Editing Koala (section)

==Characteristics==
[[File:Koala grooming.ogv|thumb|right|Scratching and grooming]]

The koala is a robust animal with a large head and [[vestigiality|vestigial]] or non-existent tail.<ref name=jackson/>{{rp|1}}<ref name="Nowak 2005">{{cite book|author=Nowak, R.|year=2005|title=Walker's Marsupials of the World|url=https://archive.org/details/walkersmarsupial00nowa|url-access=limited|publisher=Johns Hopkins University Press|pages=[https://archive.org/details/walkersmarsupial00nowa/page/n145 135]–36|isbn=978-0-8018-8211-1}}</ref> It has a body length of {{cvt|60|-|85|cm}} and a weight of {{cvt|4|-|15|kg}},<ref name="Nowak 2005"/> making it among the largest arboreal marsupials.<ref name=captive/> Koalas from Victoria are twice as heavy as those from Queensland.<ref name="Martin-Handasyde"/>{{rp|7}} The species is [[Sexual dimorphism|sexually dimorphic]]: males are 50% larger than females. Males' noses are more curved<ref name="captive" /> and sport chest glands, which are visible as bald patches.<ref name="Martin-Handasyde" />{{rp|55}} The female's pouch opening is secured by a [[sphincter]] which holds the young in.<ref>{{cite journal|last=Smith|first=M|year=1979|title=Notes on reproduction and growth in the koala, ''Phascolarctos cinereus'' (Goldfuss)|journal=Australian Wildlife Research|volume=6|issue=1|pages=5–12|doi=10.1071/WR9790005}}</ref>

The pelage of the koala is denser on the back.<ref name="captive">{{cite book|author=Jackson, S.|year=2003|title=Australian Mammals: Biology and Captive Management|publisher=CSIRO Publishing|pages=147–51|isbn=978-0-643-06635-9}}</ref> Back fur colour varies from light grey to chocolate brown.<ref name="jackson" />{{rp|1–2}} The belly fur is whitish; on the rump it is mottled whitish and dark.<ref name="Nowak 2005" /> 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.<ref>{{cite journal|author1=Degabriele, R. |author2=Dawson, T. J. |year=1979|title=Metabolism and heat balance in an arboreal marsupial, the koala (''Phascolarctos cinereus'')|journal=Journal of Comparative Physiology B|volume=134|issue=4|pages=293–301|doi=10.1007/BF00709996|s2cid=31042136 |issn=1432-1351}}</ref> The koala has curved, sharp claws well adapted for climbing trees. The large forepaws have two opposable digits (the first and second, which are opposable to the other three) 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 (which are still separate) function like a comb.<ref name="Martin-Handasyde" />{{rp|5}} 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.<ref name="moyal" />{{rp|183}}
[[File:Koala skeleton1.jpg|thumb|upright|left|Mounted skeleton]]
For a mammal, the koala has a [[Brain-to-body mass ratio|disproportionately small brain]],<ref name=jackson/>{{rp|81}} 60% smaller than that of a typical [[diprotodont]], weighing only {{cvt|19.2|g}} on average.<ref>{{cite journal |author1=Carmen de Miguel |author2=Maciej Henneberg |year=1998 |title=Encephalization of the Koala, Phascolarctos cinereus |journal=Australian Mammalogy |volume=20 |issue=3 |pages=315–320 |doi=10.1071/AM98315 |url=https://www.researchgate.net/publication/233726395 |access-date=13 October 2018 |archive-date=17 March 2022 |archive-url=https://web.archive.org/web/20220317214337/https://www.researchgate.net/publication/233726395_1998_koala_brain |url-status=live }}</ref> The brain's surface is fairly smooth and "primitive".<ref name="Martin-Handasyde"/>{{rp|52}} It does not entirely fill the cranial cavity, unlike most mammals,<ref name=jackson/>{{rp|81}} 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.<ref name="Martin-Handasyde"/>{{rp|52–53}} 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.<ref name=jackson/>{{rp|81}} 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.<ref name="Tyndale-Biscoe"/>{{rp|234}}

The koala has a broad, dark nose<ref name=Clode>{{cite book|last=Clode|first=Danielle|year=2023|title=Koala: The Extraordinary Life of an Enigmatic Animal|publisher=W. W. Norton & Company|pages=172–175|isbn=9781324036845}}</ref> with a good sense of smell, and it is known to sniff the oils of individual branchlets to assess their edibility.<ref name=jackson/>{{rp|81}} Its relatively small eyes are unusual among marsupials in that the pupils have vertical slits,<ref name=captive/> an adaptation to living on a more vertical plane. Its round ears provide it with good hearing,<ref name=Clode/><ref name="Martin-Handasyde"/>{{rp|6}} and it has a well-developed middle ear.<ref name=Louysa/> The koala larynx is located relatively low in the vocal tract and can be pulled further down. They possess unique folds in the velum (soft palate), 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.<ref name=Frey>{{cite journal|last1=Frey|first1=R|last2=Reby|first2=D|last3=Fritsch|first3=G|last4=Charlton|first4=B. D.|year=2018|title=The remarkable vocal anatomy of the koala (Phascolarctos cinereus): insights into low-frequency sound production in a marsupial species|journal=Journal of Anatomy|volume=232|issue=4|pages=575–595|doi=10.1111/joa.12770|pmid=29460389|pmc=5835795|s2cid=3708255}}</ref><ref name="Charlton 2013">{{cite journal |author1=Charlton, B. D. |author2=Frey, R. |author3=McKinnon, A. J. |author4=Fritsch, G. |author5=Fitch, W. T. |author6=Reby, D. |title=Koalas use a novel vocal organ to produce unusually low-pitched mating calls |journal=Current Biology |volume=23 |issue=23 |pages=R1035–6 |year=2013 |doi=10.1016/j.cub.2013.10.069 |pmid=24309276|doi-access=free |bibcode=2013CBio...23R1035C }}</ref>
[[File:Machoires de koala avec dents.png|thumb|right|Teeth of a koala, from left to right: [[molar (tooth)|molars]], [[premolar]]s (dark), [[diastema (dentistry)|diastema]], [[canine tooth|canines]], [[incisor]]s]]
The koala has several adaptations for its low nutrient, toxic, and fibrous diet.<ref name=jackson/>{{rp|76}} The animal's dentition consists of incisors and [[cheek teeth]] (a single premolar and four molars on each jaw) that are separated by a large gap (a characteristic feature of herbivorous mammals). The koala bites a leaf with the incisors and clips it with the premolars at the [[Petiole (botany)|petiole]], before chewing it to pieces with the cusped molars.<ref name="Martin-Handasyde"/>{{rp|46}} Koalas may store food in their [[cheek pouch]]es before it is ready to be chewed.<ref>{{cite book|author1=Lee, A. L. |author2=Martin, R. W. |year=1988|title=The Koala: A Natural History|publisher=New South Wales University Press|page=20|isbn=978-0-86840-354-0}}</ref> 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,<ref name="Tyndale-Biscoe"/>{{rp|231}} which digests the eucalyptus leaves to provide most of the animal's energy.<ref name="Martin-Handasyde"/>{{rp|47}} A koala sometimes regurgitates the food into the mouth to be chewed a second time.<ref>{{cite journal|author=Logan, M.|year=2001|title=Evidence for the occurrence of rumination-like behaviour, or merycism, in the koala (''Phascolarctos cinereus'', Goldfuss)|journal=Journal of Zoology|volume=255|issue=1|pages=83–87|doi=10.1017/S0952836901001121}}</ref>

Koalas are [[Hindgut fermentation|hindgut fermenters]], and their digestive retention can last 100 hours in the wild or 200 hours in captivity.<ref name="Martin-Handasyde"/>{{rp|48}} This is made possible by their [[caecum]]—{{cvt|200|cm}} long and {{cvt|10|cm}} in diameter—possibly the largest for an animal of its size.<ref name=moyal/>{{rp|188}} Koalas can retain food particles for longer fermentation if needed. They are more likely keep smaller particles as larger ones take longer to digest.<ref name="Martin-Handasyde"/>{{rp|48}} 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,<ref name=jackson/>{{rp|77–78}} although this can vary across seasons and sexes.<ref name="Martin-Handasyde"/>{{rp|49}} They can digest the toxic [[secondary metabolite|plant secondary metabolites]], [[phenolic compound]]s and [[terpene]]s due to their production of [[cytochrome P450]], which neutralises these poisons in the [[liver]].<ref>{{cite journal |last1=Johnson, R. N. |display-authors=etal |year=2018 |title=Adaptation and conservation insights from the koala genome |journal=Nature Genetics |volume=50 |issue=8 |pages=1102–1111 |doi=10.1038/s41588-018-0153-5 |pmid=29967444 |pmc=6197426 |hdl=2440/115861 |hdl-access=free}}</ref> The koala replaces lost water at a lower rate than species such as some possums.<ref name="Tyndale-Biscoe">{{cite book |author=Tyndale-Biscoe, H. |year=2005 |title=Life of Marsupials |publisher=CSIRO Publishing |isbn=978-0-643-06257-3 |url={{google books|plainurl=yes|id=KqtlPZJ9y8EC}}|access-date=9 November 2015 |archive-date=23 January 2016 |archive-url=https://web.archive.org/web/20160123225012/https://books.google.com/books?id=KqtlPZJ9y8EC |url-status=live }}</ref>{{rp|231}} It maintains water by absorbing it in the caecum, resulting in drier faecal pellets packed with undigested fibre.<ref name="Tyndale-Biscoe"/>{{rp|231}}<ref name=moyal/>{{rp|188}}