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Like many marsupials, the majority of diprotodonts are primarily nocturnal or at least crepuscular, but most macrop-ods 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 (Ail-urops 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 Thy-

logale) 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 tempera-tures—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 colu-gos 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

Kangaroos fighting on the beach. (Photo by © Mark A. Johnson/Corbis. Reproduced by permission.)

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 (Dendrola-gus 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. Hairy-nosed 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 vocaliza-

A koala (Phascolarctos cinereus) eating eucalyptus leaves. (Photo by Daniel Zupanc. Bruce Coleman, Inc. Reproduced by permission.)

Matschie's tree kangaroo (Dendrolagus matschiei) sits on a branch. (Photo by Art Wolfe/Photo Researchers, Inc. Reproduced by permission.)

A koala (Phascolarctos cinereus) eating eucalyptus leaves. (Photo by Daniel Zupanc. Bruce Coleman, Inc. Reproduced by permission.)

Matschie's tree kangaroo (Dendrolagus matschiei) sits on a branch. (Photo by Art Wolfe/Photo Researchers, Inc. Reproduced by permission.)

tion when distressed. Sugar gliders have an alarm call that sounds like the barking of a small dog. Squirrel gliders (P. nor-folcensis) 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.

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