The koala (Phascolarctos cinereus), often called the koala bear, is an arboreal herbivorous marsupial native to Australia. It is easily recognisable by its stout, tailless body and large head with round, fluffy ears and large, dark nose. While the term "koala bear" is commonly used, it's a misnomer, as koalas are more closely related to kangaroos, bandicoots, and possums than to true bears. The word "koala" itself is thought to originate from the Dharug language, meaning 'no drink' or 'no water', reflecting the koala's ability to obtain most of its hydration from eucalyptus leaves.
Distribution and Habitat
Koalas can only be found in the southeast and eastern parts of Australia, along the coastlines of Queensland, New South Wales, Victoria, and South Australia. They live in forests and open woodlands, typically dominated by eucalyptus tree species. Seventy percent of Australia is either arid or semi-arid land. The koala's range covers roughly 1,000,000 km2 (390,000 sq mi), and 30 ecoregions. Fossil evidence shows that the koala's range stretched as far west as southwestern Western Australia during the late Pleistocene.
Physical Characteristics
The koala has a body length of 60-85 cm (24-33 in) and weighs 4-15 kg (8.8-33.1 lb), making it among the largest arboreal marsupials. Koalas from Victoria are twice as heavy as those from Queensland. Males are typically larger than females, a phenomenon known as sexual dimorphism. The 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. 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. 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 Eucalyptus Diet: A Double-Edged Sword
Millions of years ago, koalas carved out a unique ecological niche by specializing in a diet of eucalyptus leaves. While over 600 species of Eucalyptus are available, the koala shows a strong preference for around 30. The most favoured species are Eucalyptus microcorys, E. tereticornis, and E. camaldulensis, which, on average, make up more than 20% of their diet. They prefer plant matter with higher protein than fibre and lignin.
However, this dietary choice comes with significant challenges. Eucalyptus leaves are toxic, low in nutrients, and high in hard-to-digest fibre and lignin. The Eucalyptus tree, much like many other plants, has evolved to be toxic to protect itself from being eaten by herbivores. To survive on this diet, koalas have developed several unique adaptations.
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Adaptations for Eucalyptus Consumption
Koalas possess special liver enzymes and symbiotic gut bacteria that help break down the poisonous compounds found in eucalyptus leaves. They are hindgut fermenters, with a long caecum in which the hard-to-digest leaf fragments ferment with the aid of a large quantity of microbes. Koalas can retain leaf fragments in their caecum for more than 100 hours in the wild, and up to 200 hours in captivity, allowing for more thorough digestion. The koala'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). While the hindgut is relatively large, only 10% of the animal's energy is obtained from digestion in this chamber. Koalas can retain food particles for longer fermentation if needed. They are more likely keep smaller particles as larger ones take longer to digest.
The "Pap" Diet and Microbial Inheritance
Baby koalas, known as joeys, rely on a special substance called "pap" to transition from a milk diet to solid food. This pap is a semi-liquid faeces produced by the mother, which the joey consumes directly from her cloaca. The consummation of the pap allows the baby koala to gain those microbes necessary to the digestion of eucalyptus leaves. This explains why, unlike most marsupials, the koala’s poach faces downwards, so that the baby can have an easier access to that pap.
The Impact of Diet on Koala Behavior and Physiology
The koala's specialized diet has a profound impact on its behavior and physiology.
Reduced Brain Size and Cognitive Abilities
The koala has a disproportionately small brain, 60% smaller than that of a typical diprotodont, weighing only 19.2 g (0.68 oz) 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.
One explanation for the koalas’ small brain is to do with their terrible diet. Eating such a poorly nutritious plant does not give the koala much energy. And brains need a lot energy, especially big brains. Therefore, koalas could have such small brains to preserve energy and not waste any of said precious energy on things like thinking and problem solving. They lack the capacity to adapt to new and unfamiliar situations, and they have a limited ability to learn new behaviours. For instance, koalas are only able to recognise leaves as food when those leaves are on branches. If those leaves are placed on a flat surface and then presented to the koala, the koala will not realise that those leaves are food.
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Sedentary Lifestyle and Sleep Patterns
The koala's low-energy diet dictates its sedentary lifestyle and extensive sleep patterns. They sleep up to 20 hours a day, wedged in the fork of a tree, limiting their energy loss. They eat for about 3 hours, usually at night. The remaining hour is dedicated to all the miscellaneous tasks in a koala’s life from moving to socialising. 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. On warm days, a koala may rest with its back against a branch or lie down with its limbs dangling. When it gets hot, the koala rests lower in the canopy and near the trunk, where the surface is cooler than the surrounding air. It curls up when it gets cold and wet. It resorts to a lower, thicker, branch during high winds.
Social Behavior and Communication
Koalas are asocial and spend just 15 minutes a day on social behaviours. In areas of higher density and fewer trees, home ranges are smaller and more clumped. Koala society appears to consist of "residents" and "transients": the former are mostly adult females and the latter are males. Resident males appear to be territorial and dominant. 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. Adult males occasionally venture outside their home ranges; when they do, dominant ones retain their status.
Adult males communicate with loud bellows-"a long series of deep, snoring inhalations and belching exhalations". Because of their low frequency, these bellows can travel far through the forest. Koalas may bellow at any time, particularly during the breeding season, when it serves to attract females and possibly intimidate other males. They also bellow to advertise their presence when they change trees. These sounds signal and exaggerate the male's body size; females pay more attention to bellows by larger males. 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. Younger animals squeak and older ones squawk when distraught. When another individual climbs over it, a koala makes a low closed-mouth grunt. Koalas also communicate with facial expressions.
Reproduction and Development
Koalas are seasonal breeders, and they give birth from October to May. Females in oestrus lean their heads back and shake their bodies. Despite these obvious signals, males 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. Koalas are induced ovulators. The gestation period lasts 33-35 days, and a female gives birth to one joey or occasionally, twins. The young are born tiny and barely formed, weighing no more than 0.5 g (0.018 oz). However, their lips, forelimbs, and shoulders are relatively advanced, and they can breathe, defecate, and urinate. The joey latches on to one of the female's two teats and suckles it. 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. 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 doubles in size. The eyes begin to open and hair begins to appear. At six or seven months, 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. 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. 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. Gradually, it becomes more independent. The mother becomes pregnant again after a year after the offspring reaches around 2.5 kg (5.5 lb).
Conservation Status and Threats
Koalas are listed as a vulnerable species by the International Union for Conservation of Nature. At present, koalas face multiple threats in the wild, from deforestation, drought, disease, habitat loss, fragmentation, and modification to climate change. The WWF report, entitled Australia’s 2019-2020 Bushfires: The Wildlife Toll, revealed the true scale of devastation left in the aftermath of the Australia's 2019-20 bushfires. In January 2020, the WWF UK bushfires appeal raised over £3.2 million as thousands of supporters signed up to become koala protectors and gave generous donations to help fund the recovery effort.
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