Diprotodontia

(Koala, wombats, possums, wallabies, and kangaroos)

Class Mammalia

Order Diprotodontia

Number of families 10

Number of genera, species 40 genera; 131 species

Introduction

The Australasian order Diprotodontia is not particularly large (it has just 131 described living species) but is one of the most startlingly diverse of all mammal groups. Its members include animals as superficially different as the teddybear-like koala (Phascolarctos cinereus), the tiny feather-tailed glider (Acrobates pygmaeus), and the magnificent red kangaroo (Macropus rufus). While some diprotodonts are instantly recognizable, others are rather obscure and they include among their number some of the world's rarest animals. As a group, the diprotodonts are relatively new to science—first recognized as an order less than 150 years ago in 1866.

Evolution and systematics

The earliest known diprotodont fossils date back to the Oligocene epoch 24–35 million years ago (mya), but even these constitute a diverse assemblage of forms, so the origins of the group almost certainly go back further, to the Cretaceous period. Then, like most early mammals, the ancestors of all Australasian marsupials were probably small insect-eating animals not unlike modern bandicoots (order Peramelemorphia) or the monito del monte (order Microbiotheria). These ancestors arrived in Australia from South America and Antarctica when all three were united in the massive tree-covered supercontinent of Gondwana. When Gondwana broke apart about 40 mya, Australia's marsupials were isolated and began a major radiation. In the absence of large numbers of placental mammals, they expanded to fill a huge range of ecological niches. No group diversified more than the diprotodonts.

There are currently 131 species of diprotodont in 40 genera, representing 10 families of koala, wombat, possum, cuscus, rat-kangaroo, kangaroo, wallaby, pygmy possum, ringtail, glider, honey possum, and feathertails. The suborder Vombatiformes, containing the living families Phascoloarctidae (koala) and Vombatidae (wombats) probably represents an early offshoot of the diprotodont lineage, characterized (at least in living members) by a reduced tail and backward-opening pouch. All other diprotodonts are placed the same group, suborder Phalangerida, which represents the main trunk of the phylogenetic tree. This trunk splits into five major branches (superfamilies) of which two, the Phalangeroidea (possums and cuscuses) and the Burramyoidea (pygmy possums) contain just one family each. The other three superfamilies each unite two families, the kangaroos and rat-kangaroos in the Macropodoidea, the ringtails and gliders in the Petauroidea, and the honey possum and feathertails in the Tarsipedoidea. This classification is broadly supported by molecular data in which the sequences of certain key molecules including DNA are compared. The diprotodonts closest relatives in other orders are probably the bandicoots and bilbies (order Peramelemorphia) and the marsupial moles (order Notoryctemorphia). However these relationships are largely speculative.

The discovery of previously undescribed mammal species is an increasingly rare event and yet this group includes among its ranks several species new to science in the last 25 years. These exciting newcomers included the long-footed potoroo, Potorous longipes, and the Proserpine rock wallaby, Petrogale persephone, both found in Australia and described in the early 1980s. Zoological expeditions to New Guinea and the Indonesian North Moluccas in the late 1980s and early 1990s yielded the previously unknown Telefomin cuscus, Phalanger matanim; the black tree kangaroo, Dendrolagus scottae; the dingiso or forbidden tree kangaroo, Dendrolagus mbaiso; and the Gebe cuscus, Phalanger alexandrae.

Physical characteristics

Diprotodont marsupials are united by two important characteristics that belie their great divergence in size and over-all form. The first concerns dentition (the arrangement of teeth) in the lower jaw. Koalas, wombats, kangaroos, and possums all have only two developed incisor teeth at the front of the lower jaw. These are large and often project forwards as an adaptation to cropping vegetation. A second pair of very small incisors is present in some species, but there are no lower canines, just a gap between the incisors and the cheek teeth. This arrangement is known as diprotodonty (literally translated this means "two first teeth")—hence the ordinal name Diprotodontia.

The second major unifying characteristic of diprotodonts is syndactyly. This means "fused toes," and refers to the structure of the hind feet, the second and third digits of which are always fused together forming a strange-looking double toe with two claws. The twin claws of the fused digits are retained and in most species serve the useful function of a grooming comb for removing caked dirt or other debris clinging to the animal's fur. Another shared feature of many diprotodonts is the arrangement of digits on the front paws. In most climbing species, the first two fingers oppose the other three, allowing the animals to maintain a firm grip of branches and stems. A notable exception to this rule is the brushtailed possum, whose forepaws are more like tiny nimble hands.

Members of the suborder Vombatiformes are set apart from other diprotodonts, being rather squat and heavy, with a pouch the opens to the rear. In burrowing wombats this prevents the pouch filling up with soil, whereas in koalas it is a rather inconvenient arrangement inherited from non-climbing ancestors. Other diprotodonts, members of the suborder Phalangerida, have a more athletic build with a long tail. In many possums the tail is prehensile. It may be thin as in potoroos or muscular as in large kangaroos, very long and brushy (as in Leadbeater's possum, Gymnobelideus leadbeateri) or virtually naked. Phalangerids have retained the forward opening pouch of their ancestors as a secure means of transporting young while climbing or hopping. In members of the super-family Macropodoidea (kangaroos, wallabies, and rat-kanga-roos),

the hind legs are larger and more powerful than those at the front, and the hind feet are very long (the group and generic names macropod and Macropus mean "big footed"). In most macropods the first hind toe is absent.

Diprotodonts have soft fur—that of many possums and the koala is very woolly. The majority of species are some shade of gray or brown, but a few are rather dramatically colored—for example Goodfellow's tree kangaroo (Dendrolagus goodfellowi), with a rich cinnamon red and gold coat and the yellow-footed rock wallaby (Petrogale xanthopus), whose yellow legs, feet, and ears, red and yellow-banded tail, and bold white cheek flashes contrast with gray body fur and make it one of the world's more decorative mammals.

Gliding possums are equipped with a built-in parachute, formed from a web of skin extending along each flank from the front to back legs. The precise structure of this membrane (called the patagium) varies between the three families of glider. In the pseudocheirid greater glider it stretches from elbow of the forelimb to ankle of the hind, while in the acrobatid pygmy glider it links wrist to knee. The lesser gliders (family Petauridae) have the most complete patagium, extending from wrist to ankle. None of the marsupial gliders have a tail membrane, although the tail of the pygmy glider is modified to assist the gliding process, with a vane of hairs along each side earning the species its alternative common name feathertail glider.

Convergent evolution is a recurring theme in marsupial history, and the diprotodonts are no exception. The diverse diprotodont body forms and lifestyles show striking similarities with mammals of several other orders. For example, the head of kangaroos is deer-like, with a long muzzle, erect, mobile ears and large bulging eyes situated on the side of the head. Like deer, kangaroos have good all-round hearing and vision and long, powerful legs capable of making dramatic leaps and propelling the animal at speed—all essential for avoiding predators in open habitats. The stout, badgerlike form of the wombats is an adaptation to burrowing, with strong legs, claws and jaws all contributing to the effort of excavating what can be very hard soils. With their woolly fur, round face, large, forward-facing eyes and careful climbing technique, the cuscuses are strongly reminiscent of primates such as lorises and pottos. The pygmy possums are the marsupial equivalent of European dormouse in looks and some aspects of behavior—they climb with the aid of a prehensile tail and enter deep hibernation in cold weather. The sugar glider (Petaurus breviceps) and Leadbeater's possum resemble species of flying and non-flying squirrel respectively.

Distribution

The Diprotodontia is an exclusively Australasian group, with the bulk of its members restricted to Australia itself. New Guinea and the surrounding islands are home to about 47 endemic Indonesian diprotodonts, mainly tree kangaroos, cuscuses and forest wallabies. Only five species occur on both New Guinea and Australia but are restricted to tropical north Queensland in the latter. The most widespread species are the common brushtailed and ringtail possums, the common wallaroo and the red and gray kangaroos. With a little help from humankind, several species have made it to other parts of the world. The brushtailed possum (Trichosurus vulpecula) thrives in New Zealand as an undesirable alien despite sustained attempts to eradicate it. Wallabies that have escaped or been released from zoos or private collections have established colonies as far away as Great Britain. The group also includes some animals with dangerously restricted distributions—for example, several Indonesian cuscuses and possums and the Australian Gilbert's potoroo, which with an estimated wild population of just 30 individuals living in one reserve near Albany in Western Australia, is one of the worlds most threatened animals.

Habitat

Diprotodont marsupials have successfully exploited virtually every terrestrial habitat their Australasian range has to offer. Forest diprotodonts include the cuscuses and tree kangaroos of tropical Queensland and New Guinea and Lead-beater's possum, which favors old growth highland forests of Australian mountain ash. Koalas and various wallabies and the widespread spectacled hare wallaby are animals of open woodland and bush, while rock wallabies are generally restricted to boulder slopes that less sure-footed animals find difficult to negotiate. Gray kangaroos and pademelons are grassland animals, preferring areas with regular rainfall and some tree cover, while reds make do with the sparsest vegetation in the Central Australian Desert. Honey possums (Tarsipes rostratus), quokkas (Setonix brachyurus), and various rat kangaroos are heath dwellers, while mountain pygmy possum is restricted to alpine habitats where snow covers the ground for half the year. Several species are now almost exclusively island dwellers, though not out of preference. Small wallabies such as the quokka and the rufus hare wallaby have been eradicated from much of the former range on mainland Australia by introduced predators and are only secure on offshore islands not yet colonized by foxes or cats. There are no aquatic diprotodonts, but several species of peturid, pseudochirid, and acrobatid possum readily take to the air on gliding membranes like those of flying squirrels. Not surprisingly, such species are restricted to habitats with many tall trees, from which they can launch their spectacular aerial swoops. Among the most adaptable of all marsupials, the common brushtailed possum, has adapted very well to the changes wrought by humans. It does well in suburban habitats and is a frequent visitor to gardens and outbuildings.

By no means all diprotodonts have regular nests, dens or other homes. The majority of large macropods are seminomadic and use a large home range. The red kangaroo (Macropus rufus) is especially wide ranging, and severe droughts or bush fires may force them to disperse 200 mi (300 km) or more in search of water and food. Being large, they have little need of a secure resting place, and most make do with patches of scant shade beneath desert shrubs such as saltbush. Smaller kangaroos and wallabies favor thickets of denser vegetation. While none of the large macropods build a nest as such, individuals may stake a claim to a particular spot. For example, the quokkas of Rottnest Island in Western Australia compete fiercely for the very limited number of shady resting places. The lack of potable water on the island in summer means that shade in which to avoid over-heating is at a premium, and dominant individuals will claim the best spots as their own.

Unlike their larger cousins, many rat-kangaroos and bettongs do build a nest or den. Nest builders collect bedding material such as grass and vines, sometimes carrying them clamped between the body and the tail. These are arranged in a thicket of dense vegetation or in a shallow scrape, often

excavated in the shelter of a shrub or grass tussock. The West Australian boodie, a species of bettong, digs its own burrow or uses the abandoned tunnels of rabbits. Wombats are supreme diggers—their homes are extensive burrows in forests, grasslands or scrub.

Among the arboreal diprotodonts, most use some kind of customized resting place. Cuscuses prepare special sleeping platforms of bent twigs and leaves, while honey possums and pygmy gliders weave intricate nests of grass, shredded bark and moss. Most possums and gliders build nests of twigs and leaves and many will make use of ready-made possum or bird nests, hollow logs or naturally occurring tree holes. Both males and females build nests and the majority of possums will have several nests within a home range. Possum nests are sometimes referred to as "dreys."

Behavior

Like many marsupials, the majority of diprotodonts are primarily nocturnal or at least crepuscular, but most macropods will sometimes move about in daylight, especially under the cover of forest of scrub. A good many nocturnal species will also emerge by day to bask in the sun, especially early in the morning when the warmth helps them to digest the rewards of a night's foraging. Basking is an important part of the energy efficient lifestyle of wombats, koalas, and kangaroos. Only one species is generally thought of as completely diurnal, the musky rat kangaroo (Hypsiprymnodon moschatus) of tropical Queensland. However a good many macropods, in particular the tree kangaroos and forest wallabies appear to be active both night and day. The large Celebes cuscus (Ailurops ursinus) is also reported to be at least partly diurnal. Day active species tend to be those that live in forests or other sheltered habitats where their activity is less likely to draw unwanted attention. Some Australian pademelons (genus Thylogale) tailor their activities to the time of day—grazing in open pasture by night and browsing in forest by day, pausing periodically to rest in the shelter of a dense shrub or to bask in a sunny clearing.

The annual cycles on which these various patterns of daily activity are superimposed are most apparent in animals living in temperate parts of Australia, where the seasonal climate forces some quite drastic behavioral adaptations. Surviving the winter is not simply a question of keeping warm, but of having the energy to do so. Animals that spend the warmer months feasting on seasonal foods such as fruits and certain insects must either rely on stored body fat, cache food for hard times ahead, switch to an alternative food source, or be prepared to drastically reduce energy consumption during periods of shortage. It is not surprising that the only diprotodont to use all of these strategies hails from the Southern Highlands of Australia, where the annual cycle of glut and deficiency is especially acute. The mountain pygmy possum (Burramys parvus) spends the spring gorging on the millions of bogong moths that visit the highlands to breed. When the moths are gone the possum switches to fruits and seeds, storing those it cannot eat immediately in a winter larder. In winter it hibernates for six months or more, the only marsupial to do so. It can also enter an energy-saving torpid sleep during periods of food shortage at any time of year—a good insurance policy again the vagaries of mountain weather. Several other diprotodonts capable of facultative torpor induced by food shortage or low temperatures—among them the honey possum and the pygmy glider (Acrobates pygmaeus).

Many diprotodonts are arboreal. The members of the possum families have strong grasping hands and feet. The tail is prehensile to some degree. Tails used for grasping often lack fur near the tip, especially on the underside—bare, callused skin, provides much better grip than soft fur. In species such as Leadbeater's possum the tail serves as a counterbalance, in the ringtail and dormouse possums it can be used to steady the animal in the tree, and in several the tail is strong enough to support the animal's entire weight. The koala, however, manages to climb very well with no tail to speak of. It climbs tall eucalypti by hugging the trunk, digging in with surprisingly large, hooked claws and hauling itself upwards, then reversing the process in order to descend. Koalas are also surprisingly adept at moving along the ground—this is after all the only way for them to get from one tree to another. They walk with a rolling, bowlegged gait or when pressed, proceed at a surprisingly fast, bounding gallop.

The ground is a dangerous place for most tree-dwelling animals. It is therefore not surprising that gliding as a means of traveling from tree to tree without descending to ground level has evolved several times in a variety of arboreal lineages. These include the placental flying squirrels and colugos and also the diprotodont families Pseudocheiridae, Peturidae, and Acrobatidae.

Three families of diprotodont have forsaken the trees for life at ground level—the wombats, kangaroos, and rat-kangaroos. The wombats have tackled the threat of predation by building secure underground dens and by becoming too large for most of Australia's native hunters to tackle safely. Their legs are short

but immensely strong as an adaptation to burrowing and they walk with a purposeful quadruped waddle.

Kangaroos and wallabies are famous for hopping. When moving at speed, they propel themselves forward with powerful leaps of the hind legs—the forelegs do not touch the ground at all, and the tail is used only for balance. Hopping is a remarkably efficient way to travel, and a large kangaroo can reach top speeds of up to 30 mph (48 kph), covering 30 ft (9 m) or more with every bound. Interestingly, swimming is the only form of locomotion in which a kangaroo moves its large back legs independently of one another.

None of the methods of locomotion described so far is especially novel—all are very similar to those employed by various groups of placental mammals elsewhere in the world, a fact reflected in much congruence of form between marsupial and placental animals. Some squirrels glide, primates and rodents climb, and several small rodents are great hoppers—some are even known as kangaroo mice, even though other group has committed quite so fully to life on the hop as the macropod diprotodonts.

However there is one form of marcopod locomotion that is entirely unique. When moving at slow speeds, large kangaroos are effectively five-legged. Their reduced forelegs are much too weak to support the entire body weight, and so the tail plays the role of sturdy prop. The animal leans forward onto its hands, and swings its hind legs forwards while supporting its rear end on the base of the tail. At times the tail can even be used to temporarily support the entire body weight, for example during combat. A fighting kangaroo may lean back onto its tail while attempting to inflict thunderous double blows on its opponent with both hind feet.

As if to prove that that no change is irreversible, some kangaroos have returned to an arboreal way of life. Tree kangaroos have evolved from ground-dwelling ancestors and thus lack many of the primary adaptations to climbing such as a prehensile tail. Nevertheless they move about the trees with great agility, using their tail as a counterbalance or brace, and grasping the branches with large, well-cushioned feet. They will leap from tree to tree and descend by jumping from branch to lower branch or shuffling backwards down the trunk. In one final phylogenetic twist, the Doria's tree kangaroo (Dendrolagus dorianus) has become ground dwelling once more.

Diprotodont populations exhibit a range of social structures. Among the macropods, forest-dwelling species including most of the rat kangaroos and smaller wallabies tend to live alone, except when breeding, while larger species that spend more time in open spaces are more gregarious. Kangaroo groups, known as "mobs," are rather casual associations, and members are free to come and go at will. Females are less inclined to disperse than males and often remain in the same mob as their mother. Hence the female members of a mob are often related. The size of a mob varies, with the largest groups forming when several mobs converge on a resource such as a good feeding area. Male kangaroos are socially dominant to females but they do not lead the mob—their interest in a particular group is usually confined to periods when one or more females is in or approaching breeding condition. There is no real cooperation between members of a kangaroo mob, but there is an element of safety in numbers and each individual benefits from the alertness of others.

Koalas and common wombats are solitary for most of the year. Koalas live in close proximity to one another but they require personal space. While more than one individual may use the same tree, they do so at different times. Common wombats live alone in their burrow, but they are not generally aggressive and will visit one another's homes. Hairynosed wombats are altogether more social, and up to 50 individuals have been known to share a warren of interconnected burrows.

Among the various possum species there is an inverse relationship between size and sociality. Most large species are solitary except when breeding, whereas small species such as the honey possum, the feathertails, and lesser gliders often live in pairs and may rest in quite large groups, especially during periods of cold weather when they huddle together for warmth. Leadbeater's possum is unusual in that females are socially dominant to males, and mothers drive their daughters from the territory as soon as they are old enough to fend for themselves. In this species, it tends to be males that stay put.

Diprotodonts commonly use four main types of vocalization to communicate, namely: barking, used to communicate positon to other group members; sneezing, indicative of disagreements within the group; hissing, used as a distress call; and crabbing, a sound made to convey displeasure, associated for example with being disturbed while in the nest, or while sleeping. The mountain pygmy possum makes a low guttural vocalization

when distressed. Sugar gliders have an alarm call that sounds like the barking of a small dog. Squirrel gliders (P. norfolcensis) exhibit some unique vocal communications, they produce gurgling chatters and soft, nasal grunts, also repetitive, short gurgles. The common wombat makes a loud hissing growl when annoyed. And it has been said that the very loud hissing, crackling territorial call of the male common brushtail has a definite nightmare quality.

Feeding ecology and diet

Feeding strategies among primitive diprotodonts were aimed at a generalist herbivorous diet of leaves, fruits, and roots. The prehensile tail and opposable digits of the front feet enhanced climbing ability in many, giving them access to the branches of trees as well as some security from ground dwelling predators like the thylacine. The majority of diprotodonts are still vegetarian, but whereas early diprotodonts were mostly generalists, many now specialize in a wide range of vegetarian diets. The great radiation of ancient diprotodonts was fuelled in part by changes in the Australian climate as the continent drifted slowly north having separated from Antarctica. As Australia's forest cover dwindled, competition for certain plant foods become more intense and herbivores were forced to specialize. While opportunities for carnivorous and insectivorous lifestyle remained broadly similar, changes in vegetation cover presented a range of entirely new food types for herbivores to exploit. Large areas that were formerly covered in dense forest became drier—the lush forests giving way to more open woodland, grassland and finally to desert.

The evolution of grazing habits in Australian marsupials mirrored that which occurred in placental mammals elsewhere on the planet. Grass is a tough food. It requires a lot of chewing and very thorough digestion to unlock its nutrients. However it is abundant and easy to find, and those diprotodonts that adopted the grazing habit (mostly macropods) are now among the most widespread Australian mammals. The arrival of European settlers to the continent a little over 200 years ago has spelled disaster for many native diprotodonts, but not for large grazers, which benefited from the improvement of grassland for livestock.

Leaves and grasses are convenient dietary staples—they are easy to find and collect, and in many parts of Australia and New Guinea they are available year round. However they are not particularly nutritious, and most herbivorous animals have to eat quite a lot of them to survive. However diprotodont mammals are surprisingly fuel-efficient animals. All marsupials run their body at a slightly lower temperature than placental mammals and some are able to survive on the most meager of rations. For example, red kangaroos have a very large stomach, which when full might contain up to 15% of the animal's body weight in food. But this is processed so efficiently that the kangaroo actually eats much less than an equivalent sized sheep or other placental mammal.

The prize for most effective use of low-grade food goes to the koala. Koalas are famously lethargic animals. They sleep up to 20 hours a day and move very little when awake. In the past it was speculated that koalas were effectively drugged by toxins in the eucalyptus leaves they ate. Now it is very clear that the laid-back koala lifestyle is a highly adaptive survival strategy—by expending very little energy, koalas can survive on a food resource so low in nutritional content that no other mammal even attempts to eat it. Wombats employ a similar low-energy lifestyle, resting for long periods underground and using the warmth of the sun to reheat their body in the morning, thus avoiding the need to burn fuel.

For other diprotodonts living away from desert and dry bushland, quality food is a little easier to come by, but competition is correspondingly more intense. Rat-kangaroos and smaller macropods living on heaths and in forests eat more fruits, fungi, and starchy roots, and the active possums target sugary fruits, oily seeds, and flowers containing protein-rich pollen. Some of the most successful modern forest diprotodonts have learned to tap into the internal plumbing of trees, and feed on sugar-rich sap and gum. Early gum-eaters probably took advantage of damage to trees caused by other factors such as high winds, and fed opportunistically on the sticky secretions that leaked from damaged branches. From this it is a relatively small step to deliberately inflicting wounds on the tree in order to release sap. But sap and gum flow very slowly, even from deep wounds, and an active animal like a possum cannot afford to sit around waiting for a tree to leak enough sap to make a meal. Hence many species, in particular the petaurid gliders, have developed longer-term strategies whereby they make a cut with their sharp front teeth, and return later to harvest and eat the blob of gum that has accumulated.

Most diprotodonts consume at least some animal material, mostly insects, along with their regular diet of plants. A few species have become more actively insectivorous—for example, the mountain pygmy possum, which for several months of the year eats nothing but large moths, and the pygmy glider, which catches insects to supplement its diet of nectar and fruit.

Most divergent of all is the honey possum, which lives almost exclusively on a diet of pollen and nectar. It is one of very few mammals to do so, and has a long, specially modified tongue with a brushlike tip for collecting pollen from deep tubular florets.

Reproductive biology

Marsupial mating systems are not well known. Some species may be monogamous, but most are probably polygynous or promiscuous. The marsupials are a group defined by their reproductive biology. Female marsupials give birth to live young, but at a very early stage after a very short gestation (for example 21 days in the common wombat, 17 days in the brushtail possum, 32 days in the eastern gray kangaroo). Female marsupials have two uteri and two vaginas. Young developing in one or both uteri are born through a third opening, which only develops when the female is due to give birth for the first time. In most marsupials this birth canal is a temporary structure that seals over after the birth of every litter, but in certain diprotodonts (the kangaroos and the honey possum) it becomes permanent.

Newborn young make their own way to the mother's teats, which are usually located within a pouch (the "marsupium" for which the group is named). The young latch onto a teat and continue their development sustained by milk that changes in composition to suit their needs as they grow. The diprotodonts have some of the best developed pouches—designed to carry the young securely while the mother hops, ands burrows her way through daily life. The number of teats varies between species and gives a rough guide to maximum litter size—a female cannot rear more young than she has teats. Diprotodont litters are quite small—one or two is normal for most possums, whereas single young are the norm for kangaroos, rat kangaroos, wombats, and the koala. The mountain pygmy possum may give birth to as many as eight young, but only the four strongest will find a teat and survive.

Most mothers continue to suckle to their young when they have outgrown the pouch. Possums and wombats carry larger youngsters on their back. By accompanying their mother as she forages, young animals learn by imitation what foods are good to eat. Young kangaroos that have left the pouch may

remain close to their mother and continue to suckle for many weeks, reaching into the pouch to drink from the same teat they used as a newborn. By this stage there is often a new baby in the pouch attached to a different teat, in which case the mother produces a different kind of milk for each of her two offspring.

In many diprotodonts, including the honey possum and several species of kangaroo, the interval between one young vacating the pouch and the birth of a replacement can be as little as one day. This is thanks to a remarkable phenomenon known as embryonic diapause. Because gestation is so short, it does not interrupt the normal estrus cycle, and a female can come into season very soon after her first young is born. The new mother may mate and conceive during this "post-partum" estrous, but as long as the pouch is occupied by a suckling youngster the second embryo or litter does not develop beyond a very early stage. Instead it remains in a state of suspended animation, ready to be reactivated should the first young be lost or when it is almost ready to leave the pouch. The process is controlled by the same hormones that regulate milk production.

With the exception of the honey possum, individuals of which rarely live to see their first birthday, diprotodonts are quite long-lived animals. Possums, cuscuses, ringtails, gliders, and small macropods have a life expectancy between six and 14 years, koalas often live into their late teens, and wombats and large kangaroos well into their twenties. Even the diminutive feathertails and pygmy possums can live seven and 10 years respectively.

Conservation

Six species of diprotodont marsupial are known to have gone extinct in recent years, among them the Toolache wallaby, Macropus greyi. This once abundant native of South Australia was one of the swiftest of all macropods. Its agility and its very fine pelt made it a favorite quarry for "sportsmen" and it was hunted to the brink of extinction in the 1920s. The species may have hung on in some parts of its former range until as late as the 1970s, but here have been no positive records since 1924 and the species is listed as extinct. Another victim of changing times was the broad-faced potoroo—this was first described in 1844, but was extinct within 40 years.

Of the remaining diprotodont species, approximately a quarter currently appear on the IUCN Red List of Threatened Animals. In New Guinea these include the black spotted cuscus and Goodfellow's tree kangaroo and in Australia the long-nosed potoroo, the Proserpine and yellow-footed rock wallabies, the brush-tailed bettong, and the mountain pygmy possum. Two species, the Gilbert's potoroo and the Northern hairy-nosed wombat are Critically Endangered, both with less than 50 individuals left in the wild. A further quarter of all diprotodonts are regarded as Near Threatened or data deficient. This latter designation means the animals, or at least their distribution and abundance, are so little known that conservationists have been unable to assess the extent of any potential threat.

The problems facing many threatened diprotodonts are very similar. In Australia the arrival of humans and their companion animals has had a devastating effect on native wildlife, in terms of habitat modification, hunting and predation by introduced carnivores. The first aboriginal settlers arrived between 60,000 and 40,000 years ago. They introduced the semi-domesticated dingo and began managing the landscape using burning regimes to encourage fresh plant growth. However these changes were minor compared to those that came about with the arrival of European settlers in the late eighteenth century. The advance of farming, the intensification of burning regimes, and above all the deliberate introduction of foxes, cats, and rabbits, had a profoundly destabilizing effect. In New Guinea, there are two main threats to native diprotodonts, most of which are forest-dwellers. Unrestricted logging not only destroys habitat, it also opens up areas of previously inaccessible forest to hunting.

On a more positive note, several species of diprotodont that were once considered extinct have been rediscovered alive and well. While most of these are still very rare, being endangered is still very much better than being extinct. Gilbert's potoroo was presumed extinct until 1994 when a small colony was rediscovered on the coastal heathland near Albany in Western Australia. A captive breeding program is underway. Leadbeater's possum was declared extinct in the early twentieth century, but turned up again in 1961. Ironically it appears to have been saved by the enormous bush fires that devastated large areas of the Victorian Highlands in 1939—the standing dead wood left by the fire provided a sudden surplus of suitable possum nesting holes. The mountain pygmy possum had never been seen alive before 1966. The species had been described from preserved remains and

was presumed long extinct, when one turned up alive and well in a ski lodge in the winter resort of Mount Hotham. Other resurrected diprotodonts include the parma wallaby and the Mahogany glider.

There are also some conservation success stories—perhaps most notably the koala. In 1920, the species was facing extinction due to overhunting and habitat fragmentation. Thanks to a ban on hunting and the establishment of many reserves, the trend was reversed. In fact in many ways the campaign to save the koala has been too successful. Many reserves are now so overcrowded that disease has become a serious issue. While the future of the species appears secure, the outlook for many thousands of individuals living on overcrowded reserves is rather bleak. At best they face deportation to new homes (of which there are a dwindling number), at worst a slow death from infections such as Chlamydia. In some places, wildlife managers have had to make the unhappy decision to cull some the animals they have work so hard to save in order to improve living conditions of others.

The northern hairy-nosed wombat, Lasiorhinus krefftii, is now Critically Endangered. Its southern cousin Lasiorhinus latifrons is faring a little better, but still has a rather restricted distribution. The common wombat, Vombatus ursinus, while not officially threatened, is regarded as a pest in some states and is becoming a cause for conservation concern.

Significance to humans

Australia is unique among the world's nations in having such a diverse array of highly distinctive endemic mammals. People sometimes confuse the fauna of other continents, imagining tigers in Africa and leopards in South America, but few would make the same mistake with a kangaroo or koala. As a result, these two diprotodonts in particular have achieved iconic status in the last century. They are used to market all things Australian from beer to tourism and no visit "down under" is complete without the opportunity to cuddle a koala or see a mob of kangaroos bounding across the desert.

Kangaroos are among the few marsupials to have benefited from changes in the Australian landscape since the arrival of white settlers. As pioneer farmers began developing huge areas of bushland as grazing pasture for livestock, kangaroo numbers soared. The native population benefited greatly from the improved grazing created by irrigation from artesian wells and boreholes. Being naturally adapted to arid conditions, the kangaroos fared much better than the domestic stock and when droughts hit, farmers were driven to furious despair at the sight of healthy kangaroos grazing the last dry blades of grass alongside emaciated and dying sheep and cattle. The situation now boils down to an uneasy truce in which native grazers are fenced out of core areas of pasture, where sheep and cattle are provided with feed supplements and additional water in times of drought. Kangaroos that manage to breach perimeter fences are routinely shot, but many still manage to make a good living on marginal ranchland.

Kangaroo meat is increasingly popular. It is very lean and is often marketed as a healthy alternative to beef. Large quantities are also used in the manufacture of pet foods. Kangaroo skin makes very soft leather. This is not very hard wearing but it is flexible and pleasant to touch and thus suitable for hats and other items of clothing.

Another diprotodont with a split personality—one part useful, the other a nuisance—is the common brushtailed possum. Possums are resourceful and adaptable animals and they thrive in and around human settlements, raiding garden produce and trashcans at night and generally making a nuisance of themselves. They do considerable damage to trees in plantations and carry a number of diseases. In the past some of this damage was offset by the value of the possum's fur, but this has declined of late. Possums introduced to New Zealand between 1840 and 1900 have caused untold damage to native trees and plants.

The damage caused by kangaroos or possums is as nothing compared to that inflicted by rabbits. These were introduced for food and sport in 1858 and rapidly reached plague proportions. Their depredations were as damaging to native grazers as they were to livestock and efforts at eradicating them have involved unleashing some of the most unpleasant wildlife diseases known—first myxomatosis in the 1950s and more recently rabbit hemorrhagic disease or RHD. While rabbits and kangaroos compete directly for the same foods, rabbit control measures have also affected the fate of another group of diprotodonts—the wombats. Being prolific burrowers, wombats often provide rabbits with a means to circumvent fences erected to keep them out of pastures. As a result wombats have been mercilessly persecuted.

Resources

Books

Aplin, K. P., and M. Archer. "Recent Advances in Marsupial Systematics with a New Syncretic Classification." In Possums and Opossums: Studies in Evolution. Sydney: Surrey Beatty & Sons and Royal Zoological Society of New South Wales, 1987

Flannery, T. Mammals of New Guinea. Papua, New Guinea: Robert Brown & Associates, 1995.

Flannery, T. Possums of the World. Chatswood, Australia: GEO Productions & the Australian Museum, 1994.

Macdonald, D. The New Encyclopedia of Mammals. Oxford: Oxford University Press, 2001.

Nowak, R. "Order Diprotodontia." In Walker's Mammals of the World. Vol. I. 6th edition. Baltimore: Johns Hopkins University Press, 1999.

Strahan, R. The Mammals of Australia. Carlton, Victoria: Reed New Holland, 1995.

Organizations

Austalian Conservation Foundation Inc. 340 Gore Street, Fitzroy, Victoria 3065 Australia. Phone: (3) 9416 1166. Web site: <http://www.acfonline.org.au>

Australian Museum. 6 College Street, Sydney, New South Wales 2010 Australia. Phone: (2) 9320 6000. Web site: <http://www.amonline.net.au>

IUCN—The World Conservation Union. Rue Mauverney 28, Gland, 1196 Switzerland. Phone: +41 (22) 999 0000. E-mail: mail@iucn.org Web site: <http://www.iucn.org>

South Australian Museum. North Terrace, Adelaide, South Australia 2010 Australia. Phone: (8) 8207 7500. Web site: <http://www.samuseum.sa.gov.au>

Amy-Jane Beer, PhD