived from the Greek words φάσκωλος and ἄρκτος . The specific name, cinereus, is Latin for.
Taxonomy
The generic name Phascolarctos was given in 1816 by French zoologist Henri Marie Ducrotay de Blainville, who did not give it a specific name until further review. In 1819, German zoologist Georg August Goldfuss gave it the binomial Lipurus cinereus. Because Phascolarctos was published first, according to the International Code of Zoological Nomenclature, it has priority as the official genus name. French naturalist Anselme Gaëtan Desmarest coined the name Phascolarctos fuscus in 1820, suggesting that the brown-coloured versions were a different species than the grey ones. Other names suggested by European authors included Marodactylus cinereus by Goldfuss in 1820, P. flindersii by René Primevère Lesson in 1827, and P. koala by John Edward Gray in 1827.Evolution
The koala is classified with wombats and several extinct families in the suborder Vombatiformes within the order Diprotodontia. The Vombatiformes are a sister group to a clade that includes macropods and possums. The koala's lineage possibly branched off around 40 million years ago during the Eocene.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 broader diets. Some species, such as 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. 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. 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, and had several adaptations that allowed it to live on a eucalyptus diet: the palate shifted towards the front of the skull; the upper teeth were lined by thicker bone, molars became relatively low compared to the jaw joint and with more chewing surface; the pterygoid fossa shrank; and a larger gap separated the incisor teeth and the molars.
P. cinereus may have emerged as a dwarf form of the giant koala, following the disappearance of several giant animals in the late Pleistocene. A 2008 study questioned this hypothesis, noting that P. cinereus and P. stirtoni were sympatric during the mid-late Pleistocene, and that their teeth morphology displayed the major differences. The fossil record of the modern koala extends back at least to the middle Pleistocene.
Genetics and variations
Three subspecies have been described: the Queensland koala, the New South Wales koala, and the Victorian koala. These forms are distinguished by pelage colour and thickness, body size, and skull shape. The Queensland koala is the smallest, with silver or grey short hairs and a shorter skull. The Victorian koala is the largest, with shaggier, brown fur and a wider skull. 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. In 2016, a comprehensive phylogenetic study did not support the recognition of any subspecies.Other studies have found that koala populations are highly inbred with low genetic variation. Such low genetic diversity may have been caused by population declines during the late Pleistocene. Rivers and roads limit gene flow and contribute to the isolation of southeast Queensland populations. In April 2013, scientists from the Australian Museum and Queensland University of Technology announced they had fully sequenced the koala genome.
Characteristics
The koala is a robust animal with a large head and vestigial or non-existent tail. It has a body length of and a weight of, making it among the largest arboreal marsupials. Koalas from Victoria are twice as heavy as those from Queensland. The species is sexually dimorphic: males are 50% larger than females. Males' noses are more curved and sport chest glands, which are visible as bald patches. The female's pouch opening is secured by a sphincter which holds the young in.The pelage of the koala is denser on the back. Back fur colour varies from light grey to chocolate brown. The belly fur is whitish; on the rump it is mottled whitish and dark. The koala has the most effective insulating back fur of any marsupial and is resilient to wind and rain, while the belly fur can reflect solar radiation. The koala has curved, sharp claws well adapted for climbing trees. The large forepaws have two opposable digits that allow them to grip small branches. On the hind paws, the second and third digits are fused, a typical condition for members of the Diprotodontia, and the attached claws function like a comb. The animal has a robust skeleton and a short, muscular upper body with relatively long upper limbs that contribute to its ability to climb. The thigh muscles are anchored further down the shinbone, increasing its climbing power.
For a mammal, the koala has a disproportionately small brain, 60% smaller than that of a typical diprotodont, weighing only on average. The brain's surface is fairly smooth and "primitive". It does not entirely fill the cranial cavity, unlike most mammals, and is lightened by large amounts of cerebrospinal fluid. It is possible that the fluid protects the brain should the animal fall from a tree. The koala's small brain may be an adaptation to the energy restrictions imposed by its diet, which is insufficient to sustain a larger brain. Its small brain limits its ability to perform complex behaviours. For example, it will not eat plucked eucalyptus leaves on a flat surface, which does not match its feeding routine.
The koala has a broad, dark nose with a good sense of smell, and it is known to sniff the oils of individual branchlets to assess their edibility. Its relatively small eyes are unusual among marsupials in that the pupils have vertical slits, an adaptation to living on a more vertical plane. Its round ears provide it with good hearing, and it has a well-developed middle ear. The koala larynx is located relatively low in the vocal tract and can be pulled further down. They possess unique folds in the velum, known as velar vocal folds, in addition to the typical vocal folds of the larynx. These features allow the koala to produce deeper sounds than would otherwise be possible for their size.
File:Machoires de koala avec dents.png|thumb|right|Teeth of a koala, from left to right: molars, premolars, diastema, canines, incisors
The koala has several adaptations for its low nutrient, toxic, and fibrous diet. The animal's dentition consists of incisors and cheek teeth that are separated by a large gap. The koala bites a leaf with the incisors and clips it with the premolars at the petiole, before chewing it to pieces with the cusped molars. Koalas may store food in their cheek pouches before it is ready to be chewed. The partially worn molars of koalas in their prime are optimal for breaking leaves into small particles, resulting in more efficient stomach digestion and nutrient absorption in the small intestine, which digests the eucalyptus leaves to provide most of the animal's energy. A koala sometimes regurgitates the food into the mouth to be chewed a second time.
Koalas are hindgut fermenters, and their digestive retention can last 100 hours in the wild or 200 hours in captivity. This is made possible by their caecum— long and in diameter—possibly the largest for an animal of its size. Koalas can retain food particles for longer fermentation if needed. They are more likely keep smaller particles as larger ones take longer to digest. While the hindgut is relatively large, only 10% of the animal's energy is obtained from digestion in this chamber. The koala's metabolic rate is only 50% of the typical mammalian rate, owing to its low energy intake, although this can vary across seasons and sexes. They can digest the toxic plant secondary metabolites, phenolic compounds and terpenes due to their production of cytochrome P450, which neutralises these poisons in the liver. The koala replaces lost water at a lower rate than species such as some possums. It maintains water by absorbing it in the caecum, resulting in drier faecal pellets packed with undigested fibre.