Koala, Wombats, Possums, Wallabies, and Kangaroos: Diprotodontia
KOALA, WOMBATS, POSSUMS, WALLABIES, AND KANGAROOS: Diprotodontia
Diprotodonts are an order of about 131 species of marsupial mammals that live in Australia, New Guinea, and parts of Indonesia. The order also contains a family of giant diprotodonts that are now extinct. Within this order are some of Australia's best known marsupials, including the kangaroos, koalas, and wombats, as well as some of the least known species such as cuscus and potoroos.
Diprotodonts have evolved to fill almost every terrestrial (land) ecological niche, and as a result, they have evolved special physical features that allow them to live most efficiently in their chosen environment. For example, some tree-dwelling (arboreal) gliding possums have evolved a skin membrane that stretches from wrist to ankle and acts as a parasail, allowing them to stay away from predators, animals that hunt them for food, and conserve energy by "flying" from tree to tree. Wombats have evolved strong claws and short, stocky bodies well suited for digging. Kangaroos have strong hind limbs that allow them to race across open grassland at speeds up to 35 miles (55 kilometers) per hour and to leap distances of up to 30 feet (9 meters). Possums and cuscus have evolved tails that can curve around and grasp a branch (prehensile tails).
As a result of this diversification, the species in this order look very different from one another. However they all share at least two physical characteristics that include them as diprotodonts. All members of this order have two large incisor teeth on the lower jaw. Incisors are front teeth that are modified for cutting. These teeth are also noticeable in more familiar rodents such as beavers and rabbits. Most members of this order also have three pairs of incisors on the upper jaw, and a few species have a second small pair on the lower jaw as well. In addition, members of this order have no canine teeth. Canine teeth are sharp, pointed teeth used for tearing food, and are located between the incisors in the front and the molars (grinding teeth) in the back. Diprotodontia have an empty space where canine teeth usually are located. This pattern of teeth has evolved because most members of this order are herbivores, or plant eaters. They need sharp front teeth to clip off the tough grasses and other plants that make up most of their diet, and they need molars to grind the plants, but they do not need canines to tear their food apart the way carnivores (meat-eaters) do. A few species in this order now eat insects, invertebrates, or flower nectar, but their tooth pattern suggests that at one time during their evolution, they also ate plants.
Besides sharing a common pattern of teeth, all diprotodonts have a condition in their hind limbs called syndactyly (sin-DACK-tuh-lee). Syndactyly means "fused toes." In members of this order, bones of the second and third toe on the hind feet have grown together into a single bone as far down as the claw. However, this fused bone has two separate claws—this twin claw is used for grooming. In many species in this order, the fourth hind toe is enlarged, and the fifth toe is either very small or absent.
On the front limbs of many species, the first two fingers oppose the other three. This means that these fingers, like the thumb on a human hand, can reach across and touch the tip of the other three fingers (unlike, for example, a dog paw or human foot where none of the toes can bend to touch each other). This adaptation is found mainly in species that live in trees, as it helps them grasp branches and climb.
Diprotodonts are marsupials, and like all marsupials they give birth to very poorly developed young after a short pregnancy. The young then attach to teats (nipples) in the mother's pouch and are carried for weeks or months until they mature enough to live independently. All diprotodonts have forward-opening pouches (like the kangaroo) except for wombats and koalas. Wombats are burrowing animals. A backward opening pouch is an advantage when digging, because it will not fill up with dirt. The backward opening pouch of the koala, which lives in trees, may be left over from a time when its ancestors lived on the ground and dug like the wombat.
Diprotodonts have soft fur, and many species have been hunted for their skins. Most species are earth tone colors, grays and browns, but some have quite eye-catching coloration, such as the yellow-footed rock wallaby, whose patches of red, yellow, and white contrast with its gray fur. Diprotodonts range in size from the red kangaroo, which weighs up to 187 pounds (85 kilograms) to the little pygmy possum, which weighs only about a quarter of an ounce (7 grams). In the past, the fossil record shows that there were much larger diprotodonts living in Australia. These animals became extinct about 50,000 years ago when humans first appeared in Australia.
Diprotodontia have evolved to take advantage of almost every terrestrial habitat. This expansion into different habitats is called radiation. Kangaroos graze on grasslands, cuscus and tree kangaroos live in tropical rainforest trees. Some pygmy possums live in the mountains where it snows six months out of the year. Despite the variety of habitats where members of this order can be found, some individual species live in very restricted areas, because they have evolved to use a very specific set of resources.
For the most part, diprotodonts are herbivores. Those species that do not eat leaves, fruits, and roots now, probably had ancestors that did. Many species have developed extra large or extra long digestive tracts that allow them to eat leaves and grass with low nutritional value. In addition, they have evolved behaviors that reduce their need for energy. For example, koalas sleep about twenty hours per day to conserve energy.
Some species, such as the mountain pygmy possum, feed heavily on insects. Others species eat insects, worms, and even occasionally a lizard, in addition to a mainly vegetarian diet. The honey possum has developed a long snout that allows it to feed exclusively on plant pollen and nectar.
BEHAVIOR AND REPRODUCTION
Diprotodonts are mainly active at twilight and night. The only species that is active exclusively during the day is the musky rat-kangaroo, although some diprotodonts that live in the forest tend to be active during both day and night. The mountain pygmy possum is the only diprotodont, and in fact the only marsupial, to hibernate or become inactive in cold months.
Many diprotodonts live alone, coming together only to mate, but there are exceptions. Kangaroos tend to associate in loose groups, called mobs, but there is no definite leader and no cooperation among members as there is in a structured group like a wolf pack. Common wombats visit each other's burrows and are not aggressive toward each other, but they do not live together in social groups. Likewise, koalas live near each other, but have their own personal space. On the other hand, hairy-nosed wombats may live in large groups of up to fifty animals, sharing a series of interconnected burrows. Small possums, such as the honey possum and feather-tailed possum, may huddle together for warmth, but larger species of possum live alone. Diprotodonts can be very noisy. They use barking, sneezing, hissing, grunting, gurgling, and growling to mark their territories and communicate their moods to other members of their own species.
In terms of reproduction, diprotodonts, like all marsupials, have short (two weeks to one month) pregnancies. At birth, the newborn is tiny (in some species, as small as a jelly bean). The young are carried in the mother's pouch for weeks or months until they can survive in the outside world. Many species continue to nurse their young after they leave the pouch. Wombats and possums carry their young on their back after they outgrow the pouch. In many species the young may remain with the mother outside the pouch for up to several months before becoming completely independent.
DIPROTODONTS AND PEOPLE
Two members of this order, the kangaroo and the koala, and are national symbols of Australia, and are used heavily in tourist promotions. Kangaroos have been hunted since the first humans arrived in Australia. Today there is a market for kangaroo meat, both for use in pet food and for humans, and leather made of kangaroo skins. The common brushtail possum has adapted to suburban environments, and is considered a nuisance. Introduced into New Zealand in 1840, the brush-tailed possum is an invasive alien (introduced, non-native species) that damages native plants and animal habitats. Many farmers also see the wombat as a pest, since its burrows allow rabbits (invasive aliens in Australia) to cross under fences intended to keep them out of grasslands. Kangaroos, however, have benefited from the colonization of Australia by Europeans. Europeans cleared the land for grazing livestock. This increased the amount of grassland habitat favorable to kangaroos and allowed their populations to increase.
The arrival of Europeans and the animals they introduced (rabbits, red foxes, cats, sheep, cattle) significantly changed the habitats of some diprotodonts and put others in direct competition with these introduced animals for food. Hunting, clearing the land for farming, changing patterns of burning grassland, and economic development have put pressure on these animals, often forcing them into marginal habitats, reducing their range or fragmenting them into isolated populations.
One might think that with our ability to go to every corner of the planet, all the marsupials in Australia and New Guinea would have been discovered. Imagine scientists' surprise and excitement in the 1980s when two new species of diprotodonts were discovered in Australia. Then, in the 1990s, four new diprotodonts were found in New Guinea. It is possible that in the twenty-first century, other adventurous scientists will find still more new species from this order.
About 25 percent of the species in this order are considered threatened or potentially in danger of extinction. Six species have gone extinct in recent years. However, three other species thought to be extinct have been found to be still alive, although considered Critically Endangered, facing an extremely high risk of extinction. The northern hairy-nosed wombat is also Critically Endangered, with possibly fewer than 100 individuals left in the wild. Its cousin, the southern hairy-nosed wombat, is Endangered, facing a very high risk of extinction, because of its limited range. On the other hand, the koala, once threatened with extinction in 1920, has been the target of successful conservation (though in some areas, koalas are dying or being relocated because of overcrowding).
FOR MORE INFORMATION
Finney, Tim F. Mammals of New Guinea, 2nd ed. Ithaca, NY: Cornell University Press, 1995.
Menkhorst, Frank. A Field Guide to the Mammals of Australia, 2nd ed. Oxford, U.K.: Oxford University Press, 2001.
Nowak, Ronald M. Walker's Mammals of the World. Baltimore: Johns Hopkins University Press, 1995.
Australian National Parks and Wildlife Service. http://www.nationalparks.nsw.gov.au/ (accessed on June 30, 2004).
Marsupial Society of Australia. "Fact Sheets." http://www.marsupialsociety.org (accessed on June 30, 2004).
Parks and Wildlife Service Tasmania. http://www.parks.tas.gov.au (accessed on June 30, 2004).
Queensland Government Environmental Protection Agency/Queensland Parks and Wildlife Service. "Nature Conservation." http://www.epa.qld.gov.au/nature_conservation (accessed on June 30, 2004).