Devoir de Philosophie

Koala - biology.

Publié le 11/05/2013

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Koala - biology. I INTRODUCTION Koala, tree-dwelling marsupial with a specialized diet consisting almost exclusively of the leaves of eucalyptus trees. Koalas are native to Australia, where they are sometimes called koala bears or native bears, although koalas are not related to bears. Bears are placental mammals--their young develop fully inside the mother's body, nourished by a spongy organ called the placenta. As marsupial mammals, koala young are born while still in an undeveloped state. Newborn koalas complete their development inside a pouch on the mother's abdomen. Few wild animals rival the koala bear in popularity and recognition. Its teddy-bear appearance inspires human affection and intense concern for its survival, as more and more eucalyptus forests are cleared to make room for farmland, housing, and industry. II RANGE AND HABITAT Koalas depend on eucalyptus trees, also known as gum trees, for shelter and food. Before Europeans colonized Australia, koalas were found in eucalyptus forests throughout the continent. When European settlers arrived in Australia in the 19th century, they began clearing eucalyptus forests to create farmland. By the end of the 20th century, more than 80 percent of Australia's eucalyptus forests were destroyed. Today koalas survive in the remaining small patches of eucalyptus forests throughout eastern Australia, from the tropical northeast to the cooler southeast. These patches of forest are widely separated from each other, preventing koalas from moving from one patch to another to find new breeding partners or additional eucalyptus trees. There are more than 800 species of eucalyptus trees, but koalas are particular--they prefer the leaves from only about 35 species. Different species of eucalyptus trees grow in different parts of Australia, so koalas in the north feed on different species than koalas in the south. For instance, in the south koalas are more abundant in forests where the dominant eucalyptus tree is the manna gum tree (Eucalyptus viminalis). In the north koala populations are largest in forests where forest red gum (Eucalyptus tereticornis) or river red gum (Eucalyptus camaldulensis) trees are prevalent. Koalas feed only on healthy trees and will shun trees damaged by poor soil, cold temperatures, or low rainfall. III PHYSICAL DESCRIPTION Koalas resemble a miniature bear, with a large head above a stocky body, a tiny tail, and long muscular limbs. They have a round face with forward-facing eyes; a blunt, prominent nose; and two round ears on top of the head. Koalas range in size from 69 to 79 cm (27 to 31 in) in length, and from 5 to 12 kg (11 to 26 lb) in weight. Males are generally larger than females, and koalas in the south tend to be larger than their counterparts in the north. Thick, woolly fur covers the koala's body. For most koalas, white fur covers the throat and chest and fringes their rounded ears, but elsewhere on the body fur color and length vary depending on where they live. Koalas living in the cooler climates of the south have long fur that is gray-brown or cinnamon in color. Those found in tropical northern ranges have shorter fur that is light silvery-gray in color. Koala paws are well adapted for gripping and climbing trees. The koala's two forepaws end in five digits (fingers or toes), each with stout, curved claws used for climbing. The first two digits are opposite the other three, as if the koala has two thumbs and three fingers. This paw structure enables the koala to grasp tree branches. Each of the koala's two hind paws also has five digits. A broad, clawless first digit is opposite the other four digits. The second and third digits are fused into a single, double-clawed toe that is used like a comb to groom fur. Mature male koalas have a brown scent gland in the center of the chest. This gland produces a smelly substance that leaves the surrounding fur wet and dark. Males rub this gland on tree trunks and branches, and the scent left behind may play a role in communication among koalas. Characteristic of marsupials, female koalas have a pouch on the belly where a developing baby, known as a joey, lives for the first seven months after it is born. The pouch opens at the top, although the weight of a growing joey forces the pouch opening downward. The eyes of koalas face forward on the head, enabling the animal to focus both eyes on a single object. This ability, known as binocular vision, allows koalas to accurately judge distances, an important skill for an animal that leaps between tree branches. Koalas appear to have a good sense of smell--they sniff leaves before selecting one to eat, suggesting that they can smell a difference among individual leaves. They also sniff the gland secretions left by males at the base of trees and on branches. Both wild and captive female koalas may live 13 to 20 years, while males generally have a maximum life span of about 12 years. In the wild, however, most koalas die younger due to predation, disease, or accidents with motor vehicles. IV DIET AND DIGESTION Koalas eat about 200 to 500 g (7 to 18 oz) of eucalyptus leaves per day. Eucalyptus leaves are low in nutrition, however, because they contain little protein and largely consist of fiber and other indigestible compounds. The koala has evolved unique adaptations to exist on this diet, and these adaptations influence many aspects of the animal's digestive process, from the shape of teeth to the speed of digestion. The teeth of koalas include four incisors at the front of the mouth, two in the upper jaw and two in the lower jaw. Cheek teeth, consisting of one premolar and four molars, are found in both the upper and lower jaws. A gap, called the diastema, separates the incisors and cheek teeth. Koalas use the incisors to grasp a single leaf and pull it into the mouth through the diastema. The animals then bite the leaf with their premolar, separating the leaf from its branch. Sharp structures called cusps, which are found on the surface of each of the four molars, act like scissors to cut the leaf into tiny particles when the koala chews. The smaller the leaf pieces, the easier it is to extract nutrients from the leaf's cells during digestion. The molars of older koalas lose their cusps after years of wear, and these animals can no longer chew their food into fine pieces. Worn cusps may cause older koalas to starve even though they ingest the same quantities of leaves as koalas with sharp cusps. Koalas have a slow metabolic rate. As a result, food particles remain in the digestive system for up to 100 hours. This extended period permits koalas to derive the most energy possible from nutrient-poor eucalyptus leaves. During digestion, food particles pass through the koala's stomach to the small intestine, which extracts most of the food's nutrients. Food particles then move into the fermentation chamber composed of a spacious fiber-digesting organ, called the caecum, and part of the colon. The fermentation chamber contains millions of microbes that break down food, removing any remaining nutrients. Another part of the colon removes water from food particles--koalas rarely drink water as they receive all the water they need from the leaves they eat. V REPRODUCTION Female koalas become sexually mature around 18 to 24 months of age. They can produce one offspring a year until they reach about 13 years of age. Males begin to produce sperm around age 2 and, in the absence of older, stronger males, they may breed at that young age. More often, however, a male must grow big enough to compete with other males for females, and mating generally begins for males at about 4 years of age. The breeding season for koalas is from October to May, during the spring and summer months in Australia. This seasonal breeding allows young to emerge from the mother's pouch when food is abundant. Mating occurs during estrus, the brief stage in the female's reproductive cycle when eggs are released and she is receptive to mating. Koalas partake in courtship behaviors that look a lot like fighting. With much bellowing, a male attacks and tries to mate with every female in the area. The male seems unable to tell the difference between a female that is in estrus and one that is not, an unusual disability for a mammal. If a female is not in estrus, she will drive the male away by biting and scratching to escape his advances, while screaming loudly. If a female is in estrus mating occurs, usually at night. Males and females do not stay together after mating and males play no role in raising young. After mating, gestation (the time between conception and birth) lasts about 35 days. Usually only one young is born; twins are rare. The newborn joey is about the size of a small grape and weighs less than 6 g (0.2 oz). At birth the joey is hairless, its eyes are closed, and it has no ears. Its hind limbs are just buds, but its shoulders and front legs are more developed, with tiny claws. The newborn uses these front legs to crawl from the birth canal through its mother's belly fur to her pouch opening. Once in the warm pouch, the joey attaches its lips to one of the mother's two nipples. The joey will stay in the pouch for about seven months, growing rapidly. After about three months, the joey weighs nearly 57 g (2 oz), its eyes have opened, its ears have developed, and fur begins to appear. At about six months, the joey has full body fur and begins to poke its head outside the pouch. After seven months the joey no longer fits in the pouch and rides around on its mother's back. A joey drinks only its mother's milk until around six months of life, when it begins to eat leaves that have been digested and eliminated by its mother. These mushy yellowish-green leaves, called pap, help the joey acquire the intestinal microbes necessary for the digestion of eucalyptus leaves. The joey eventually begins to eat raw leaves, foraging from its mother's back. Joeys become fully weaned around 12 months. At this time, the mother is usually pregnant again and stops feeding the joey. The joey may stay near the mother for another 6 to 12 months. By age 2, females are sexually mature and they may settle near their mothers, but males are often driven away by other adult males and must find a new territory. VI BEHAVIOR Koalas spend much of their time in a tree, resting in order to conserve energy to compensate for their nutrient-poor, low-energy diet. They sleep about 20 hours a day, sit and munch food for 1 to 3 hours a day, and spend only about four minutes a day actively moving between branches or trees, grooming, and partaking in social activities. Most feeding occurs from about 5 PM to midnight. Koalas feed in several "bouts," each lasting an average of 20 minutes. Koalas sleep in the same trees that they feed in, resting in the forks of trees and occasionally stretching along a branch. They change trees once a day or less, typically climbing down a tree feet first and walking awkwardly on the ground to another tree. At a new tree koalas spring from the ground to catch their claws in the bark and they then use their muscular limbs to scramble up the trunk. In trees, large pythons are the koala's primary predators, and owls and eagles sometimes snatch young koalas off their mothers' back. While on the ground the koala is vulnerable to predators such as dingoes and domestic dogs. Some koalas are hit by motor vehicles when they cross a road to get to another tree. Except for the mother and her young, koalas live alone. Each koala maintains a home range consisting of a number of trees. Within its home range, a koala uses only a small number of trees, with up to a third of its time spent in one favorite tree. Home ranges vary in size depending on the nature of the habitat, and male home ranges are larger in size than those of females. In areas of good habitat--marked by fertile soil, warmer temperatures, plentiful rainfall, and the preferred eucalyptus species--home ranges average 1 to 2 hectares (3 to 4 acres); in moderately good habitat home ranges average 2 to 3 hectares (5 to 8 acres); and in very poor habitat a male home range may reach nearly 200 hectares (500 acres). The home range of a koala may overlap with the home ranges of other koalas. When there are many individuals in one area, adults will share a single tree, but only briefly. It is in the shared trees that most social interaction occurs among koalas. Koalas communicate with vocalizations, including loud, low bellows emitted by adult males. Males bellow year-round, but most often during the breeding season when males fight with other males to gain dominance. Occasionally a koala may move a long distance, especially a young animal leaving its mother to strike out on its own. One study reported a male koala moving more than 8 km (5 mi) during a six-week period. VII STATUS OF KOALAS Koalas are not classified as an endangered species under Australia's Endangered Species Protection Act. They are categorized as "lower risk/near threatened," in the Red List of Threatened Species compiled by the World Conservation Union (IUCN), a nongovernmental organization that compiles global information on endangered species. While these classifications indicate a relatively low level of concern, the number of koalas in Australia and their endangered status is a matter of debate among government officials and conservationists. Wildlife management experts in the Australian federal and state governments believe koalas are abundant and in little danger of extinction nationally, although these authorities do believe that some local populations may be threatened. Other scientists and conservationists hotly dispute this view because they believe that the koala is in danger of extinction, primarily threatened by habitat loss. Over the last 200 years more than 80 percent of eucalyptus forests have been cleared to make room for farming, housing, and other human development. Forest fires have also destroyed koala habitat. As more and more eucalyptus forests have been decimated, koala populations have become fragmented, surviving in the few remaining patches of forests. Unable to travel to other forests, koalas have a limited choice of breeding partners, increasing the chance for inbreeding (mating between closely related animals). Biologists are concerned that the genetic diversity of koala populations will steadily decline as a result of inbreeding. This growing genetic uniformity can have serious consequences. Because inbred animals share so many genes, they are likely to be equally vulnerable to disease and other threats. If a potentially lethal disease strikes a koala population with a similar genetic makeup, the entire group may die. Fragmented populations can also lead to overpopulation of a limited habitat. When a koala population grows too large for a patch of forest, the animals over browse leaves and destroy the very trees that sustain the koala population, resulting in starvation. Experts believe that stress due to habitat loss, overpopulation, and other threats increases the incidence of symptoms caused by chronic infections from various strains of the bacterium Chlamydia. One strain of this bacterium causes eye disease and blindness; another causes reproductive tract disease that leaves females sterile. Mating and other contact among koalas may spread the infection. Chlamydia may also pass from an infected mother to her young during birth and also when the young consume pap. Coping with koala overpopulation is a vexing problem for conservationists and governments. Sometimes koalas are relocated to new habitats, but this is costly and provides only a short-term solution since koala habitats continue to be lost. Programs that sterilize koalas have been effective in reducing overpopulation in certain areas, but these programs are also expensive, require long-term monitoring of sterilized animals, and are a realistic solution for only small populations. Culling (killing a prescribed number of koalas) is often proposed as a solution to overpopulation, but this option has never been implemented because many people object to killing these beloved animals. Conservationists agree that the best, most cost effective solution is to protect and restore the habitat of koalas. Most of the remaining eucalyptus forests in Australia are not protected by federal law. Conservationists in Australia are working to pass federal laws that protect koala habitat. In 2000 the United States Fish and Wildlife Service listed the koala as threatened under the U.S. Endangered Species Act. While the U.S. listing does not provide practical aid to the koala, it has raised awareness of the koala's plight. Scientific classification: The koala is the only member of the family Phascolarctidae. It is classified as Phascolarctos cinereus. Contributed By: John Seidensticker Susan Lumpkin Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.

« V REPRODUCTION Female koalas become sexually mature around 18 to 24 months of age.

They can produce one offspring a year until they reach about 13 years of age.

Males begin toproduce sperm around age 2 and, in the absence of older, stronger males, they may breed at that young age.

More often, however, a male must grow big enough tocompete with other males for females, and mating generally begins for males at about 4 years of age. The breeding season for koalas is from October to May, during the spring and summer months in Australia.

This seasonal breeding allows young to emerge from themother’s pouch when food is abundant.

Mating occurs during estrus, the brief stage in the female’s reproductive cycle when eggs are released and she is receptive tomating. Koalas partake in courtship behaviors that look a lot like fighting.

With much bellowing, a male attacks and tries to mate with every female in the area.

The male seemsunable to tell the difference between a female that is in estrus and one that is not, an unusual disability for a mammal.

If a female is not in estrus, she will drive themale away by biting and scratching to escape his advances, while screaming loudly.

If a female is in estrus mating occurs, usually at night.

Males and females do notstay together after mating and males play no role in raising young. After mating, gestation (the time between conception and birth) lasts about 35 days.

Usually only one young is born; twins are rare.

The newborn joey is about the sizeof a small grape and weighs less than 6 g (0.2 oz).

At birth the joey is hairless, its eyes are closed, and it has no ears.

Its hind limbs are just buds, but its shouldersand front legs are more developed, with tiny claws.

The newborn uses these front legs to crawl from the birth canal through its mother’s belly fur to her pouch opening.Once in the warm pouch, the joey attaches its lips to one of the mother’s two nipples. The joey will stay in the pouch for about seven months, growing rapidly.

After about three months, the joey weighs nearly 57 g (2 oz), its eyes have opened, its earshave developed, and fur begins to appear.

At about six months, the joey has full body fur and begins to poke its head outside the pouch.

After seven months the joeyno longer fits in the pouch and rides around on its mother’s back. A joey drinks only its mother’s milk until around six months of life, when it begins to eat leaves that have been digested and eliminated by its mother.

These mushyyellowish-green leaves, called pap, help the joey acquire the intestinal microbes necessary for the digestion of eucalyptus leaves.

The joey eventually begins to eat rawleaves, foraging from its mother’s back. Joeys become fully weaned around 12 months.

At this time, the mother is usually pregnant again and stops feeding the joey.

The joey may stay near the mother foranother 6 to 12 months.

By age 2, females are sexually mature and they may settle near their mothers, but males are often driven away by other adult males andmust find a new territory. VI BEHAVIOR Koalas spend much of their time in a tree, resting in order to conserve energy to compensate for their nutrient-poor, low-energy diet.

They sleep about 20 hours a day,sit and munch food for 1 to 3 hours a day, and spend only about four minutes a day actively moving between branches or trees, grooming, and partaking in socialactivities.

Most feeding occurs from about 5 PM to midnight.

Koalas feed in several “bouts,” each lasting an average of 20 minutes. Koalas sleep in the same trees that they feed in, resting in the forks of trees and occasionally stretching along a branch.

They change trees once a day or less, typicallyclimbing down a tree feet first and walking awkwardly on the ground to another tree.

At a new tree koalas spring from the ground to catch their claws in the bark andthey then use their muscular limbs to scramble up the trunk.

In trees, large pythons are the koala’s primary predators, and owls and eagles sometimes snatch youngkoalas off their mothers’ back.

While on the ground the koala is vulnerable to predators such as dingoes and domestic dogs.

Some koalas are hit by motor vehicleswhen they cross a road to get to another tree. Except for the mother and her young, koalas live alone.

Each koala maintains a home range consisting of a number of trees.

Within its home range, a koala uses only asmall number of trees, with up to a third of its time spent in one favorite tree.

Home ranges vary in size depending on the nature of the habitat, and male home rangesare larger in size than those of females.

In areas of good habitat—marked by fertile soil, warmer temperatures, plentiful rainfall, and the preferred eucalyptusspecies—home ranges average 1 to 2 hectares (3 to 4 acres); in moderately good habitat home ranges average 2 to 3 hectares (5 to 8 acres); and in very poor habitata male home range may reach nearly 200 hectares (500 acres). The home range of a koala may overlap with the home ranges of other koalas.

When there are many individuals in one area, adults will share a single tree, but onlybriefly.

It is in the shared trees that most social interaction occurs among koalas.

Koalas communicate with vocalizations, including loud, low bellows emitted by adultmales.

Males bellow year-round, but most often during the breeding season when males fight with other males to gain dominance.

Occasionally a koala may move along distance, especially a young animal leaving its mother to strike out on its own.

One study reported a male koala moving more than 8 km (5 mi) during a six-weekperiod. VII STATUS OF KOALAS Koalas are not classified as an endangered species under Australia's Endangered Species Protection Act.

They are categorized as “lower risk/near threatened,” in theRed List of Threatened Species compiled by the World Conservation Union (IUCN), a nongovernmental organization that compiles global information on endangeredspecies.

While these classifications indicate a relatively low level of concern, the number of koalas in Australia and their endangered status is a matter of debate amonggovernment officials and conservationists.

Wildlife management experts in the Australian federal and state governments believe koalas are abundant and in little dangerof extinction nationally, although these authorities do believe that some local populations may be threatened.

Other scientists and conservationists hotly dispute thisview because they believe that the koala is in danger of extinction, primarily threatened by habitat loss. Over the last 200 years more than 80 percent of eucalyptus forests have been cleared to make room for farming, housing, and other human development.

Forest fireshave also destroyed koala habitat.

As more and more eucalyptus forests have been decimated, koala populations have become fragmented, surviving in the fewremaining patches of forests. Unable to travel to other forests, koalas have a limited choice of breeding partners, increasing the chance for inbreeding (mating between closely related animals).Biologists are concerned that the genetic diversity of koala populations will steadily decline as a result of inbreeding.

This growing genetic uniformity can have seriousconsequences.

Because inbred animals share so many genes, they are likely to be equally vulnerable to disease and other threats.

If a potentially lethal disease strikesa koala population with a similar genetic makeup, the entire group may die. Fragmented populations can also lead to overpopulation of a limited habitat.

When a koala population grows too large for a patch of forest, the animals over browseleaves and destroy the very trees that sustain the koala population, resulting in starvation.. »

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