Kangaroo - biology.

Kangaroo - biology. I INTRODUCTION Kangaroo, common name for a group of mammals found in Australia and neighboring islands. Kangaroos are marsupials, a type of mammal that gives birth to undeveloped young. In kangaroos and many other marsupials, the young are carried and nurtured in a special pouch on the mother's body. More than 50 different kinds of animals are grouped together in two kangaroo families. The large kangaroos include red kangaroos and gray kangaroos, wallabies, pademelons, and quokka; they belong in the family Macropodidae. The other family, Potoroidae, is made up of assorted smaller species, such as various rat-kangaroos, bettongs, and potoroos. The largest kangaroos are the gray kangaroo and red kangaroo, which can stand up to 2 m (6.5 ft) tall and weigh up to 85 kg (187 lb). The smallest are the musky rat-kangaroos, ratlike animals measuring about 30 cm (12 in), not including the tail. Kangaroos are found only in mainland Australia and New Guinea and on some of their offshore islands, such as Tasmania. A few species have been introduced to other countries, such as tammars in New Zealand and a rock wallaby in Hawaii. Kangaroos inhabit every imaginable type of habitat throughout Australia and New Guinea. Red kangaroos and gray kangaroos are found in grasslands, savannas, and open woodlands. Rock wallabies live on nearly vertical rock walls in the southern desert. Bettongs inhabit burrows in arid scrubland. As their name implies, tree kangaroos are found high in the rain forest canopy, while rat-kangaroos scamper nimbly through the dense, wet understory below. Red-necked wallabies live in many habitats, including the frigid peaks of Tasmania's mountains, and the endangered quokka's last refuge is just two windblown islands off the southwestern Australia coast. Several species of kangaroos are so adaptable that they are common inhabitants of public parkland, suburban gardens, and even golf courses. Fossil evidence shows that the first kangaroos appeared in Australia about 15 million years ago, during the Miocene Epoch. As a result of climate changes about 8 million years ago, Australia's rain forests gave way to open woodlands and savanna, and kangaroos evolved into many of the forms we know today. The now extinct giant kangaroos stood up to 3 m (10 ft) high and weighed 200 kg (440 lb). II PHYSICAL CHARACTERISTICS Most kangaroo species have large ears and relatively small heads. Their small front feet have five digits of unequal length, while the back feet typically have four digits. One of the toes on each hind foot has a sharp claw used in defense. Many species are drably colored to closely match their surroundings, helping them to hide from predators. Males of the larger kangaroo species are noticeably bigger in size than the females, a trait known as sexual dimorphism. A physical feature that distinguishes kangaroos from other marsupials is the adaptation of the hind legs and tail for hopping. In red kangaroos, for example, the hind legs are very large--roughly ten times the size of the small front limbs. When moving slowly, kangaroos use all four legs. When they need to move fast, they rise up on their muscular hind legs and start hopping. The leaps of some kangaroos are legendary. Red kangaroos moving at top speeds can cover 5 m (16 ft) or more in a single bound. They are able to reach speeds of more than 50 km/h (31 mph) for short periods. The kangaroo's heavy, muscular tail is also important for locomotion. It serves as a counterbalance and a rudder during hopping, and provides support when the animal sits. The only member of the kangaroo family that does not hop is the musky rat-kangaroo. Scientists have tried to learn why hopping is so widespread among kangaroos but is so rare in other mammals. Studies have shown that hopping is energy efficient for kangaroos, partly because of springlike tendons in their legs. The energy required to start hopping is quite high, but very little extra energy is needed to hop faster and faster. In contrast, mammals that run on all fours require increasing amounts of energy as speed increases. III REPRODUCTION Like all marsupials, kangaroos lack a true placenta, the structure in a pregnant female's womb that provides nutrients for and removes waste from the growing embryo. Instead of a fully developed placenta, kangaroo females form a type of yolk sac in the womb. The embryo absorbs nutrients from this yolk sac for four to five weeks and then emerges from the birth canal, still in an embryo-like form but with well-developed forelimbs. The tiny animal, weighing about 1 g (0.04 oz), crawls along its mother's body and into an abdominal pouch. Once inside, the offspring attaches firmly to a nipple, suckling milk. It remains there for several months until it is more fully formed and can move about on its own. The young kangaroo, sometimes called a joey, may remain with the mother, climbing into her pouch for nourishment or safety, until it is more than a year old. All kangaroos produce a single young per litter except for the musky rat-kangaroo, which usually produces two young. Depending on environmental conditions and food resources, one litter is typically born each year. Some kangaroo species living in dry regions have evolved the ability to breed whenever food and water resources are favorable. This may result in normal reproduction or no litters for several years if conditions are poor. The unpredictable nature of resources has led to the evolution of another specialized reproductive feature in some kangaroos. As soon as the tiny offspring emerges from the womb, the female is able to mate again. While the newborn suckles a nipple in the mother's pouch, a fertilized egg begins developing in her womb. The development of the fertilized egg stops abruptly when the egg becomes a simple, hollow ball of about 100 cells, called a blastocyst. The blastocyst remains in a state of suspended development in the female's womb while the offspring in the pouch continues to grow. After the pouch offspring is more mature and is able to leave the mother's pouch (up to six or seven months later), the blastocyst resumes development and soon emerges from the womb and moves into the now empty pouch. This reproductive feature, called embryonic diapause, enables kangaroos to care for up to three litters simultaneously during desirable conditions: an older, weaned offspring still under the mother's care, another maturing in the pouch, and a third in suspended development in the uterus. Should adverse conditions cause the weaned young to die, other offspring are still available for rearing. The life span of the larger kangaroos ranges from 12 to 18 years in the wild, but this is greatly influenced by climate and food supply. In captivity, large kangaroos are known to live as long as 28 years. Smaller species have a somewhat shorter life span. IV DIET Most kangaroos are herbivores, or plant-eaters. The larger species, such as the red kangaroos and gray kangaroos, are grazers, feeding on grasses and other vegetation in open forests and savannas. Medium-sized, forest-dwelling species--such as the swamp wallaby, tree kangaroo, and wallaroo--browse on leaves, shoots, and twigs. The small forest inhabitants--rat-kangaroos, bettongs, and musky rat-kangaroos--tend to be omnivores, eating both plants and small animals. Some of these small forest-dwellers specialize in eating fungi, tree gum residues, insects, worms, and even dead animals, or carrion. Kangaroos are similar to hoofed mammals, or ungulates, such as the deer and antelope, in their ability to digest plant matter that is high in fiber and low in protein. Like ungulates, they have evolved multi-chambered stomachs containing bacteria that can break down plant cell walls and release the nutritious cell contents. V SOCIAL ORGANIZATION Kangaroo social life is poorly understood, as few species have been studied in detail. Small species, such as potoroos, bettongs, and musky rat-kangaroos, tend to be solitary and widely dispersed, with a male's territory encompassing those of several females. The animals come together briefly for mating, and mother and offspring remain together only until the young can live on their own. In contrast, red kangaroos and gray kangaroos are highly social, forming large groups, called mobs, of ten or more males and females. Males form a hierarchy in the mob based largely on age and size. The most dominant male, called a boomer, has exclusive access to the females for mating. He spends much of his time wandering in and out of the mob, checking the reproductive condition of the females and also intimidating other males who attempt to mate with them. It may take a male ten years to achieve the status of a boomer, but he will remain in this position for a year at most. A younger, more vigorous male soon forces him out of his position and even out of the mob, and he soon dies. Larger kangaroos, although ordinarily timid, are dangerous when threatened by rivals. In a form of ritualized wrestling, kangaroos support themselves on their hind legs and tail and pummel attackers with forepaws and kick with their powerful hind legs. VI SURVIVAL Aboriginal hunters once relied on kangaroos for their meat and hides but caused minimal impact on kangaroo populations. As Europeans settled the Australian continent, overhunting coupled with the introduction of livestock that ate the grasslands providing cover for some smaller species caused some kangaroo populations to decline. Today some smaller, more specialized kangaroos are threatened by habitat destruction, which reduces the areas where kangaroo species can survive. Predatory mammals introduced to Australia, such as the fox, domestic dog, and domestic cat, prey on small kangaroos. And other introduced mammals, especially rabbits and livestock, compete with kangaroos for food resources. The larger kangaroos are commonly hunted and poisoned by ranchers, who consider them pests that overgraze land. Despite these pressures, the larger kangaroo species are still thriving. About ten are considered endangered, including several rat-kangaroos, two hare wallabies, and two nailtail wallabies. Scientific classification: Kangaroos make up the two kangaroo families, Macropodidae and Potoroidae, in the order Marsupialia. The red kangaroo is classified as Macropus rufus; the two species of gray kangaroo are classified as Macropus giganteus and Macropus fuliginosus; the ring-tailed rock wallaby as Petrogale xanthopus, the bettong as Bettongia penicillata, and the musky rat-kangaroo as Hypsiprymnodon moschatus. Contributed By: Miles Roberts Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.