As a rule, he selects a comfortable low fork in a manna gum as sleeping quarters during the daytime. Being mainly nocturnal, the koala usually sleeps or dozes away the hours between dawn and sunset, but often wakes up for a snack. When the supply of tender leaf shoots fails, a move is made to a neighbouring tree.1
Despite appearances, the koala's life is not an easy one. Its preferred food is so tough and abrasive to chew that its teeth are eventually worn down to stumps. The same food is also so toxic that it would kill most other animals and has so little nutritional value that, quite simply, the koala has not enough energy to do more than sit in its tree. How then, has the koala evolved to live in a diverse range of habitats throughout a large distribution and to flourish to a comparatively ripe old age? To better understand the koala we need to take a closer look at the ecology of the animal that Serventy and Serventy call a 'triumph of evolution'.2
Koalas have long been known as fussy eaters. Of the 600 or so species of eucalypts in Australia, the koala has been observed to feed upon or found sitting in only 120 species, but only 14 of these can be considered to be primary food sources. It has also been observed in less than 30 non-eucalypt trees, including Acacia, Allocasuarina, Banksia, Callitris, Hakea, Leptospermum and Melaleuca.3 As limited as the diet of the koala appears, when compared to other marsupial species, some biologists consider the koala to be a 'generalist' feeder. The greater glider, for example, is even more selective in its choice of eucalypt leaves. Koalas feed primarily on a subgenus of Eucalyptus called Sym-phyomyrtus that includes some 400 species, while greater gliders feed on eucalypts of the subgenus Monocalyptus, of which there are only 150 species.4
Eucalypt leaves contain a wide variety of compounds. Some, such as water and protein, are essential, but most are anything but. Steve Cork, from CSIRO's Division of Wildlife and Ecology in Canberra, summarised the digestibility of eucalypt leaves when he wrote that they contain only subsistence levels of protein and high concentrations of tannins.5 Eucalypt leaves are also very high in lignified fibre (which makes them difficult to digest) and contain strong-smelling oils and cya-nidic compounds (i.e. substances that are converted through the metabolism process into cyanide) that make them not only unpalatable but poisonous to most mammals. On the positive side, however, the leaves have a high water content, which explains why the koala has little need to drink.6
If we were to try to eat what a koala does in a single day, we would be very ill and perhaps even die. The available information on the toxicity of eucalypt leaves appears to be limited to anecdotal reports of extracted Eucalyptus oils, whose major component is a compound called 1,8-cineole or eucalyptol. Overdoses of the oil in humans cause gastrointestinal burning, vomiting, dizziness, convulsions, diarrhoea, depressed respiration, lack of coordination and seizures, and can result in coma and death.7 It is difficult to establish a fatal dose, however, as children are known to have died after ingesting as little as 3.5 millilitres, with major poisoning also having been observed after doses of 5 to 10 millilitres. In contrast, there are cases of people being ill but recovering after consuming as much as 120 to 220 millilitres.8
The role of chemicals such as tannins in plants is hotly debated, with many believing that they form part of an 'arms race' between plants and animals. As animals develop ways of getting around plants' defences, so the plants evolve better defences, until the plants become highly toxic and the animals have to develop advanced metabolic mechanisms in order to neutralise these toxins. Such arms races are generally considered primarily to be between plants and insects, with mammals being caught in the crossfire. However plants may also be targeting mammals, as insect outbreaks typically are limited to certain times of the year, whereas mammals such as koalas consume foliage all year. Also, some mammals can eat entire plants in a single mouthful. Eucalypt foliage offers an abundant food source, but the koala has had to evolve a number of physiological strategies in order to tolerate its low nutritional value, high toxicity and high proportion of dietary fibre.9
Eucalypt foliage is so high in fibre that the koala's stomach is quickly filled, thus reducing the amount of food that can be consumed. A high-fibre breakfast of wholegrain bread or muesli will fill your stomach and give you plenty of energy for the day ahead. Eucalypt leaves, however, have three to six times more fibre than muesli, but much less energy.10 To make matters worse, the smaller the animal, the more energy per kilogram of body weight it needs to consume. Dietary fibre is composed of sugars linked together in long, intertwined chains by a compound called lignin, hence 'lignified'. The dietary enzymes of koalas and other herbivorous mammals such as kangaroos and cattle are powerless against this fibre. For this reason, the koala has a very different intestinal system to that of other non-herbivorous mammals. The koala's large intestines are full of bacteria and other micro-organisms that break apart the leaves' dietary fibre by a process akin to that of fermentation. The large intestine also includes an enormous caecum—like a human appendix but much larger—that helps the koala to digest its food. This fermentation process produces alcohol, however in amounts too small to have an intoxicating effect, so the popular conception that the koala's lack of energy is due to it being permanently under the influence is completely incorrect.11
The koala gets the bulk of its energy requirements through the digestion of cell contents in the stomach and the small intestine. This means that it does not have to rely on bacterial fermentation of cell wall cellulose in its hindgut, which is surprising, given that the koala's hindgut (which includes an enormous caecum and proximal colon) is proportionally the largest fermentation chamber of any herbivore. The digestion of cellulose in the hindgut provides only about 10 per cent of the
Digestive system of the koala showing the enormous caecum. (Taken from Lee & Carrick (1989))
koala's overall daily energy intake, and much of the indigestible material is rapidly excreted.12
The mass of dietary fibre is sorted by the large intestine. Only the smaller particles are retained for fermentation, the largest and therefore least digestible being excreted from the gut. Unfortunately this type of fermentation process releases energy slowly and, in the case of eucalypt foliage, very slowly. The koala gains such a limited amount of energy from its food that it has evolved a metabolic rate half that of other mammals. This allows it to survive on a very low intake of protein and energy, but the koala is delicately balanced between being the minimum size for its liver and digestive system to cope with such a nutritionally poor diet and the maximum size for it to have enough mobility actually to gather its leaves, hence its slow movements.13 The koala's metabolism is so slow that it must remain inactive for some 20 hours per day!
As the fermentation process releases energy slowly strategies have been developed to maximise the efficiency of the koala's digestive system including making the particles of leaf ingested as small as possible. This is why the koala's molars are high crowned and ridged, to allow it to grind its food as finely as possible. In addition to chewing its food thoroughly before it swallows, the koala has been observed to regurgitate the contents of its stomach and chew them again. This process is known as merycism, and is similar to a cow's chewing the cud.14
Molar tooth of a young koala, showing the cusps that help to grind its food. (Taken from Lee & Carrick (1989))
Research by Janet Lanyon and Gordon Sanson from Monash University has revealed that the surface area of the molars' chewing surface and, therefore, their efficiency, increases as the koala gets older. At approximately six years old, however, the koala's teeth pass the point of maximum surface area and become increasingly smooth. This means that they become increasingly less efficient grinders and the ageing koala consumes an increasing proportion of large leaf particles that are difficult to digest, so the nutritional value of its food steadily decreases. Lanyon and Sanson also found that, on a daily basis, koalas with the highest degree of tooth wear were consuming 41 per cent more leaves and masticating that leaf material
25 per cent more than koalas with the lowest degree of tooth wear. In other words, an old koala must eat considerably more, and chew its food considerably longer, than a young koala. This only hastens the wear on its teeth and, ultimately, will result in complete nutritional failure and starvation.15 One of the most common causes of mortality in elderly koalas, apart from predator attacks, dehydration, heat exhaustion or disease, is starvation.
The koala's low metabolic rate also helps explain the animal's disproportionately small brain. Most mammals have a brain equal in volume to the available space in the skull. The koala's brain occupies only 61 per cent of this space and is unusual in that the cerebral hemispheres are smooth.16 In his classic book, The Future Eaters, Tim Flannery explained that the koala's unusually small brain size is an adaptation for conserving energy, as the brain (and the liver) has the highest energy requirements of any organ in the body.17 The size of the koala's brain may mean that it cannot be considered an intellectual giant, but on the other hand, it can only be advantageous for an animal who lives high in swaying eucalypt branches to have a brain protected by an insulating layer of fluid in the event of a fall.
The koala might have one of the smallest mammalian brains, but that is not to say that its senses are undeveloped. Koalas have been observed smelling individual branchlets of leaves before deciding whether or not to eat them, and research has shown that they can detect the foliage's chemical components by scent alone. The fact that the koalas smell the leaves suggests that they are using the leaves' volatile and scented oils as a cue for their other constituents. Koalas have also shown marked preferences within species at different times of the year. One species that has been the particular focus of researcher's attention is the manna gum, and the first person to explore the foliage's changing toxicity was Ambrose Pratt who recorded that although the leaves were the staple food for Victorian koalas for most of the year, at certain times of the year, juvenile and young leaves were capable of producing highly toxic hydrocyanic acid.18 Pratt's anecdotal records are supported by a study of leaves from a plantation in Western Australia. Local koalas ate the manna gum leaves for most of the year, except during the wetter months of July and August, when they tended to choose other leaf sources. Tests showed that the manna gum leaves contained increased levels of cyanidic compounds during these months.19
Many observers have suggested that the koala prefers younger leaves to older ones, but this does not appear to apply to all trees and, as we saw above, young manna gum foliage can contain increased levels of toxic compounds. To date, the only long-term study to assess the proportions of young and old eucalypt leaves in the wild koala's diet is that conducted by U Nyo Tun who found that over a 12-month period, four rehabilitated koalas ate considerably more mature foliage than young foliage.20 The main criterion for leaf selection, rather than age, seems to be water content. Younger leaves contain more water and nitrogen and less fibre than mature leaves, but they can also contain more phenolic compounds that the koala would prefer to avoid. The koala will not eat any leaves, regardless of their age, that contain less than 55 per cent water, which perhaps reflects the koala's own minimum water requirements during warmer weather.21Alistair Melzer obtained similar results when studying koalas in central Queensland. He found the animal's threshold water content was 51 per cent in the cooler months but this increased to 63 per cent in the hot dry months of summer.22 The water needs of the trees themselves appear to modify the koalas' habitat utilisation and diet, perhaps in response to their own metabolic water needs.23
Despite the anomalies, the koalas' observed preference for younger foliage seems to be borne out by studies at San Diego Zoo, which found that koalas offered foliage from 11 species of eucalypts preferred younger leaves. These leaves had significantly higher concentrations of crude protein, non-structural carbohydrates, phosphorous, sodium and potassium, and lower concentrations of lipid cell-wall polysaccharides, lignin, calcium and selenium than the leaves the koalas rejected.24 Researchers at San Francisco Zoo found koalas' leaf consumption to correlate positively with higher nitrogen levels and lower fibre content in the leaves.25 These studies of captive koalas and several other stud-ies26 suggest that nitrogen is a limiting factor so that a nitrogen threshold might have a valuable role to play in determining koala diet selection and in turn their broader distribution.
As yet, however, no study has identified a single factor that will consistently predict koala leaf preference either across euca-lypt species or between individual trees. Until recently every attempt to identify the koala's food trees on the basis of either leaf chemistry or environmental characteristics failed to provide a full explanation of food tree preferences. William Foley and his team at the Australian National University in Canberra took a different approach. They discovered that certain eucalypt phenol compounds known as formylated phloroglucinol compounds (FPCs) adversely affect the leaves' palatability and have a major influence on koala food tree selection. They found the koala's food preferences to correlate closely with lower concentrations not only of FPCs, but also of volatile terpenes such as 1,8-cineole which co-varies with FPCs. As koalas feed almost exclusively on eucalypts from the subgenus Symphyomyrtus and FPCs occur at variable concentrations in all species of this subgenus, this is a major factor affecting food choice not only in captivity, but also in the wild where most of the trees koalas feed upon have moderate levels of FPCs.27 Other factors that appear to influence the koala's preferences for different eucalypt plants include tree size, higher nitrogen levels, and probably the animal's changing need for, and access to, water.28 Such a hypothesis is reinforced by studies in semi-arid regions, where koalas have been observed to select leaves with a higher than average water content.
So it would appear that the koala is an extreme example of mammalian evolutionary adaptation to overcome plant antiherbivore defences.29 Not only has the koala evolved to gain the maximum possible nutritive value from a toxic, indigestible food source, it has also learned to recognise and avoid those eucalypts which contain sufficiently high concentrations of FPCs to make it, or any other marsupial who tries to eat them, very sick.30
So important are the components of the leaves the koala eats that, generally speaking, its distribution is limited to regions with better quality soils.31 However, this may not be linked to the concentrations of FPCs in the foliage, as shown by studies on tallowwood carried out by Ben Moore and his colleagues. They found that trees growing in poor-quality soil had lower levels of FPCs than those enjoying high-quality soil.32 This would seem to suggest that the koala's criteria for selecting high-quality soil are greater quantity of foliage, higher leaf nutrient levels and higher water content.33 Throughout its distribution the koala's population densities appear to reflect soil quality, with higher densities in good-quality soils and low densities in poorer soils. In New South Wales, the koala is found in regions such as Grafton, Coffs Harbour, Port Macquarie, Newcastle and the Bega Valley, all of which have good-quality soils. Unfortunately, the higher-quality soil in these regions makes them prime agricultural land and the resulting vegetation clearance has had a significant impact on the koala's distribution, and is an example of a direct conflict between the needs of the koala and those of humans.
We have seen that the species of eucalypt, the chemical content of the individual leaf, the aridity of the region and soil quality all impact on the koala's food preferences, but how much does a koala eat? A simple question, but one that is not easy to answer. Various studies have tried different ways in which to establish the koala's daily food intake and found it to vary between 14 and 71 grams of foliage per kilogram of body weight. However, these animals were fed for less than 24 hours and were not necessarily given very palatable food.34 After checking literally hundreds of food consumption records, Ben Moore concluded that a ten-kilogram koala typically would eat between 600 and 800 grams of fresh leaves per day.35
The amount of food a koala consumes is governed by more than its body size. As we saw above, increasing tooth wear means that an older koala has to eat more leaves than a younger animal in order to gain the same amount of energy. The koala also has to eat more in colder weather, to have enough energy to keep warm, and the female koala has to eat more to compensate for the increased demands made on her body by a long period of lactation. Dr Andrew Krockenberger discovered that free-ranging lactating females ate 36 per cent more leaf than non-lactating females. He also discovered that female koalas selected leaves in some months that contained less total phenolic compounds and a higher ratio of nitrogen to total phenolic compounds.36
Lactation places considerable demands on the female koala, since the joey spends up to eight months in its mother's pouch, being fed exclusively on milk. The female koala gives birth to only one, or very rarely two, young after a gestation period of just 34 to 36 days. At birth, the neonate is the size of a jelly bean, only 19 millimetres long, and weighs less than half a gram. The neonate travels up to its mother's pouch from her cloaca using only its innate sense of smell and tiny forelimbs. Once in the pouch, it stays attached to one of its mother's teats for five to six months. During this period the female cleans the joey and pouch, including waste products, by licking it. When it is about six months old, the joey is fully furred and sticks its head out of its mother's backward opening pouch for the first time. During the 6—8 months the joey spends in its mother's pouch, the components of the female koala's milk change. Initially, the milk is high in carbohydrates and low in lipids and protein but as lactation continues the level of carbohydrates decreases steadily, while those of protein and, especially, lipids increase.37
The joey's first solid food is semi-liquid faeces ('caecotrophs') called pap, which it eats direct from its mother's rectum. Pap is a green, jelly-like substance that contains viable bacteria and is much higher in water content (at about 80 per cent) than normal koala faeces (around 50 per cent). Pap appears to be derived directly from the contents of the mother's caecum and is thought to inoculate the joey's alimentary tract with symbiotic bacteria. This allows the joey to make the transition from its mother's milk to the fibre-rich eucalypt foliage.38 The pap appears to be generated on demand. The joey leans its head out of the pouch with one or both forelimbs and begins to nuzzle its mother's cloaca, causing her to defecate. The joey ignores the normal faecal pellets but after 30—60 minutes (depending on when she last defecated) the character of the faeces begins to change, from pellets to relatively unformed faecal matter, which gradually becomes more and more liquid. The joey ingests this pap eagerly for over an hour, using biting motions and pausing only to chew and swallow. The joey's consumption of pap lasts from several days to up to a week and marks the beginning of its transition from being permanently confined within its mother's pouch to spending more and more time out of the pouch.
The joey's first teeth appear at the same time as the weaning process begins. When the joey first leaves its mother's pouch, at about seven months old it is, effectively, born again as it is now at a similar developmental stage to a placental mammal newborn. The joey clings to its mother's stomach or back as she eats and travels around, but returns to the pouch to feed. By the time it is nine months old, the joey has permanently left the pouch, and it is fully weaned at approximately 12 months old, at which time its mother can become aggressive towards it. After weaning, the young koala will often occupy a 'home range' within its mother's territory until it is two years old (we will look at home ranges in more detail in the next chapter). From two to four years of age, most young koalas, male and female, move away from their mothers to seek their own territories. The joey's unusually long dependent period means that female koalas tend to breed every second year, although they can breed every year if they are in favourable locations where they have high-quality food.
The lactating female koala, like all koalas, sources her foliage within her home range, and the optimal foraging area needs to be just large enough to yield an adequate food supply but not so large that the koala wastes energy unnecessarily or is at increased risk from predators. Researchers have found that within their home ranges, koalas exhibit obvious preferences for certain species of eucalypts. In the Brisbane Ranges, north of Melbourne, for example, Mark Hindell and Anthony Lee found a clear preference for the manna gum, swamp gum and red stringybark.39 Home ranges can vary enormously in size, as can be seen in the excellent review by Alistair Melzer and Wayne Houston. They showed that in Victoria, home ranges can cover less than a hectare for both male and female koalas, as opposed to 135 hectares in central Queensland for male koalas and 101 hectares for females. These figures reflect the significantly lower density of nutritional food in Queensland and correspondingly lower density of animals.40
As we have seen, the koala's physiology and behaviour has evolved and adapted to allow it to thrive on an exclusive diet that is not only almost devoid of nutritional value but also high in toxic compounds. As the koala gets older, it has to spend more and more time and energy gathering and chewing its food, just to survive. So how long does a koala live? There are few detailed studies of longevity in the wild, although an average life expectancy would appear to be approximately 12 years for males and up to 15 for females. A female tagged on French Island was still breeding at ten years of age and one male at Walkerville, Victoria, was estimated to have been 16 years old when it died. There are also records of females living for 17 to 18 years.41 As we shall see in the following chapters, various factors determine how long a koala lives, most of which, unfortunately, are the result of negative interaction with humans. Apart from starvation as a result of tooth wear, many koalas succumb to disease and predators such as dingoes, dogs, wedge-tailed eagles, powerful owls and goan-nas.42 Others fall victim to drought and fires. As the koala's physiology has evolved to compensate for its toxic, indigestible diet, so its behaviour has evolved to protect it from some of the threats it faces in the wild.
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