Kangaroos and Wallabies
Kangaroos and Wallabies
The difficult life of a newborn kangaroo
Kangaroos and wallabies are pouched mammals, or marsupials, of Australia and nearby islands. They have hind legs enlarged for leaping. Most species live on the ground, and some in trees. The name kangaroo is usually used for larger species, and wallaby for smaller ones. They all belong to the family Macropodidae, meaning “big footed,” and they are herbivorous, or plant-eating animals.
Kangaroos and wallabies hold the same place in their ecosystem as ruminants, such as deer. They graze and have similar mechanisms for chewing and digesting plants. Most members of the family are nocturnal, feeding at night.
The kangaroo’s hand has five clawed fingers, all approximately the same length. It can be used for grasping. The hind feet are quite different, being extremely large and having only four toes. The first two are tiny and are joined together at the bone but not at the claw. These claws are useful for grooming. The third toe is huge, with a strong, sharp claw. When fighting, the animal may use this claw as a weapon. The fourth toe is again small, but not as small as the grooming toes.
Kangaroos are famous for their prodigious leaps— sometimes up to 30 ft (9.2 m) long and 6 ft (1.8 m) high by the gray kangaroo. Because the spring–like tendons in their hind legs store energy for leaps, they are sometimes called “living pogo sticks.” It has been calculated that kangaroos actually use less energy hopping than a horse uses in running. When they are grazing, kangaroos tend to move in a leisurely fashion using all four feet plus the hefty tail for balance. They move the hind legs while balancing on the front legs and tail, then move the front legs while balanced on the hind legs, rather like a person walking on crutches. They often rest by reclining on their side, leaning on an elbow.
Most kangaroos are unable to walk in normal fashion, moving the hind legs at separate times. However, tree–dwelling kangaroos have the ability to move their hind legs at different times as they move among the branches.
Like all marsupials, female kangaroos have a protective flap of fur–covered skin that shields the offspring as they suckle on teats. The kangaroo’s marsupium, as this pouch is called, opens toward the head. The pouch is supported by two bones, called marsupial bones, attached to the pelvis. No other mammals have these bones, but even male kangaroos do, despite the fact that they do not have pouches.
Some kangaroos live in social groups and others are solitary. In general, the larger animals and the ones that live in open grasslands are more social. Within a group, called a mob, the individuals are safer. In a mob, the dominant male competes with the others to become the father of most of the offspring, called joeys. Because the dominant male is larger than the other males (called boomers), over many generations, through sexual selection the males have evolved to become considerably larger than the females (called does).
The difficult life of a newborn kangaroo
For such a large animal, the gestation period of kangaroos is incredibly short. The longest among the kangaroos is that of the eastern gray kangaroo (Macropus giganteus), in which the baby is born after only 38 days. However, it is less than an inch long, blind, and hairless like the newborns of all marsupials. It may weigh as little as 0.01 oz (0.3 g).
The kangaroo has virtually no hind legs when born. In fact, the front legs, which are clawed, look as if they are going to be mammoth. These relatively large front paws serve the purpose of pulling the tiny, little–formed creature through its mother’s fur and into her pouch. Instinct guides the tiny infant, who moves with no help from its mother. If it moves in the wrong direction, the mother ignores it. If it moves slowly, it may die from exposure. These tiny creatures are born with disproportionately large nostrils, so smell apparently plays a major role in guiding the path to the mother’s pouch.
A newborn kangaroo has a longer distance to travel than most marsupials. Most others have a pouch that faces backward, or is an open flap of skin where it is easier for the baby to find the teats. In the kangaroos, the baby must climb up to the top of the pouch, crawl over the edge, and then back down inside to reach a teat.
If successful in reaching the pouch, the baby’s tiny mouth clamps onto a teat, which swells into the mouth so that the infant cannot release it. The infant’s
esophagus expands so it can receive both nourishment and oxygen. The baby, now called a pouch embryo, cannot let go even if it wants to. It will be a month or more before its jaw develops enough to open. Only the teat that the baby is attached to actually produces milk.
After the infant is born and moves into the pouch, a female red kangaroo (Macropus rufus) may mate again. This time, however, the fertilized egg goes into a resting state and does not develop until the female stops nursing the first young. Her body signals that change to the zygote, which then starts developing again. This time lag, called diapause, has great advantages to the species in that if one young dies, another embryo can quickly take its place. Diapause does not occur in the eastern gray kangaroo. The pouch embryo will continue to develop as it would if inside a uterus. In the big kangaroos, it takes 10 months or more before the joey emerges for the first time (often falling out by accident). It gradually stays out for longer and longer periods, remaining by its mother’s side until about 18 months old. A male great kangaroo reaches sexual maturity at about two or three years, a female not for several years more.
The great kangaroos
One fossil kangaroo, Procoptodon goliah, was at least 10 ft (3.1 m) tall and weighed about 500 lb (227 kg). Today, the largest of the species is the male red kangaroo, which may have a head–and–body length of almost 6 ft (1.8 m), with a tail about 3.5 ft (107 cm) long. It may weigh 200 lb (90 kg).
Fourteen species of living kangaroos belong to the genus Macropus. They include the largest living marsupials. In varying contexts and times they have been regarded as pests, or as among the treasures of Australia. Farmers have argued that kangaroos take food from sheep and cattle, but actually kangaroos tend to select different plants from domestic livestock. Today, only a few are seen near urban areas, but they are widespread in the countryside, where they are still a favorite target of hunters, who sell their meat and skins.
The eastern gray kangaroo and its western relative (M. fuliginosus), which is actually brown in color, occupy forest areas throughout the eastern half and the southwest region of Australia. The forest–living species mostly eat grasses. Their young are born at more predictable times than those of the red kangaroos and they take longer to develop. They spend about 40 weeks in the pouch, and the mother does not mate again until the joey becomes independent and mobile.
The red kangaroo shares the western gray kangaroo’s habitat. As European settlers developed the land, forests were cut, reducing habitat for the gray kangaroos, but increasing the red kangaroo populations. Only the male red is actually brick red; the female is bluish gray, giving it the nickname “blue flier.” It has the ability to care for three young at different stages of their lives at once. It can have an egg in diapause in its uterus, while a tiny pouch embryo can be attached to one teat. Then, another teat elongates so that it is available outside the pouch, where an older, mobile offspring can take it for nourishment. This situation probably evolved in response to the dryness of the red kangaroo’s desert environment, which can easily kill young animals.
In the continental interior, the red kangaroo lives in open dry land, while wallaroos, also called euros (M. robustus), live around rock outcroppings. The wallaroos, which have longer and shaggier hair than the larger kangaroos, are adapted for surviving with minimal water for nourishment. When water is not available, the animal reduces the body’s need for it by hiding in cool rock shelters, and their urinary system concentrate the urine so that little liquid is lost.
The smaller wallabies
Smaller kangaroos are usually called wallabies. The name is especially used for any kangaroo with a hind foot less than 10 in (25 cm) long. Several species of Macropus are regarded as wallabies as well as species in other genera. The two smallest are the tammar wallaby (M. euge–nii) of southwestern Australia and adjacent islands and the parma wallaby (M. parma) of New South Wales. Their head and body are about 20 in (50 cm) long with tail slightly longer. The tammar wallaby has been known to drink saltwater. The whiptail wallaby (M. parryi) is the most social of all marsupials. It lives in mobs of up to 50 individuals, and several mobs may occupy the same territory, making up an even bigger population.
Rock wallabies (Petrogale species) have soft fur that is usually colored to blend in with the dry, rocky surroundings in which they live. However, the yellow–footed rock wallaby (P. xanthopus) is a colorful gray with a white strip on its face, yellow on its ears, dark down its back, yellow legs, and a ringed yellow–and–brown tail. Rock wallabies have thinner tails than other wallabies and use them only for balance, not for propping themselves up. They are very agile when moving among the rocks. Some have been known to leap straight up a rock face 13 ft (4 m) or more. Rock wallabies have sometimes been kept in zoos, where they live and breed in communal groups.
The smaller hare wallabies are herbivores that feed mostly on grasses and can run fast and make agile jumps. Close study of the hare wallaby called the quokka (Setonix brachyurus) provided naturalists with their first solid information about marsupials. With a head–body length of about 20 in (50 cm), plus a 10–in (25–cm) tail, this rodent like creature lives in swampy areas in southwestern Australia. Today it lives mainly on neighboring islands. Several other species are rapidly disappearing and one, the central hare wallaby (Lagorchestes asomatus), is known from only one specimen found in 1932. It is thought to be extinct.
Several wallabies that were widespread in the past are probably extinct. Nail–tailed wallabies (Onychogalea species) had tough, horny tips to their tails. These 2 ft (61 cm) tall marsupials lost their habitat to grazing livestock and agricultural pursuits, and were also hunted. Nail–tails were also called organ grinders, because their forearms rotated while they were hopping.
Five species of wallabies (Dorcopsis species) live only in the rainforest of New Guinea. They do not hop as well as other kangaroos because their hind feet are not much larger than their front. Somewhat smaller kangaroos called pademelons (Thylogale species) live in New Guinea as well as on the Australian continent.
Tree kangaroos
The tree kangaroos (Dendrolagus species) are herbivores that live in trees in mountainous forest of New Guinea and Australia. They have fairly long fur and live in small groups. Some of them have the ability to leap between strong branches of trees as much as 30 ft (9.2 m) apart.
Tree kangaroos have longer forearms and longer tails. Although their tail is not truly prehensile, or grasping, they may wrap it around a branch for support. Unlike other kangaroos, their tail is about the same thickness from base to tip. Tree kangaroos are hunted as food and so they are decreasing in numbers. The single young stays in the pouch for almost a year and suckles even longer, so the rate of reproduction is rather slow.
Rat-kangaroos
A subfamily of smaller, more ancient marsupials is called rat–kangaroos. Most scientists classify the rat–kangaroos in a separate family, the Potoroidae. These animals are omnivorous, eating a variety of foods.
The musky rat-kangaroo (Hypsiprymnodon moschatus) is the smallest kangaroo, with a head–body length of only about 10 in (25 cm) plus a furless tail (the only one in the family) of about 5 in (12.7 cm). This species also has front and hind feet closer to the same size than any other member of the family. It eats some insects along with grasses and other plants. (Some scientists separate the musky rat–kangaroo into its own family, the Hypsiprymnodontidae, due to differences in the teeth and skull.) The potoroos (Potorous species) are about twice as large as the musky rat–kangaroos and display a more advanced leaping ability.
The desert rat-kangaroo (Caloprymnus campestris) was first seen in southern Australia in 1843, but not again until 1931. There have been no reliable records of this species since 1935 and it is thought to be extinct. The northern rat-kangaroo (Bettongia tropica) of Queensland was observed in the 1930s, but not again until 1971. It has huge hind feet, which cover half the length of its body. The species is endangered for reasons that are unclear and two captive breeding population have been established to learn more about the species.
The bettong, also called the woylie or brush-tailed rat-kangaroo (Bettongia penicillata), has a prehensile tail, which it uses to carry the dry grasses used in building a nest. Woylies were quite common over southern Australia, but as human populations have increased, it has become extirpated over most of its original range. Similarly threatened is the boodie or short-nosed rat-kangaroo (B. lesueur). The only kangaroo that digs burrows, where it gathers in a family group, it is now restricted to several islands in western Australia’s Shark Bay. Unlike the other members of the kangaroo family, the boodie never uses its front feet while walking.
Clearly, many of the smaller kangaroos are endangered and even nearing extinction. Apparently, they are vulnerable to even small changes in their habitat. The great kangaroos, on the other hand,
KEY TERMS
Diapause —A period during which a fertilized egg does not implant and start to develop. A change in the mother’s hormone system, perhaps triggered by favorable weather conditions, signals the egg to start developing.
Embryo —A stage in development after fertilization.
Herbivorous —An animal that only eats plant foods.
Marsupium —The pouch or skin flap that protects the growing embryo of a marsupial.
Prehensile —Of a tail, able to be used for grasping.
Ruminant —A cud-chewing animal with a four-chambered stomach and even-toed hooves.
Uterus —Organ in female mammals in which embryo and fetus grow to maturity.
appear to be thriving as long as their habitats remain protected and hunting for their skin and meat is conducted on a sustainable basis.
Resources
BOOKS
Dawson, T.J. Kangaroos: The Biology of the Largest Marsupials. Kensingston, Australia: University of New South Wales/Ithaca, 2002.
Knight, Linsay. The Sierra Club Book of Great Mammals.San Francisco: Sierra Club Books for Children, 1992.
Lyne, Gordon. Marsupials and Monotremes of Australia. New York: Taplinger, 1967.
Nowak, Ronald M. Walker’s Mammals of the World. 6th ed. Baltimore: The Johns Hopkins University Press, 1999.
Strahan, R.The Mammals of Australia. 2nd ed. Sydney: Reed New Holland, 2002.
Stidworthy, John. Mammals : The Large Plant-Eaters Encyclopedia of the Animal World. New York: Facts On File, 1988.
Tyndale-Biscoe, Hugh. Life of Marsupials. New York: American Elsevier Publishing Co., 1973.
Jean F. Blashfield