Australia: The Land Where Time Began
Of the great variety of large herbivores that evolved in Australia to fill the vacant niches opening up as grasslands replaced rainforests in the Miocene, the wombat is the only survivor. They are bulk feeders, fermenting their food in their large colon. It is believed the large marsupials, such as diprotodonts, also fermented their food in the colon. The kangaroos are also large herbivores that evolved to take advantage of the expanding grasslands, but they ferment their food in a forestomach, and not in the colon. There are 72 species of kangaroos but only 3 surviving species of wombat. The same decline of the hindgut fermenteers that is seen among the marsupials, Phascolonus gigas, Phascolonus medius, and possibly the diprotodonts, etc., occurs in the placentals such as the horse, rhinoceros, etc. The foregut fermenting macropods flourished, as did the placental foregut fermenters, such as the antelopes, cattle, etc.
Ancestral wombats are distinguished from other marsupial grazers that occurred in the Miocene by their possession of a single pair of incisors in the upper jaw and teeth with open roots (Murray, 1998). Found in fossil deposits from 23 Ma, the earliest known of the wombat line was Rhizophascolonus, from the Late Oligocene. From the Later Miocene, all later species have open-rooted teeth. Warendja, the smallest species, was the least specialised, and occurred from the Late Miocene to the Pleistocene. As a result of its dentition and burrowing adaptations, this animal resembled a very large rabbit. Giant wombats arose 5 Ma in the Pliocene, at around the same time as the diprotodonts and giant kangaroos. Ramsayia and Phascolomys medius roamed over large parts of Australia, from south and central parts of the continent to north Queensland and western New South Wales. The giant wombat, Phascolonus gigas, had an even wider distribution that included Tasmania and Western Australia.
The Pleistocene, about 2 Ma, was the time when the living species of wombat, of the Vombatus and Lasiorhinus genera, first made their appearance. The largest of the living species, the common wombat, Vombatus ursinus, from the moist forests of southeastern Australia, reaches a size of up to 40 kg. It is restricted to altitudes above 600 m at the northern limit of its range. In the southern parts of its range, it ranges from sea level to the tree line in alpine regions. In the past it had occupied areas throughout southeastern Australia, extending to the Murray River in South Australia and Queensland in the north, as well as Tasmania and several Bass Strait islands. Hackett's wombat, Vombatus hacketti, was present in western Australia until about 40,000 years ago, indicating that a common ancestor probably extended all across the southern parts of the continent. Both species are believed to have been restricted to the more southerly regions as the climate warmed over the last 12,000 years, the western species going extinct when its habitat changed too much or too fast for it to adapt to the new conditions. The living species may have moved to higher altitudes at the northern limits of its range as a response to the same climatic changes.
The southern hairy-nosed wombat, Lasiorhinus latifrons, weighing 32 kg, and the northern hairy-nosed wombat, Lasiorhinus krefftii, weighing 35 kg, both had a much larger range in the past, that took in much of the central areas of the continent. At present, the southern species is restricted to small isolated populations in South Australia and the northern species to similar sized populations in Queensland. The competition for native grasses with introduced stock and rabbits has reduced their range even further. The northern species now survives as a single population of 100 individuals in a reserve. It is not as well known as the other 2 extant species.
It has been suggested (Johnson, 1998) that the habit of burrowing may have saved the 3 wombat species from extinction when the rest of the hindgut fermenters, as the aridification of the continent progressed over the last 2 million years, possibly developing the habit as a response to the deteriorating conditions during that time. They are the only large grass-eating animal to burrow. Among the animals in the range of 10 g- 10 kg body mass there are many burrowers. The other larger animals that burrow are either carnivores, such as the badger, or armadillos, pangolins, etc. that eat colonial insects, and porcupines that eat roots, tubers and fallen fruit (Tyndale-Biscoe, 2005).
The energy required to dig a burrow increases by the cube of the soil volume removed, while the muscles used in digging increase by the square power. As a result of the requirement for digging, the diggers have short, powerful limbs for digging that require a lot of energy from food. In all except the wombat, a grass eater, this energy comes in a concentrated form from the highly nutritious insects, but grass is far less nutritious, and requires large amounts to produce the necessary energy. Another feature of the wombat's lifestyle that makes it an unlikely burrower is the need for such a large animal to cover a wide area to obtain enough grass for its needs, its short limbs not being the best way to cover such large distances, especially when at the end of each foraging trip it must return to its burrow. Presumably there was a strong selection pressure for it to burrow while retaining it food preferences.
The tough native perennial grasses and sedges have been shown to be the preferred food of all wombat species, though when food is scarce they will eat introduced pasture grasses and forbs, leaves of woody shrubs, eucalypts such as mallee (Hume & Barboza, 1998). When occupying eucalypt woodland, the common wombat prefers species of Poa, Danthonia and Themada, as well as Lomandra, a rush, found in a home range that is usually of about 20 ha. Stipa nitidia, growing around the main warren complex, is the main food of the southern hairy-nosed wombat. The grass near the warren complex is close-cropped, which induces it to sprout new shoots that are preferred by the wombat. The home range of this species is about 4 ha, much smaller than the home range of western grey kangaroo, Macropus fuliginosus, that inhabits the same type of environment. As the plant growth declines and the wombats need to range further to get enough food, these 'grazing halos' around the warren complex increase in area. The northern hairy-nosed wombat also prefers tough native perennial grasses, preferring species of Heteropogon, Enneapogon and Aristida. It usually has a core area of about 4 ha, and a total home range of about 25 ha. The metabolic requirements of wombats appear to be much less than those of other herbivores, as indicated by the comparatively small size of their home range. This would also explain one of the constraints of burrowing among large herbivores (Tyndale-Biscoe, 2005).
Dentition for bulk plant food
The adaptation of the wombat to the processing of tough plant material is seen in its teeth and the structure of its skull. The single pair of upper incisors, set transversely, form a sharp cutting tool against the lower incisors. The split in the upper lip allows the animal to crop the grass very close to the ground. There is a large diastema, a gap, between the incisors and the cheek teeth, as canines are completely lacking. The cheek teeth are comprised of 1 premolar and 4 molars in each jaw. The open rooted teeth are all long and curved, and continuously growing, as occurs in rabbits and rodents. The upper molars, that curve outwards, meet the lower molars, that curve inwards, very close to the midline of the skull (Murray, 1998).
The molars are 4-lobed in young wombats, similar to the condition in koalas, Phascolarctos cinereus. Teeth of the 2 species differs in that the teeth of the wombats the enamel quickly wears off the crowns to expose the dentine. The dentine wears down faster than the enamel at the side of the teeth, which causes the enamel of the inner curvature of the teeth to form a strong, sharp edge, while on the outer curvature a thinner crest.
The blades of the upper teeth meet those of the lower teeth as the lower jaw moves upwards and sideways as the animal chews with powerful strokes to cut the grass stems, the resulting pieces of stem being held in the opposing basins as pressure is applied, being sheared off as the ridges pass each other. This chewing is powered by large jaw muscles that are highly specialised, chewing occurring on alternate sides by using only the muscles on that side. Originating on the zygomatic arch under the eye socket are very large masseter muscles, that insert on the back end of the lower jaw. The molar row is set near the midline, and the zygomatic arch is set well away from the midline. As a result of this arrangement, a very strong lateral force is exerted in the lower jaw by the masseter muscles, as well as the compression force. The broad flat appearance of the wombat skull is a result of this arrangement of chewing muscles and skull structure.
Similar features are seen in large rodents, such as marmots, but not in other grass-eaters such as kangaroos that have a different arrangement of teeth and jaw muscles. The extinct wombats from the Middle Miocene had a similar arrangement of teeth and skull structure, indicating that they probably also ate similar plants such as tough grasses.
Adaptations for coarse food
The southern hairy-nosed wombat grinds its food to about half the particle size as the western grey kangaroo when both species are grazing on the same plant species (Wells, 1989), making more of the cell contents available for digestion. The simple stomach digests proteins and soluble carbohydrates, after which they are absorbed in the small intestine, that is relatively short (Hume, 1999). The protein needs of the wombat are provided by this process, and bacterial fermentation of the residual plants material, mostly cell walls, provides much of its energy needs. The fermentation takes place in the hindgut, as occurs in the koala and the greater glider, Patauroides volans. It differs from those species in its caecum being very small, the fermentation taking place in the colon, the largest part of the wombat gut.
The colon is divided into 2 parts, the fermentation takes place in the anterior portion, and water is reabsorbed in the posterior section. The southern hairy-nosed wombat is desert adapted, having a small fermentation chamber and a large water reabsorption posterior section. In the common wombat, a forest dweller, the proportions of the colon sections are reversed, probably because it has less need to conserve water. In the southern hairy-nosed wombat, fluid takes 49 hours to pass through the gut, particulate matter taking 70 hours, much slower than in the eastern grey kangaroo, where the times are 15 hours and 35 hours, respectively (Hume, 1999), the opposite of the koala. The slowly moving food passing through the large capacity colon allows a longer time for the bacteria to hydrolyse the cellulose to short chain fatty acids, SCFA, which is then absorbed into the bloodstream.
Urea is recycled by being excreted into the gut, bacteria in the colon then utilise it to synthesise their own protein. The same process occurs in the koala and the common ring-tailed possum. As with these animals and the macropods, urea recycling reduces the amount of water required to excrete it in the urine. The wombat does not digest the bacteria, as the macropods do, or as the common ringtail possum does, by re-ingesting caecal contents. The wombat benefits by the urea that is recycled leading to greater proliferation of the bacteria, which increases the rate of fermentation, making more SCFA available, and conserving nitrogen, that is at low levels in the preferred food of wombats. The result is that the wombat can thrive on food nitrogen levels that are very low. Figures for the common wombat and the southern hairy-nosed wombat are 75 mg/kg0.75/day & 116 mg/kg0.75/day respectively. These amounts are lower than any herbivorous placental mammal and at the lower end of the marsupial range. Wombat urine is not unusually concentrated, yet their water turnover rate is among the lowest of any herbivorous mammal (Wells & Green, 1998). This is achieved by a high rate of resorption of water from the colon, the dung water content going as low as 40 %, reaching the lowest levels in summer.
Southern hairy-nosed wombats are able to maintain body mass while eating very low quality food, even in times of drought, as a result of their levels of conservation of water, nitrogen and energy. Because of the lower metabolic rate and slower passage of food through their gut, they are able to survive on low quality food better than the donkey, which is a placental hindgut fermenter that has adapted to live in similar conditions. When temperatures rise above 30oC the wombat becomes distressed, but the donkey has evolved to live in the open where it handles the high temperatures better.
The burrowing habit of wombats has allowed them to survive in the hot, dry environment of their normal habitat, avoiding ambient temperatures outside their thermoneutral zone (Tyndale-Biscoe, 2005). They can remain in their burrow for longer periods during drought, which conserves energy and water. In cooler weather they conserve energy by basking in the sun, as reptiles do, using the sun to raise their temperature, saving the energy that would otherwise be used to warm their bodies. All wombat species cannot tolerate high temperatures, moving deeper into their burrows when the temperature increases (Brown, 1984). A study of the common wombat has found that when a forest was clear-felled and burnt the wombats remained in their burrows and all survived (McIlroy, 1973). The burrows provide protection from forest fires in their normal environment (Tyndale-Biscoe, 2005).
Wombats in captivity were found to have a standard metabolic rate (SMR) of 130 kl/kg0.75/day, which is 64 % of the marsupial average, and 42 % f the average for placentals (Wells, 1978; Wells & Green, 1998). The thyroid hormone levels for wombats are the lowest found in any mammal. These hormone levels reflect the SMR. The common wombat living in good conditions was found to have an SMR close to that of the marsupial average, 210 kl/kg0.75/day (Gowland, 1973). A study of the field metabolic rate (FMR) of the southern hairy-nosed wombat, based on water turnover and the water content of food plants, comparing the daily minimal energy requirements and the daily grass intake, found that the forage consumed contained 2.8 times the energy required for maintenance. This ratio between the SMR and the FMR is very similar to that found for the koala. If there is enough food available the southern hairy-nosed wombat was found to gain enough energy from its preferred diet of perennial grasses to support late stage lactation.
The burrow allows wombats to survive in their habitat, regardless of the extremes of temperature and humidity, even surviving droughts and the inevitable bushfires. Disadvantages of the burrowing habit are the energy costs of digging and maintaining it, and the limited distance the wombat can forage around the burrow, as it needs to return to the burrow before daybreak.
The first detailed description of a wombat burrow was by a 14-year-old boy, Peter Nicholson, who crawled down a common wombat burrow, reporting his findings in his school magazine (Nicholson, 1963). In his thorough description he found the burrow was up to 18 m long, with short branches, and a number of nest chambers, some of which were lined with bracken and eucalypt leaves. He also found that the wombat moved deeper during the day and gradually back towards the entrance as light dimmed (in Tyndale-Biscoe, 2005).
Later studies of the common wombat found that the burrows were about 20 m long, most of the large burrows had no branches, but had 2 or more resting chambers. The burrows often had a resting chamber about 2 m from the entrance where the animal rested until the conditions were right for it to emerge. Most burrows of the common wombat are dug in sloping hillsides near streams, so they are usually much deeper than those of the 2 species of hairy-nosed wombat. The burrows of both species of hairy-nosed wombat are usually dug in flat ground, leading off from a crater. The burrows of the southern species usually reaches a depth of about 2 m, and those of the northern species about 4 m.
When studying the burrows of both species of hairy-nosed wombats it was found that most burrows had only 1 entrance and their lengths ranged from about 4-28 m. Larger warrens were found to be more complex, with up to 28 entrances in the largest, with a total tunnel length of 89 m, often on several levels. Soil type demined the depth of the tunnels, 1 m in firm soil and up to 2 m in sandy soil (Shimmin et al, 2002).
The warrens of northern hairy-nosed wombats were found to be simpler, of 5 warrens studied 4 had 1 entrance with tunnels reaching 10-15 m, while the 5th had 3 entrances and 54 m of tunnels. All were deeper than the tunnels of the southern species, reaching depths of up to 4 m (Shimmin, 2001).
The temperature in burrows of southern hairy-nosed wombats was found to remain about 14oC in mid winter and 26oC in mid summer, though the air temperature above ground varied between 2oC and 38oC, both temperatures being within the thermoneutral zone of wombats. More then 3 m from the entrance the temperature remained constant, regardless of often large variations outside the burrow throughout the day (Shimmin et al, 2002).
Airflow in complex burrows varied only slightly, and in simple burrows with a single entrance it was negligible, which led to a higher humidity than outside the burrow, a few metres from the entrance, rising progressively in from the entrance, to almost saturation after some distance along the tunnel. This high humidity contributed to water conservation. The lack of temperature variation along the burrow has led to the belief that it is probably the increasing humidity with depth that determines the depth of a burrow being greater than 3 m (Tyndale-Biscoe, 2005).
When wombats return from forging for the night they move to deep within the burrow. The next night they move towards the entrance, but if the temperature is too high or the humidity is too low they move back deeper in the burrow. When the ambient temperatures are low in winter they remain about 3 m from the entrance until sunset (Wells, 1978). This pattern was confirmed by a study over 20 years in which swinging flaps were placed over all entrances of a number of warren complexes. It was found that the wombats emerge at night when the temperature outside the burrow matches that of the burrow, returning when the ambient temperature is still below the burrow temperature in the early morning. This indicates that wombats avoid the temperature extremes by exploiting the microclimate in their burrows, always being inside their thermoneutral zone, and conserving water and energy in the process (Taylor, 1998).
In closed wombat burrow that are unoccupied the oxygen and carbon dioxide levels are the same as in the air outside the burrow, diffusion being sufficient to keep the air inside and outside in equilibrium. When a burrow is occupied, the carbon dioxide level rose from the normal 0.04 % of air to 2.6 %, oxygen falling from 21 % to 16 %. Wombats have a much higher tolerance of low oxygen and high carbon dioxide levels than non-burrowers, but when digging in the far end of their tunnels, they emerge to 'get some fresh air' after about 5 hours of digging (Frappell et al, 2002). Being able to burrow for such a long period in such atmospheric conditions is believed to be at least partly made possible by their low metabolic rate, especially in their tolerance of such high CO2 levels.
It has been calculated that the volume of soil removed in the construction of a large warren was about 8-13 m3, a subadult being observed digging a 4 m deep burrow in 30 minutes (Steele & Temple-Smith, 1998), which amounted to about 1 tonne of soil, the feat being more extraordinary as the soil removed near the end of the tunnel needed to be pushed out along a section of pre-existing tunnel. It is assumed that this particular wombat was working at top speed as it was trying to escape from the destructive excavation of the tunnel behind it. The greatest length of new tunnel observed in an undisturbed warren was 10 m in 4 weeks (Shimmin et al., 2002). Once a burrow is completed, the animal usually makes only minor changes and enlargements. It has been estimated that the actual volume of a burrow is greater than the volume of the soil in the mound that is removed, leading to the conclusion that the soil in the burrow must be compressed. The floor of the burrow is compressed by the movement of the wombat along it to and from its foraging excursions, they also increase the height of the tunnel by pressing their heads and hips against the roof (Steele & Temple-Smith, 1998). It has been claimed that the wombat also uses its flat head to crush the head of attacking dogs against the ceiling of the burrow.
Construction of a burrow takes a very large amount of time and energy, estimated to be about 14 times the SMR. It is believed old burrow complexes have probably been used by multiple generations of wombats. There is some evidence that northern hairy-nosed wombats allow their young to remain in the parental warren, adult females establishing new burrows (Johnson & Crossman, 1991). In the southern hairy-nosed wombats the young are driven from the parental warren when they are 2 years old. This has been suggested as a possibly cause of the very high mortality among this age group (Wells, 1978).
All 3 wombat species have a backward-facing pouch with 2 teats, and produce a single young at a time, that remains in the pouch for a long time, after which it remains close to its mother for a further year. As a result of this long period between the birth of the young and final separation from the mother, wombats produce young only every second year at best. This level of reproduction is only achieved if there is sufficient high quality forage near the burrow complex at the time of late lactation.
The southern hairy-nosed wombats
Up to 10 individuals share a large warren with many entrances that are interconnected, surrounding which is a circle of simple burrows, about 100-115 m from the main warren, that are small and simple (Wells, 1978; Steele & Temple-Smith, 1998). The wombats occupying a warren graze over an area of about 4 ha between the warren and the circle of small burrows. The grass in this grazed area is cropped close to the ground, responding by putting out new shoots continuously. The small burrows are used temporarily, allowing the wombats to extend their grazing area, and they are also used by young wombats that have been expelled from the main warren. It is not certain how the social structure of the warren residents operates, but there appears to be system of division of the grazing area among the warren members. The males display territorial behaviour against outsiders, defending their grazing territory from wombats outside the warren group, probably to avoid competition on the grazing halo, and also to prevent outsiders from occupying their ring of refuge burrows. They engage in scent marking of their territory with their droppings, and also with scratches.
The ovulation of the females, between August and October, only occurs in years when there is sufficient rain to produce sufficient plant growth. At the same time, males have raised testosterone levels and their prostate glands are enlarged. When there is less than 200 mm of rain in a year, neither females nor males undergo the changes necessary to lead to reproduction. A study has found that the ejaculate of males was aspermous at times other the between August and March (Taggart et al., 1998).
Mating takes place in the warren, followed by a gestation of 22 days. At birth the young attaches to a teat in the pouch. October is the month when most young are born, though it can also occur 2 months before or after October. The young remains in the pouch for about 6 months, after which is spends some time out of the pouch, leaving it permanently in July or August. At this time it emerges from the burrow for the first time and begins feeding on grass. It is fully weaned at 1 year, reaching adult size at 3 years of age. The winter and spring of the year following its birth is the most critical time of its life cycle, at a time when it is changing over to a diet of grass, but still depends heavily on a good milk supply to survive. The studies that have been carried out found no young between 1 and 3 years old, leading to the belief that many do not survive (Gaughwin et al., 1998).
Recruitment of young fails completely in times of severe drought, either as a result of females not ovulating or males lacking sperm. A drought occurred in the middle year of a study carried out 1976-1978. 90 % of females produced young in the first year and the third year, but between June 1977 and February 1978 only 1 of the 28 females in the study produced a young, the others remaining anoestrus. During all 3 years the body mass and the mesenteric fat of the females changed little, suggesting that poor nutrition was not the cause of the anoestrus state, but a factor associated with plant growth was probably responsible. It was found that the proportion carrying pouch young between December and July correlated well with a plant growth index for the previous 6 months (Gaughwin et al., 1998). It has been suggested that inhibition of reproduction may occur to conserve nutrients, as any young produced during these conditions would die, wasting the nutrients used to produce it. Droughts occur regularly in the habitats occupied by the southern hairy-nosed wombat. Rainfall would have been sufficient to allow pouch young to survive that stage in only 20 of the last 100 years (Wells, 1989). The extra pressure of competition of sheep and rabbits also impacts on wombat reproduction.
It has been suggested that a cycle of wet and dry years may be an essential component of the dryland ecosystems, the good years leading to increasing populations of herbivores, while draining the scarce nutrients from the impoverished soils, that occurs over most of Australia, the nutrients being returned to the soil in drought years to power the next cycle of growth when the rains return (Wells, 1982).
Northern hairy-nosed wombats
As with the southern hairy-nosed wombat, this species has adapted to the hot dry climate by living in deep burrows. The only time they leave their burrow is for a few hours at night (Johnson, 1991). In winter they graze for 6 hours, but in summer the grasses are more productive, allowing them to get all the food they need in 2 hours. The home range of members of this species is up to 25 ha, but the core area is about 6 ha. A tracking study carried out on 10 wombats of this species for a year found that during that year they used 28 burrows. A cluster of burrows within a discrete area was used by small groups exclusively. 70 % of the time individuals occupied these burrows alone, the remainder of the time they shared with 1 or 2 others, most frequently females shared with other females, not often with males, the males tending to be solitary (Johnson & Crossman, 1991).
The feeding ranges of individuals don't overlap with others of the same sex, but the ranges of females do overlap with those of males, usually more than 1. The entrances and the boundaries of feeding ranges are marked by dung piles and urine that has a strong smell. During the study period the group centred on the warren apparently had a stable use pattern, but more than half the inhabitants of the warren had moved to a new burrow cluster when they were checked 2.5 years later.
This species is now found at a single site in Queensland. It is known from fossils that it once ranged across Victoria, southwestern New South Wales and central Queensland, declining to 3 isolated populations by the time of the arrival of Europeans in Australia. It died out in 2 other sites in Queensland in 1921 and 1939 during droughts, the last remaining population being in the Epping Forest National Park, where it declined to less than 30 % of its former range, only increasing when cattle were excluded from its habitat (Crossman et al., 1994).
Reproduction of this species is similar to that of the southern hairy-nosed wombat.
The common wombat
This species has a similar pattern of burrow use, with home ranges from 5-23 ha, using different burrows on successive nights. The group of wombats in a study area were found to use 40 burrows, individuals using or sharing 3-11 burrows, with 5-7 other members of the group, in various parts of its home range. The population was estimated from the number of individuals caught or by counting the number of burrows used. It was found that less than 12 burrows were being used or suspected of being used at times during the study (McIlroy, 1973, 1976). The home ranges of common wombats in Tasmania was found to be about 10 ha for males and about 20 ha for females, though there was overlap between the sexes (Taylor, 1993). The environments occupied by this species was very different from that of the 2 species of hairy-nosed wombats, but the limited home range size and the pattern of use was very similar between all 3 species. This has led to the suggestion that the large amount of energy and time involved in constructing burrows imposes real constraints on how far an animal far can move from such an investment, especially as it is the only refuge the animal has from climatic extremes, and reproduction is also constrained (Tyndale-Biscoe, 2005).
This species can breed throughout the year, though births mostly occur between December and March. It had been found that the testes of the males were larger between September and December (McIlroy, 1973). Of the matings that have been observed, 2 in the wild (Triggs, 1996) and 1 in a German zoo (Boer, 1998), all 3 occurring outside the burrow. Each began with vigorous chasing, after which the male bit the female, bringing her to the ground. Once down she lay prone, the male lying on his side as they mated. Birth occurs 22 days after mating. If conception fails to take place the female returns to oestrus about 32-34 days later. Small pouch young are found in the pouch between May and August, fully furred pouch young being present between September and October. At 8 months they emerge from the burrow to being feeding on grass. A young wombat that was observed in a zoo eating moist faecal pellets produced by the mother that differed from her normal dung (Boer, 1998). An orphaned wombat of the same age was seen to to do the same (Triggs, 1996). This is believed to be the method by which young animals obtain the bacteria necessary to carry out the fermentation they use to digest the tough grass, as well as possibly providing partly digested plant food, as it is changing from an exclusively milk diet.
The common wombat lives in a much less severe environment than the southern hairy-nosed wombat, where droughts are much less common or severe, but they share the same pattern of growth and development, and they are also badly affected by competition from other herbivores, especially during the period of late lactation.
Tyndale-Biscoe, Hugh, 2005, Life of Marsupials, CSIRO Publishing.
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