بچه ها سلام.ببخشید انقدر طول کشید تا این مطلبو بذارم.به دلایل مختلفی انقدر طول کشید که چون حال ندارم فارسی تایپ کنم بیخیال گفتنش میشم.من تعدادی از کلمه های سختشو ته متن ترجمه کردم به خصوص اونایی که تو دیکشنریه بلاگ نبود.من اول می خواستم یه مطلب که در مورد تکامل کوالا بودو بذارم که دیدم کوالا برای ما ایرانیا همچینم جونور شناخته شده ای نیست پس اول یه مطلبم را جع به خود کوالا گذاشتم و بعد مطلب تکاملو گذاشتم.کلا" کوالا پستانداری ساکن استرالیاست (البته نه همشون)که برای تنبلی تکامل پیدا کرده و بیشتر عمرشو مثل درخت ساکنه.کوالا به طرزه عجیبی از برگهای اوکالیپتوس که مزه زهره مار میده تغذیه میکنه و بیشتر بالای درخها میشه پیداشون کرد.برای فهمیدنه اطلاعاته بیشتر در باره این کیسه داره لپ کشانی ادامه مطلبو بخونید. راستی من کلی این متنو ویرایش کردم و از این بهتر نمی تونستم اگه مشکلی بود ببخشید.
Several different factors have had influencial pressure on the morphology of koalas. In particular, the koala's poorly balanced diet and its arboreal lifestyle have played key roles in the evolution of its specialized morphological characteristics. The koala's arboreal lifestyle, low metabolic rate, and low energy requirements can all be partially attributed to their nutrient poor diet of eucalyptus leaves. Koalas evolved with gum trees as their primary source of nourishment, and their bodies began to adapt and specialize to cope with the diet (Australian Koala Foundation). As this happened, evolution selected for koalas to remain in the tree tops where they didn't have to expend large amounts of energy trying to escape from predators on the ground. By being arboreal, koalas can afford to have low metabolic rates and low energy requirements. As a result, the morphology of the koala is very reflective of these different evolutionary pressures. The general body shape of the koala, its paws, and its fur have all become highly specialized to support "lazy" life in the trees.
General Characteristics: Koalas are relatively small to medium size marsupials. Several theories relating to the adaptive size of koalas have been proposed by researchers. First, they suggest that because koalas rely on fermentation of eucalyptus leaves to obtain energy, their body size must remain relatively small in order to increase the relative volume of fermentation chambers with respect to the rest of their body. This ensures that koalas are able to meet their energy requirements. Secondly, because koalas are arboreal, there must be a consideration for the energetic costs required to shift a body against the forces of gravity. If koalas grew too large, it would increase the necessary metabolism and energy required for climbing. Under the influence of these considerations, koalas have reached a relatively optimal size for a herbivorous, arboreal mammal (Degabriele and Dawson, 1979).
The size of a mature koalas is slightly variable, and males are typically larger than females. Full grown males range between 20 & 30 pounds while females generally range between 15 & 22 pounds. Males usually grow to be between 29 & 33 inches, and females grow to be between about 26 & 29 inches (Sea World). Koalas have stocky, pear-shaped bodies. Their wide base is advantageous for balance and wedging themselves into the forks of trees. Koalas also have a reduced tail and relatively long limbs which benefit in sitting and climbing
Paws: Due to their arboreal lifestyle, koalas must be excellent climbers. The paws of the koala have adapted and specialized to facilitate locomotion through the trees. Koalas have two digits on on their front paws that function as thumbs and are opposable to the other three digits. This adaptation allows them to maintain an excellent grip while climbing or eating. The hind paw has an opposable digit as well. The koala's paws are also equipped with long claws and rough pads that are well adapt for climbing (Sea World).
Fur: Phascolarctos cinereus is one of the few arboreal species that does not utilize at least some form of shelter for protection from the elements (Australian Koala Hospital Foundation). Because of the koala's poor diet, low metabolism, and low energy requirements, building a nest would be a time consuming and difficult task. Although the koala doesn't use shelter, it manages to be widely dispersed across the varying range of environmental conditions that characterize the Eastern coast of Austrialia. Life in the exposed tree tops could potentially present the koala with a number of thermoregulatory problems, but the fur of the koala has become highly specialized and efficient at supporting this highly exposed, low energy, arboreal lifestyle.A study performed by Degabriele and Dawson examined the pelt of the koala in comparison to other marsupials. Their results were very interesting and help explain how the koala prevents possible thermoregulatory problems that could arise from their high exposure and low metabolic rates. They found that the insulation of the koala's fur was higher than that of any other marsupial. By closely examining the fur, they found that the dorsal surface of the koala was twice as dense as the ventral surface and much less reflective of solar radiation. The ventral body surface is noteably lighter in colour and reflects 52.3% of solar radiation while the darker dorsal, or back, surface was found to have 38.3% reflectance. They also found that the wind had little influence of the on insulation ofthe koala's fur. At wind speeds up to 4 m/s, the insulation of the dorsal fur only decreased by 14.2%. The density of the fur was variable across different sections of the body, but the more thinly furred surfaces occupied a much smaller proportion of the total body area. By changing their body posture, as in the picture above, koalas can protect or expose the thinly furred surfaces of their body to help achieve ideal temperatures. It was also discovered that koalas maintain slightly depressed body temperatures around 35.7°C while most mammals regulate their temperature around 37.0°C. This makes it a little easier for koalas to maintain constant temperatures, especially with a low metabolic rate. Different postures can drammatically enhance or reduce the surface area of the koala exposing desired areas of the body. When they ball up to keep warm, koalas reduce their surface area and expose their densely furred, poorly-reflective, and wind resistanct dorsal regions. To cool off, they allow their arms and legs to dangle and increase the exposure of more thinly furred surfaces. Panting is also a means of cooling for koalas (1979). So, koalas rely heavily on their fur to protect them in various types of environmental conditions. The koala's fur is highly adapt and specialized to prevent excessive heat loss or gain and support its low energy, arboreal lifestyle
اینم ادامه مطلب راجع به سیر تکاملیش. این متنه دوم یه کم تخصصی تره
however, the new findings published as the featured cover article in the current issue of The Journal of Vertebrate Paleontology suggest that the two species of koalas from the Miocene (24 to five million years ago) did not share the uniquely specialized eucalyptus leaf diet of the modern koala (Phascolarctos cinereus).
The shift to a wholly eucalyptus diet by modern koalas was an adaptation that probably came later as Australia drifted north, causing its rainforests to retreat and Eucalypts to become the dominant tree of most Australian forests and woodlands.
Modern koalas -- the sole living member of the diprotodontian marsupial family Phascolarctidae -are among the largest of all arboreal leaf-eaters. To attain this remarkable condition on a diet of eucalyptus leaves, a notoriously poor and somewhat toxic food source, the tree-dwelling marsupials developed unique anatomical and physiological adaptations including specialized chewing and digestive anatomies and a highly sedentary lifestyle. The dramatic differences between the skulls of extinct and modern koalas, especially in the facial region, are probably related to the change to a tougher diet of eucalyptus leaves.
Researchers from the University of New South Wales and the CSIRO have drawn these conclusions after making dozens of detailed anatomical comparisons between the brush-tailed possum, the modern koala and the two fossil species (Litokoala kutjamarpensis and Nimiokoala greystanesi).
The fossil species were unearthed from the Riversleigh World Heritage site in Queensland, Australia. The comparisons reveal similarities in the back of the skull between the modern and fossil koalas, but substantial differences in their teeth, palate and jaws.
Koalas are most closely related among living marsupials to wombats but the two species diverged some 30-40 million years ago. Among fossil koalas there are 18 named species representing five genera spanning the period from the late Oligocene (37 million years ago) to the present.
However, they are generally scarce in the fossil record and most species are only known from a few isolated teeth or jaw fragments. Therefore, it has been difficult to develop an accurate picture of their behaviour, diet and evolution.
The researchers believe that the prehistoric koalas also shared with their modern cousins the ability to produce loud "bellows" based on similar large bony prominences -- the auditory bullae -- that enclose structures in the middle and inner ear. However the auditory bullae of the extinct Nimiokoala and Litokoala species are not as exaggerated as in the modern koala, according to team member UNSW Professor Mike Archer.
"Modern koalas are extremely sedentary and vocal animals," says Archer, who is perhaps best known for leading research into the extraordinary Riversleigh fossil deposits in Queensland, which led to the site being listed on the World Heritage Register.
"They produce low frequency vocalisations that pass through vegetation and can be heard up to 800 metres away -- far exceeding the home range limits of male koalas. The fossil koalas share similar large bony ear structures to the modern koala and would have been well adapted to detecting vocalisations in the rainforest environment of Riversleigh in the Miocene era."
"In order to accommodate both the mechanical demands of their new diet, as well as maintaining their auditory sophistication, the koala underwent substantial changes to its cranial anatomy, in particular that of the facial skeleton," says Dr Julien Louys of UNSW's School of Biological, Earth and Environmental Sciences. "The unique cranial configuration of the modern koala is therefore the result of accommodating their masticatory adaptations without compromising their auditory system."
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