24 Temmuz 2007 Salı

Snake's evolution

A snake is a scaly, limbless, elongate reptile from the order Squamata. A literary word for snake is serpent (a Middle English word which comes from Old French, and ultimately from *serp-, "to creep"[1]); in modern usage this usually refers to a mythic or symbolic snake, and information about such creatures can be found under serpent (symbolism). This article deals mostly with the biology of snakes.

The phylogeny of snakes is poorly known because snake skeletons are typically small and fragile, making fossilization unlikely. It has however been generally agreed, on the basis of morphology, that snakes descended from lizard-like ancestors. Recent research based on genetics and biochemistry confirms this; snakes form a venom clade with several extant lizard families.

Recent fossil evidence suggests that snakes directly evolved from burrowing lizards, either varanids or some other group. An early fossil snake, Najash rionegrina, was a two-legged burrowing animal with a sacrum, fully terrestrial. One extant analog of these putative ancestors is the earless monitor Lanthanotus of Borneo, although it also is semi-aquatic. As these ancestors became more subterranean, they lost their limbs and became more streamlined for burrowing. Features such as the transparent, fused eyelids (brille) and loss of external ears, according to this hypothesis, evolved to combat subterranean conditions (scratched corneas, dirt in the ears). According to this hypothesis, snakes re-emerged onto the surface of the land much as they are today. Other primitive snakes are known to have possessed hindlimbs but lacked a direct connection of the pelvic bones to the vertebrae, including Haasiophis, Pachyrhachis and Eupodophis) which are slightly older than Najash.

Primitive groups among the modern snakes, pythons and boas, do have vestigial hind limbs, tiny, clawed digits known as anal spurs and used to grasp during mating. Leptotyphlopidae and Typhlopidae are other examples where remnants of the pelvic girdle are still present, in Leptotyphlopidae sometimes as horny projections or not visible at all. The frontal limbs in all snakes are gone because of the evolution of the Hox genes in this area. The axial skeleton of the snakes' common ancestor had like most other tetrapods the familiar regional specializations consisting of cervical (neck), thoracic (chest), lumbar (lower back), sacral (pelvic) and caudal (tail) vertebrae. But the Hox gene expression in the axial skeleton responsible for the development of the thorax became dominant early in snake evolution. As a result, the vertebrae anterior to the hindlimb buds (when present) all have the same thoracic-like identity (except from the atlas, axis and 1-3 neck vertebrae), meaning most of the snake's skeleton is actually made up of an extremely extended thorax. Ribs are found exclusively on the thoracic vertebrae. The neck, lumbar and pelvic vertebrae are very reduced in number (only 2-10 lumbar and pelvic vertebrae are still present), while only a short tail remains of the caudal vertebrae, although the tail is still long enough to be of good use in many species, and is modified in some aquatic and tree dwelling species. Because the front (thoracic) limbs in tetrapods appear in the area between the neck and the thorax, a location that is now almost absent in snakes, there is simply no longer any room left where they can develop.

The alternative hypothesis, based on morphology, suggests that ancestors were related to mosasaurs — extinct aquatic reptiles from the Cretaceous — which in turn are thought to have derived from varanid lizards. Under this hypothesis, the fused, transparent eyelids of snakes are thought to have evolved to combat marine conditions (corneal water loss through osmosis), while the external ears were lost through disuse in an aquatic environment, ultimately leading to an animal similar in appearance to sea snakes of today. In the Late Cretaceous, snakes re-colonized the land much like they are today. Fossil snake remains are known from early Late Cretaceous marine sediments, which is consistent with this hypothesis, particularly as they are older than the terrestrial Najash rionegrina. Similar skull structure; reduced/absent limbs; and other anatomical features found in both mosasaurs and snakes lead to a positive cladistical correlation, though some features are also shared with varanids. Supposedly similar locomotion for both groups is also used as support for this hypothesis. Genetic studies have indicated that snakes are not especially related to monitor lizards, and (it has been claimed) therefore not to mosasaurs, the proposed ancestor in the aquatic scenario of their evolution. However, there is more evidence linking mosasaurs to snakes than to varanids. Fragmentary remains that have been found from the Jurassic and Early Cretaceous indicate deeper fossil records for these groups, which may eventually refute either hypothesis.

The great diversity of modern snakes appeared in the Paleocene, probably correlated with the adaptive radiation of mammals following the extinction of the dinosaurs.

Digestion and diet
Snake eating a rat
Snake eating a rat

All snakes are carnivorous, eating small animals including lizards and other snakes, rodents and other small mammals, birds, eggs or insects. Some snakes have a venomous bite, which they use to kill their prey before eating it. Other snakes kill their prey by constriction. Still others swallow their prey whole and alive. Pareas iwesakii and other snail-eating Colubrids of subfamily Pareatinae have more teeth on the right side of their mouths than on the left, as the shells of their prey usually spiral clockwise[2]. Most snakes are very easy to feed in captivity.

Snakes do not chew their food and have a very flexible lower jaw, the two halves of which are not rigidly attached, and numerous other joints in their skull (see snake skull), allowing them to open their mouths wide enough to swallow their prey whole, even if it is larger in diameter than the snake itself. It is a common misconception that snakes actually dislocate their lower jaw to consume large prey.

After eating, snakes become torpid while the process of digestion takes place. Digestion is an intensive activity, especially after the consumption of very large prey. In species that feed only sporadically, the entire intestine enters a reduced state between meals to conserve energy, and the digestive system is 'up-regulated' to full capacity within 48 hours of prey consumption. So much metabolic energy is involved in digestion that in Crotalus durissus, the Mexican rattlesnake, an increase of body temperature to as much as 14 degrees Celsius above the surrounding environment has been observed.[3] Because of this, a snake disturbed after having eaten recently will often regurgitate its prey in order to be able to escape the perceived threat. However, when undisturbed, the digestive process is highly efficient, dissolving and absorbing everything but hair and claws, which are excreted along with uric acid waste. Snakes have been known to occasionally die from trying to swallow an animal that is too big. Snake digestive fluids are unable to digest most plant matter, which passes through the digestive system mostly untouched.

Snakes do not normally prey on people, but there are instances of small children being eaten by large constrictors in the jungle.[citation needed] While some particularly aggressive species exist, most will not attack humans unless startled or injured, preferring instead to avoid contact. The majority of snakes are either non-venomous or possess venom that is not harmful to humans.

As a general rule, snakes eat rodents. There are exceptions to this, such as the natal green snake, which eats insects. Snakes generally specialise in a few food types (for example, royal pythons will generally eat mice and gerbils in the wild). However, they do not need to hunt every day. A big meal will keep some snakes content for a long time. Anacondas and pythons can live for a year after eating large prey.


The skin is covered in scales. Many people are surprised to find that snakeskin has a smooth, dry texture, instead of a slimy texture as might be expected. Some people are afraid to touch them because they confuse snakes with worms. Most snakes use specialized belly scales to travel, gripping surfaces. The body scales may be smooth, keeled, or granular. Their eyelids are transparent "spectacle" scales which remain permanently closed, called brille. They shed their skin periodically. Unlike other reptiles, this is done in one piece, like pulling off a sock, with the snake rubbing its nose against something rough, like a rock, for instance, creating a rip in the skin around the nose and the mouth until the skin is completely removed.[1] The primary purpose of shedding is to grow; shedding also removes external parasites. This periodic renewal has led to the snake being a symbol of healing and medicine, as pictured in the Rod of Asclepius. In "advanced" (Caenophidian) snakes, the broad belly scales and rows of dorsal scales correspond to the vertebrae, allowing scientists to count the vertebrae without dissection. If there is not enough humidity in the air while snakes are shedding their skin, it can be very dangerous for the snake, because the dry skin does not shed. Skin that remains attached to the snake can harbour diseases and parasites. A tail tip that is not removed can constrict as the snake grows, cutting off the blood supply to the end of the tail causing it to drop off. A retained spectacle can cause blindness in the affected eye.

Thermographic image of a snake eating a mouse.
Thermographic image of a snake eating a mouse.

While snake vision is unremarkable (generally being best in arboreal species and worst in burrowing species), it is able to detect movement. Some snakes, like the Asian vine snake, have binocular vision. In most snakes, the lens moves back and forth within the eyeball to focus. In addition to their eyes, some snakes (pit vipers, pythons, and some boas) have infrared-sensitive receptors in deep grooves between the nostril and eye which allow them to "see" the radiated heat.

Snakes have no external ears, but they do have a bone called the quadrate under the skin on either side of the head which focuses sound into the cochlea.[2] Their sense of hearing is most sensitive to frequencies around 200–300 Hz.

A snake smells by using its forked tongue to collect airborne particles then passing them to the Jacobson's organ or the Vomeronasal organ in the mouth for examination. The fork in the tongue gives the snake a sort of directional sense of smell. The part of the body which is in direct contact with the surface of the ground is very sensitive to vibration, thus a snake is able to sense other animals approaching.

Internal organs

Anatomy of a snake. 1 esophagus, 2 trachea, 3 tracheal lungs, 4 rudimentary left lung, 5 right lung, 6 heart, 7 liver, 8 stomach, 9 air sac, 10 gallbladder, 11 pancreas, 12 spleen, 13 intestine, 14 testicles, 15 kidneys.
Anatomy of a snake. 1 esophagus, 2 trachea, 3 tracheal lungs, 4 rudimentary left lung, 5 right lung, 6 heart, 7 liver, 8 stomach, 9 air sac, 10 gallbladder, 11 pancreas, 12 spleen, 13 intestine, 14 testicles, 15 kidneys.

The left lung is very small or sometimes even absent, as snakes' tubular bodies require all of their organs to be long and thin. To accommodate them all, only one lung is functional. This lung contains a vascularized anterior portion and a posterior portion which does not function in gas exchange. This 'saccular lung' may be used to adjust buoyancy in some aquatic snakes and its function remains unknown in terrestrial species. Also, many organs that are paired, such as kidneys or reproductive organs, are staggered within the body, with one located ahead of the other. Snakes have no urinary bladder.


Snakes utilize a variety of methods of movement which allows them substantial mobility in spite of their legless condition. All snakes are capable of lateral undulation, in which the body is flexed side-to-side, and the flexed areas propagate posteriorly, giving the overall shape of a posteriorly.


A wide range of reproductive modes are used by snakes. All snakes employ internal fertilization, accomplished by means of paired, forked hemipenes, which are stored inverted in the male's tail. Most snakes lay eggs, and of those most species abandon them shortly after laying; however, some species are ovoviviparous and retain the eggs within their bodies until they are almost ready to hatch. Recently, it has been confirmed that several species of snake are fully viviparous, such as the green anaconda, nourishing their young through a placenta as well as a yolk sac, highly unusual among reptiles, or indeed anything else outside of placental mammals. Retention of eggs and live birth are commonly, but not exclusively, associated with cold environments, as the retention of the young within the female allows her to control their temperature more effectively than if the developing young were in external eggs.


See also: Snake venom

A venomous snake is a snake that uses modified saliva, venom, delivered through fangs in its mouth, to immobilize or kill its prey. Venomous snakes include several families of snakes and do not constitute a formal classification group used in taxonomy. The term poisonous snake is false - poison is inhaled or ingested whereas venom is injected. (In contrast, a few non-venomous species are constrictors such as pythons, anacondas, and boa constrictors which suffocate their prey.) Snake venom can contain many different active agents, and can potentially be a mix of neurotoxins (which attack the nervous system), hemotoxins (which attack the circulatory system), cytotoxins, bungarotoxins and many other toxins that affect the body in different ways. Snake venom is never a single type of toxin[citation needed].

Venomous snakes that use hemotoxins usually have their fangs to secrete the venom in the front of their mouths, making it easier for them to inject the venom into their victims. Snakes that use neurotoxins, such as the mildly venomous mangrove snake, have their fangs located in the back of their mouths, with the fangs curled backwards. This makes it both difficult for the snake to use its venom and for scientists to milk them.[citation needed]

It has recently been suggested that all snakes are in fact venomous to some degree.[citation needed] Snakes all evolved from a common lizard ancestor that was venomous, from which venomous lizards like the gila monster and beaded lizard also derived. The research suggests that snakes all have venom glands, even species thought totally harmless such as the Corn Snake, commonly kept as a pet. What differentiates 'venomous' from 'non-venomous' is the evolution of a venom delivery system, the most advanced being that of vipers, with fangs that are hinged to prevent self envenomation, and curl out as the snake strikes.

Venomous snakes are generally classified in three taxonomic families:

* Elapids - cobras, king cobras, kraits, mambas, Australian copperheads, sea snakes, and coral snakes.
* Viperids - vipers, rattlesnakes, copperheads/cottonmouths, adders and bushmasters.
* Colubrids - boomslangs, tree snakes, vine snakes, mangrove snakes, and many others, though not all colubrids are venomous.

Snakes biting humans

Main article: Snake bite

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Documented deaths resulting from snake bites are uncommon (about 1/1000 in most areas of the world. Only about 450 species of snakes are venomous (with only about 250 that are able to kill a human), and among the 7,000 Americans bitten by venomous snakes every year, fewer than fifteen die. See snakebites for more information, including prevention of snake bites and first aid treatment.
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Brown Snake
Brown Snake

Snake charmers

In some parts of the world, especially in India and Pakistan, snake charming is a roadside show performed by a charmer. In this, the snake charmer carries a basket that contains a snake which he seemingly charms by playing tunes from his flute-like musical instrument, to which the snake responds. However, snakes lack ears, either external or internal. Therefore they are unable to hear the music from the flute.

Researchers have pointed out that many of these snake charmers are good sleight-of-hand artists. The snake moves corresponding to the flute movement and the vibrations from the tapping of the charmer's foot which is not noticed by the public. They rarely catch their snakes and the snakes are either nonvenomous or defanged cobras. Sometimes these people exploit the fear of snakes by releasing snakes into the neighbourhood and then offering to rid the residence of snakes. Other snake charmers also have a snake and mongoose show, where both the animals have a mock fight; however, this is not very common, as the snakes, as well as the mongooses, may be seriously injured or killed.

Snake charming as a profession is now dissuaded[unclear] in India as a contribution to forest and snake conservation. In fact in some places in India snake charming is banned by law.[citation needed]

Snake trapping

Despite the existence of snake charmers, there have also been professional snake catchers or wranglers. The tribals of "Irulas" from Andhra Pradesh and Tamil Nadu in India have been practicing this art for generations. They generally don't use gimmicks and with the help of a simple stick catch the snakes from the fields or houses. They are also known to eat some of the snakes they catch and are very useful in rat extermination in the villages.[citation needed] Their knowledge of snakes and their behaviour is uncanny. Modern day snake trapping involves a herpetologist using a long stick with a "V" shaped end. Some like Steve Irwin, Bill Hasst, Joel La Rocque, and Jeff Corwin prefer to catch them using bare hands. At least one tribe of natives uses a specialized form of snake catching as a rite of passage to manhood.[citation needed] The young man of interest will wrap his leg heavily in some type of cloth all the way to the inseam. He will then stick his leg in a burrow containing a large python, typically a reticulated python. After the snake swallows most of his leg several other members of the tribe will pull him out of the hole along with the snake. The snake is then killed and the man's leg removed from the snake. These snakes can be over 20 ft long and it is possible for the man to have his leg dislocated. The scent of a prey animal may be used to help convince the snake to swallow the leg. Snakes have a single-track digestive system, but the digestion process actually takes longer.

Human consumption of snakes

In some cultures, the consumption of snakes is acceptable,[3] or even considered a delicacy,[4] prized for its alleged pharmaceutical effect of warming the heart. Western cultures document the consumption of snakes under extreme circumstances of hunger.[5] However, human consumption of snake meat, especially when eaten raw, may lead to dangerous parasitic infections. Cooked rattlesnake meat is an exception, which is commonly consumed in the Western United States. In Asian countries, drinking the blood of snakes, particularly the cobra, is believed to increase sexual virility. The blood is drained while the cobra is still alive when possible, and is usually mixed with some form of liquor to improve the taste.

In some Asian countries, the use of snakes in alcohol is also accepted. In such cases, the body of a snake or several snakes is left to steep in a jar or container of liquor. It is claimed that this makes the liquor stronger (as well as more expensive). One example of this is the Habu snake sometimes placed in the Okinawan liquor Awamori.

Main article: Serpent (symbolism)

In Egyptian history, the snake occupies a primary role with the Nile cobra adorning the crown of the pharaoh in ancient times. It was worshipped as one of the Gods and was also used for sinister purposes: murder of an adversary and ritual suicide (Cleopatra).

In Greek Mythology snakes are often associated with deadly and dangerous antagonists. The 9 headed Hydra Hercules defeated and the three Gorgon sisters are literary examples. Medusa was one of the three Gorgon sisters who Perseus defeated. Medusa is described as a hideous mortal, with snakes instead of hair and the power to turn men to stone with her gaze.

Three medical symbols involving snakes that are still used today are Bowl of Hygieia, symbolizing pharmacy, and the Caduceus and Rod of Asclepius, which are symbols denoting medicine in general.

India is often called the land of snakes and is steeped in tradition regarding snakes. Snakes are worshipped as gods even today with many women pouring milk on snake pits (despite snakes' aversion for milk). The cobra is seen on the neck of Shiva and Vishnu is depicted often as sleeping only on a 7 headed snake. There are also several temples in India solely for cobras sometimes called Nagraj (King of Snakes) and it is believed that snakes are symbols of fertility. There is a Hindu festival called Nagpanchami each year on which day snakes are venerated and prayed to. See also Nāga.

In Christianity and Judaism the snake makes its infamous appearance in the first book (Genesis) of the Bible when a snake appears before the first couple Adam and Eve and tempts them with the forbidden fruit. It is also seen in Exodus when Moses, as a sign of God's power, turns his stick into a snake; snakes are similarly produced by the pharaoh's magic-practicing priests, but Moses' snake devours them. Later Moses made Nehushtan, a bronze snake on a pole that when looked at cured the people of bites from the snakes that plagued them in the desert. Jesus instructed his disciples to be as shrewd as snakes and as innocent as doves.

According to the Bible, the snake is the symbol of Jesus Christ, in His quality of being the Redeemer: And as Moses lifted up the serpent in the wilderness, even so must the Son of man be lifted up: John 3:14

The Ouroboros is a symbol that is associated with many different religions and customs, and is also claimed to be related to Alchemy. The Ouroboros or Oroboros is a snake eating its own tail in a clock-wise direction (from the head to the tail) in the shape of a circle, representing manifestation of one's own life and rebirth, leading to immortality.

Snake belongs to one of the 12 celestial animals of Chinese Zodiac, in the Chinese calendar.

23 Temmuz 2007 Pazartesi


addax antelope
Addax (wild animal with crooked horns) nasomaculatus (nasus - the nose, macula - a spot or mark)

Both sexes have horns, mat of brown hair on forehead, rest of body grayish-white
Head and body length = 150-170 cm (59.1-66.9 in)
Height at shoulders = 105-114 cm (3.5-3.8 ft)
Height at shoulders = 93-108 cm (3.1-3.6 ft)

99-123.75 kg (220-275 lbs)
60-125 kg (132-275 lbs)
Desert succulents, grasses and herbs, leaves of small bushes
257-264 days; one offspring per birth weighing 4.7-6.75 kg (10.5-15 lbs)

At 3 years
At 1.5 years
Up to 19 years
Northern Africa (Chad, Mali, Mauritania, and Niger)
Sand and stony desert, semi-desert, and barren steppes
Approximately 500 left in the wild
Critically Endangered
Appendix I

Addax possess broad, flat hooves with flat soles that help prevent them from sinking into the desert sand.
These antelope are one of the few species where male and females have horns of the same size.
These desert antelopes' coat color changes from dark grayish-brown in winter to white in the summer - an efficient method of maintaining body temperatures.
Addax will dig depressions in the sand in which to rest. These are often located in the shade of boulders for protection from the wind and sun.
Often considered the most-well adapted antelope to a desert environment, addax rarely need to drink since they are able to get most of the water they need from the plants they eat.
Addax herds would typically consist of 5-20 individuals, led by one dominant male. Female herd members establish their own dominance hierarchy, with the oldest individuals achieving the highest rank. It should be noted that this group structure is not as standard anymore due to their near extinction in the wild. Most addax now travel in small clusters of only a few individuals.
Addax are one of the most endangered mammals in the world. Current estimates show there to be less than 500 individuals left in the wild.

Addax are nearly extinct in the wild, having been eliminated from much of their original range. These antelope have been hunted for their valuable meat and skin. They have also been destroyed by farmers and cattlemen, so as not to compete with their cattle for grazing land. Much of the addax population was decimated during the World Wars. Probably the only reason they are still alive in the wild at all is the fact that they can live in uninhabitable places with extreme heat, extensive sand dunes, and other harsh conditions where it is extremely difficult for humans to reach. Since they are so heavily built they are not capable of great speeds and are easily overtaken by horses, dogs, and, of course, vehicles. Antelope are important to habitats as grazers and browsers. They are also important as prey for carnivores. Addax reintroductions to a park in Tunisia have been successful as well as reintroductions to Niger. In a cooperative effort with other AZA (American Association of Zoos and Aquariums) institutions, Busch Gardens closely manages addax populations through a program called the Species Survival Plan (SSP), which works to improve the genetic diversity of managed animal populations. Busch Gardens currently has 30 SSP animals.

29 Haziran 2007 Cuma


Latin name:
Phascolarctos cinereus
(Greek: phaskolos means pouch; arktos means bear. Latin: cinereus means ash-colored.)
Koalas are marsupials, females having a pouch in which their young first develop. Their pouch faces the rear and has a drawstringlike muscle that the mother can tighten. They are the sole member of the family Phascolarctidae.

Kingdom: Animal
Phylum: Vertibrata
Class: Mammalia
Sub-Class: Marsupialia
Order: Diprotodontia
Sub-Order: Vombatiformes
Infra order: Phascolarctomorphia
Family: Phascolarctidae
Genus: Phascolarctos Cinereus

There are three subspeies:
P.c. victor (Victoria)
P.c. cinereus (New South Wales)
P.c. adustus (Queensland).

Size is larger in the southern regions. Head-body length in the south average 30.7 in./78 cm for males and 28 in./72 cm for females.

Average 26 lbs/11.8 kg for southern males and 17.4 lbs/7.9 kg for southern females. In the north, males average 14.3 lbs/6.5 kg; females 11.2 lbs/5.1 kg. At birth young weighs only 0.5 gm. (This is no typo; it is amazing how small they are at birth, about the size of a bee.)

The fur of the koala in southern region is thick and woolly and is thicker and longer on the back than on the belly. Koalas in northern region have a short coat; this gives them a naked appearnace. The color and pattern of the coat varies considerably between individuals and with age.

Thickest of the marsupials. Gray to tawny: white on the chin, chest, and forelimbs. Rump consists of tougher connective tissue dappled with white patches. Fluffy ears with longer white hairs. Coat is shorter and lighter in color toward northern regions.

Gestation period:
34-36 days.

Life span:
Their life span today varies considerably due to stress factors, probably averaging 13-18 years.

Principal predator:

First described in 1908 by E. Home.

A Brief History
There were once several different kinds of koala - all but one of which had died out. The earliest known member of the koala family was a browser, which lived 15 millions years ago. Evidence of a 'giant' koala, twice the size of its modern descendant, exists in fossils dating back more than 40,000 years. The sequence of koalas in the fossil record:

* 15 million years ago: Perikoala palankarinnica

* 10 million years ago: Litokoala kutjamarpensis

* 5 million years ago: Koobor notabilis & Koobor jimbarratti

* 0 million years ago: Phascolarctos cinereus & Phascolarctos stirtoni

* 1798, January 26: The 1st record of a koala being seen by an European, named John Price.

* 1803, August 21: The first detailed account of a koala was published in Sydney Gazette.

* 1816: the French naturalist de Blainwill gave the koala its scientific name, Phascolarctos, from the Greek words for 'leather pouch' and 'bear'. Later, the German naturalist Goldfuss gave it the specific name cinereus, meaning 'ash-coloured', after the color of the original specimen.

From: Koala Handbook by Simon Hunter

History of the name: "koala"
In their history of the koala, Tom Iredale and Gilbert Whitley (1934) suggest that the common name "koala" was derived from an Aboriginal dialect of eastern New South Wales. Ronald Strahan (1978) lists cullewine, koolewong, colo, colah, koolah, kaola and koala as published dialectal variations of the name in that region, "complicated by problems of transliteration and printers" errors.

The early settlers referred to koalas as sloths, monkeys, bears, and even monkey bears, adopting the unfortunate practice of transposing the names of animals which were already familiar to Europeans to Australian lookalikes. The virtual absence of a tail, together with their stocky build and their relatively long legs, gives the koalas a bear-like appearance, and undoubtedly led to their being referred to as, "koala bears", or, "native bears".

From: The Koala: A Natural History by Anthony Lee & Roger Martin

Birth and Pouch Life

* Usually, koalas produce only a single young, rarely twins are born.

* At birth, the young is about 19 mm in length and weighs about 0.5 g.

* At 7 weeks, the young has a head length of about 26 mm. The head is large in proportion to the rest of the body.

* By 13 weeks, the young has attained a body weight of about 50 g and a head length of 50 mm.

* At about 22 weeks of age, the eyes open and the young begins to poke its head out of the pouch for the first time.

* By 24 weeks of age, the cub is fully furred and the first teeth erupt.

* At 30 weeks, the cub weighs about 0.5 kg and has a head length of 70 mm. It now spends most of the time out of the pouch clinging to the mother's belly.

* Some 6 weeks later, the cub weighs 1 kg and no longer enters the pouch. It spends much of the time sitting on the mother's back, but returns to the mother's belly in cold, wet weather and to sleep.

* At 37 weeks, the cub moves from contact with the mother; the excursions were brief and quickly terminated if the mother moved.

* At 44 weeks, the cub still ventures less than a meter from the mother.

* By 48 weeks, the cub is more adventurous and no longer squeaked when the mother was removed. At this age, mother and cub are often seen sleeping back to back.

* The cub remains with the mother until about 12 months of age when it weighs a little over 2 kg.

From: The Koala: A Natural History by Anthony Lee & Roger Martin

Koala Physiology

* Unlike bears or seals, koalas do not rely on a layer of fat below the skin as a form of insulation.

* Blood flow to extremities in cold weather, as in humans, can be reduced, resulting in the conservation of heat.

* Shivering has also been observed in cold climes as a means of creating heat through rapid contractions of certain muscles. This seems to occur when outdoor temperatures drop below 10 degrees Centigrade.

* When temperatures exceed 25-30 degrees Centigrade, koalas use evaporative cooling in their airways to regulate body temperature by increasing respiration rates. They can simultaneously reduce water loss by decreasing the amount of water in their urine.

From: Koalas - Australia's Ancient Ones by Ken Phillips

More Koala Facts
picture of an albino koala

* Koalas don't live in families, but are solitary animals.

* Koalas sleep as long as 18 hours a day and have a low-energy diet of eucalyptus leaves.

* Koalas are not "drunk" or otherwise intoxicated by their leaves.

* Although koalas obtain most of their water from leaves -- the name koala is thought to mean "no drink" in several native Aboriginal tongues -- they do occasionally drink water at the edges of streams.

25 Haziran 2007 Pazartesi


Animals are a major group of organisms, classified as the kingdom Animalia or Metazoa. In general they are multi­cellular, responsive to their environment, and feed by consuming other organisms or parts of them. Their body plan becomes fixed as they develop, usually early on in their development as embryos, although some undergo a process of metamorphosis later on.

The word "animal" comes from the Latin word animal, of which animalia is the plural, and is derived from anima, meaning vital breath or soul. In everyday colloquial usage, the word usually refers to non-human animals. The biological definition of the word refers to all members of the Kingdom Animalia. Therefore, when the word "animal" is used in a biological context, humans are included.

Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and usually multicellular (although see Myxozoa), which separates them from bacteria and most protists. They are heterotrophic, generally digesting food in an internal chamber, which separates them from plants and algae. They are also distinguished from plants, algae, and fungi by lacking cell walls. All animals are motile, if only at certain life stages. Embryos pass through a blastula stage, which is a characteristic exclusive to animals.


With a few exceptions, most notably the sponges (Phylum Porifera), animals have bodies differentiated into separate tissues. These include muscles, which are able to contract and control locomotion, and nerve tissue, which sends and processes signals. There is also typically an internal digestive chamber, with one or two openings. Animals with this sort of organization are called metazoans, or eumetazoans when the former is used for animals in general.

All animals have eukaryotic cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. This may be calcified to form structures like shells, bones, and spicules. During development it forms a relatively flexible framework upon which cells can move about and be reorganized, making complex structures possible. In contrast, other multicellular organisms like plants and fungi have cells held in place by cell walls, and so develop by progressive growth. Also, unique to animal cells are the following intercellular junctions: tight junctions, gap junctions, and desmosomes.

Reproduction and development

Nearly all animals undergo some form of sexual reproduction. Adults are diploid or polyploid. They have a few specialized reproductive cells, which undergo meiosis to produce smaller motile spermatozoa or larger non-motile ova. These fuse to form zygotes, which develop into new individuals.

Many animals are also capable of asexual reproduction. This may take place through parthenogenesis, where fertile eggs are produced without mating, or in some cases through fragmentation.
A newt lung cell stained with fluorescent dyes undergoing mitosis, specifically early anaphase.
A newt lung cell stained with fluorescent dyes undergoing mitosis, specifically early anaphase.

A zygote initially develops into a hollow sphere, called a blastula, which undergoes rearrangement and differentiation. In sponges, blastula larvae swim to a new location and develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement. It first invaginates to form a gastrula with a digestive chamber, and two separate germ layers - an external ectoderm and an internal endoderm. In most cases, a mesoderm also develops between them. These germ layers then differentiate to form tissues and organs.

Most animals grow by indirectly using the energy of sunlight. Plants use this energy to convert sunlight into simple sugars using a process known as photosynthesis. Starting with the molecules carbon dioxide (CO2) and water (H2O), photosynthesis converts the energy of sunlight into chemical energy stored in the bonds of glucose (C6H12O6) and releases oxygen (O2). These sugars are then used as the building blocks which allow the plant to grow. When animals eat these plants (or eat other animals which have eaten plants), the sugars produced by the plant are used by the animal. They are either used directly to help the animal grow, or broken down, releasing stored solar energy, and giving the animal the energy required for motion. This process is known as glycolysis.

Animals who live close to hydrothermal vents and cold seeps on the ocean floor are not dependent on the energy of sunlight. Instead, chemosynthetic archaea and eubacteria form the base of the food chain.

Origin and fossil record

Animals are generally considered to have evolved from a flagellated eukaryote. Their closest known living relatives are the choanoflagellates, collared flagellates that have a morphology similar to the choanocytes of certain sponges. Molecular studies place animals in a supergroup called the opisthokonts, which also include the choanoflagellates, fungi and a few small parasitic protists. The name comes from the posterior location of the flagellum in motile cells, such as most animal spermatozoa, whereas other eukaryotes tend to have anterior flagella.

The first fossils that might represent animals appear towards the end of the Precambrian, around 575 million years ago, and are known as the Ediacaran or Vendian biota. These are difficult to relate to later fossils, however. Some may represent precursors of modern phyla, but they may be separate groups, and it is possible they are not really animals at all. Aside from them, most known animal phyla make a more or less simultaneous appearance during the Cambrian period, about 542 million years ago. It is still disputed whether this event, called the Cambrian explosion, represents a rapid divergence between different groups or a change in conditions that made fossilization possible.

Groups of animals

The sponges (Porifera) diverged from other animals early. As mentioned above, they lack the complex organization found in most other phyla. Their cells are differentiated, but in most cases not organized into distinct tissues. Sponges are sessile and typically feed by drawing in water through pores. Archaeocyatha, which have fused skeletons, may represent sponges or a separate phylum.

Among the eumetazoan phyla, two are radially symmetric and have digestive chambers with a single opening, which serves as both the mouth and the anus. These are the Cnidaria, which include sea anemones, corals, and jellyfish, and the Ctenophora or comb jellies. Both have distinct tissues, but they are not organized into organs. There are only two main germ layers, the ectoderm and endoderm, with only scattered cells between them. As such, these animals are sometimes called diploblastic. The tiny Placozoans are similar, but they do not have a permanent digestive chamber.

The remaining animals form a monophyletic group called the Bilateria. For the most part, they are bilaterally symmetric, and often have a specialized head with feeding and sensory organs. The body is triploblastic, i.e. all three germ layers are well-developed, and tissues form distinct organs. The digestive chamber has two openings, a mouth and an anus, and there is also an internal body cavity called a coelom or pseudocoelom. There are exceptions to each of these characteristics, however - for instance adult echinoderms are radially symmetric, and certain parasitic worms have extremely simplified body structures.

Genetic studies have considerably changed our understanding of the relationships within the Bilateria. Most appear to belong to four major lineages:

1. Deuterostomes
2. Ecdysozoa
3. Platyzoa
4. Lophotrochozoa

In addition to these, there are a few small groups of bilaterians with relatively similar structure that appear to have diverged before these major groups. These include the Acoelomorpha, Rhombozoa, and Orthonectida. The Myxozoa, single-celled parasites that were originally considered Protozoa, are now believed to have developed from the Bilateria as well.

White's Tree Frog, Litoria caerulea.
White's Tree Frog, Litoria caerulea.

Deuterostomes differ from the other Bilateria, called protostomes, in several ways. In both cases there is a complete digestive tract. However, in protostomes the initial opening (the archenteron) develops into the mouth, and an anus forms separately. In deuterostomes this is reversed. In most protostomes cells simply fill in the interior of the gastrula to form the mesoderm, called schizocoelous development, but in deuterostomes it forms through invagination of the endoderm, called enterocoelic pouching. Deuterostomes also have a dorsal, rather than a ventral, nerve chord and their embryos undergo different cleavage.

All this suggests the deuterostomes and protostomes are separate, monophyletic lineages. The main phyla of deuterostomes are the Echinodermata and Chordata. The former are radially symmetric and exclusively marine, such as starfish, sea urchins, and sea cucumbers. The latter are dominated by the vertebrates, animals with backbones. These include fish, amphibians, reptiles, birds, and mammals.

In addition to these, the deuterostomes also include the Hemichordata or acorn worms. Although they are not especially prominent today, the important fossil graptolites may belong to this group.

The Chaetognatha or arrow worms may also be deuterostomes, but more recent studies suggest protostome affinities.

Yellow-winged Darter, Sympetrum flaveolum
Yellow-winged Darter, Sympetrum flaveolum

The Ecdysozoa are protostomes, named after the common trait of growth by moulting or ecdysis. The largest animal phylum belongs here, the Arthropoda, including insects, spiders, crabs, and their kin. All these organisms have a body divided into repeating segments, typically with paired appendages. Two smaller phyla, the Onychophora and Tardigrada, are close relatives of the arthropods and share these traits.

The ecdysozoans also include the Nematoda or roundworms, the second largest animal phylum. Roundworms are typically microscopic, and occur in nearly every environment where there is water. A number are important parasites. Smaller phyla related to them are the Nematomorpha or horsehair worms, which are invisible to the unaided eye, and the Kinorhyncha, Priapulida, and Loricifera. These groups have a reduced coelom, called a pseudocoelom.

The remaining two groups of protostomes are sometimes grouped together as the Spiralia, since in both embryos develop with spiral cleavage.

Bedford's flatworm, Pseudobiceros bedfordi
Bedford's flatworm, Pseudobiceros bedfordi

The Platyzoa include the phylum Platyhelminthes, the flatworms. These were originally considered some of the most primitive Bilateria, but it now appears they developed from more complex ancestors.

A number of parasites are included in this group, such as the flukes and tapeworms. Flatworms lack a coelom, as do their closest relatives, the microscopic Gastrotricha.

The other platyzoan phyla are microscopic and pseudocoelomate. The most prominent are the Rotifera or rotifers, which are common in aqueous environments. They also include the Acanthocephala or spiny-headed worms, the Gnathostomulida, Micrognathozoa, and possibly the Cycliophora. These groups share the presence of complex jaws, from which they are called the Gnathifera.

Big blue octopus, Octopus cyanea
Big blue octopus, Octopus cyanea

The Lophotrochozoa include two of the most successful animal phyla, the Mollusca and Annelida. The former includes animals such as snails, clams, and squids, and the latter comprises the segmented worms, such as earthworms and leeches. These two groups have long been considered close relatives because of the common presence of trochophore larvae, but the annelids were considered closer to the arthropods, because they are both segmented. Now this is generally considered convergent evolution, owing to many morphological and genetic differences between the two phyla.

The Lophotrochozoa also include the Nemertea or ribbon worms, the Sipuncula, and several phyla that have a fan of cilia around the mouth, called a lophophore. These were traditionally grouped together as the lophophorates, but it now appears they are paraphyletic, some closer to the Nemertea and some to the Mollusca and Annelida. They include the Brachiopoda or lamp shells, which are prominent in the fossil record, the Entoprocta, the Phoronida, and possibly the Bryozoa or moss animals.


La ardilla es un mamífero roedor Mide entre 35 y 45 cm de longitud, de las que casi la mitad pertenecen a la cola. En las extremidades delanteras, el pulgar es reducido, pero los otros 4 dedos están bien desarrollados y dotados de largas, curvadas y afiladas uñas. Su cabeza es graciosa; en ella sobresalen los ojos brillantes y la boca en la que los dientes están muy desarrollados y salientes. Si se les captura jóvenes a las ardillas, es posible domesticarlas y una ardilla resulta un animal doméstico muy sensible y cariñoso.

Hay un gran número de especies de ardillas, muy distintas en color y tamaño, propagadas por todo el mundo excepto en Australia. Se alimentan de semillas, cortezas, frutos secos, brotes tiernos y bellotas, que entierran durante el verano en el suelo a unos centímetros de profundidad, para luego alimentarse de ellos durante el invierno. Este almacenamiento no lo hacen las ardillas en un solo punto, sino que lo reparten en una extensa zona del lugar en el que viven.

Las ardillas establecen su nido en los huecos de los arboles o en un hueco del ramaje, y en ocasiones, en los nidos desocupados de algún grajo (pájaro parecido al cuervo), urraca o ave de rapiña. Es curioso ver como cubren su nido por arriba con una cúpula de ramas muy entrelazadas, que tapizan del musgo para evitar que entre la lluvia en su casa.

El nido de las ardillas tiene dos entradas y dentro de esta casa las ardillas hembras dan luz entre 3 y 4 crías. Las crías de ardilla son muy pequeñas; no tienen pelo cuando nacen y tienen los ojos cerrados. La madre los amamanta durante 10 semanas y, poco a poco, les enseña moverse entre las ramas del árbol en el que viven. Su adiestramiento depende del buen desarrollo de su cola, ya que en la ardilla esta constituya el elemento esencial para mantener el equilibrio en los asombrosos saltos que efectúa de un árbol a otro. A los 5 o 6 meses ya se les puede considerar adultas, pues su cuerpo y cola ya habrán adquirido su total desarrollo.

Las ardillas de países muy fríos hibernan. Esto quiere decir que duermen durante todo el invierno envueltos en su cola. Sin embargo, su sueño nunca es muy profundo, y cualquier peligro, o sencillamente su apetito, les hace espabilar rápidamente para escapar o hurgar en el suelo a la búsqueda de su dispensa subterránea de frutos que enterró durante el verano.


De toda la familia de gatos, el león siempre ha sido considerado por el hombre como el Rey de los animales. Admirado por su nobleza, lealtad, habilidades, fuerza y valentía a lo largo de la historia, se han encontrado dibujos de el tigre por hombres hace más de 15.000 años.

No obstante, el león es solo el segundo más grande de los felinos (el más grande es el tigre) y tampoco es el más rápido. Los leones machos en su hábitat natural pueden llegar a medir hasta 3 metros, incluyendo su cola, y las leonas un poco menos. Pesan entre 150 y 225 kilos y viven entre 15 y 20 de años.

A pesar de su fama de animal feroz, los leones se muestran muy sociables dentro de su manada. Siempre hay más leonas que leones en una manada que puede tener desde 3 hasta 40 animales.

Son las leonas que hacen la mayor parte de la caza y que cuidan a los cachorros y les enseñan a cazar. cachorro leonAdemás suelen parir sus cachorros al mismo tiempo (normalmente de 3 a 4 cachorros por leona) y se ayudan entre ellas con la crianza de los pequeños. Siempre hay mucho más hembras que machos en una manada. Cuando un macho se convierte en líder de su grupo, es frecuente que lo primero que haga es matar a todos los cachorros, para eliminar la herencia de su predecesor y asegurarse de que todos los futuros cachorros tengan sus genes.

El papel más importante de los machos en una manada de leones es defender el territorio.

LeonesEl rugido de un león se puede escuchar a una distancia de hasta 8 kilómetros, y está diseñado para advertir a posibles intrusos y para llamar a miembros de su manada para que vuelvan al grupo.

También se utiliza para comunicarse con otras manadas de leones que viven en el vecindario.

A pesar de su buena fama como cazador, el león es relativamente torpe y solo logra cazar su objetivo en un 20-30% de sus intentos. Cazan en equipo y, si no logran alcanzar nada, no tienen inconveniente en apoderarse de los restos de un animal cazado por otro especie. Por eso se les llaman "oportunistas". Después de la caza, los leones suelen comer primero, luego las leonas (muchas veces con peleas entre ellas) y por último, los cachorros.

Los leones que viven en África se consideran animales en peligro y los pocos que sobreviven en Asia se considera especie en peligro de extinción.

En este vídeo se pueden ver 3 leones jovenes persiguiendo a un búfalo.