Biodiversity

Sunday, April 30, 2006

THE SERPENTS

Snakes are reptiles (Class: Sauropsida). They share this class with the orders Crocodilia, Rhynchocephalia (tuataras), Testudines (tortoises, turtles, terrapins); with their order being the Squamata that includes snakes, lizards and worm lizards. Their order is by far the most successful in terms of specie diversity. (6)

The fossil record of snakes is poor, but on the basis of their morphology, snakes most likely had lizard-like reptile ancestors. (1) There ate two hypothesis; one where the snakes evolved from some kind of burrowing lizards, possibly the varanids (Monitor lizards), the other that they evolved from the mosasaurs (extinct aquatic reptiles). (1) The transparent, fused eyelids of the snakes and the loss of an external ear fit both hypotheses. For a subterranean lifestyle, their eyes and ears were protected from dirt and the loss of limbs was an adaptation to becoming more streamlined for burrowing. (1) A few fossil records seem to support this hypothesis, with two-legged burrowing animal fossils having been found. (1)

For an aquatic lifestyle, the adaptations to eyes, ears and loss of limbs also fit. Fossil records in marine sediments from the early Late Cretaceous period seem to support this hypothesis, especially because they are older than the burrowing lizard fossils. (1) The diversity in snakes only developed in the Paleocene epoch, when many niches were empty after the extinction of the dinosaurs.

The anatomy of snakes differs from lizards in several ways. The lower jaw of snakes has two halves, joined by an elastic ligament. (1) They also posses numerous other joints in their skull allowing them to swallow prey, often much larger than their head.(2) They do not have a pectoral girdle, for they do not have forelimbs that need to be supported. Although they do not have hind limbs either, some still have remnants thereof like the pythons and boas that have tiny spurs, where the limbs once were. Snakes do not posses a urinary bladder, their brain case is closed anteriorly and as said before, they have transparent, fused eyelids and no external ear opening.

Since the snakes had lost their limbs, they had to develop other ways of locomotion. Strong flank muscles that contract in alternating bands produce a series of s-shaped curves along the length of the snake’s body, allowing it to move forward. (1) Snakes can also move along in a rather straight line, allowing them to climb trees or move along in tunnels. Here their scales help them to grasp. This concertina movement is achieved by grasping with their posterior part of their body, while extending the front part of their body, then grasping with the front part of the body and contacting the back, so that it is pulled forward. (1) Some snakes use a side-winding motion to move forward. This is used on slippery surfaces or loose sand as in desserts. (1)

Snakes have taken on many different reproductive strategies. They are all internally fertilized through the males hemipenis, which are kept inverted in their tails, (they have two, using only one at a time). (3) Most snakes lay eggs and abandon them, but some have taken on parental care by keeping the eggs internally (usually associated with cold climates). Some have even abandoned the egg shells and are nourishing the young through a placenta apart from the yoke sac, (1) giving birth to live young.

All snakes are carnivores. (1) They eat a wide variety of animals from small mammals, lizards, birds, insects, eggs and even other snakes. They do not chew their food but rather swallow it whole. Some snakes kill or paralyze their prey first, others use constriction to kill the prey and others still eat their prey alive. (1) Once the prey has been swallowed the snake becomes sluggish, as it takes a lot of energy to digest its meal. If they are disturbed, they will often regurgitate their meal to allow them to flee. They are able to digest everything but hair and claws. (1)

Snakes have adapted to many different lifestyles and habitats from tropical oceans, to desserts to jungles. They are highly successful reptiles, each adapted to its specific surroundings. The sidewinder of the Namib dessert has perfected its locomotion to overcome the hot loose sand of the dessert. The “flying” snakes of southern Asia. (4) The sea snakes, with their paddle like tails have taken to a purely aquatic lifestyle. (5) The garter snakes of Canada that hibernate through the long winters, congregate in huge numbers during the short mating season and have taken parental care as far as giving birth to live snakes.

References:

1. Wikipedia contributors. Snake [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 29, 14:04 UTC [cited 2006 Apr 30]. Available from: http://en.wikipedia.org/w/index.php?title=Snake&oldid=50737793.
2. Wikipedia contributors. Snake skull [Internet]. Wikipedia, The Free Encyclopedia; 2005 Sep 18, 07:07 UTC [cited 2006 Apr 30]. Available from: http://en.wikipedia.org/w/index.php?title=Snake_skull&oldid=23447655.
3. Wikipedia contributors. Chrysopelea [Internet]. Wikipedia, The Free Encyclopedia; 2006 Mar 19, 16:52 UTC [cited 2006 Apr 30]. Available from: http://en.wikipedia.org/w/index.php?title=Chrysopelea&oldid=44519989.
4. Wikipedia contributors. Sea snake [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 21, 20:15 UTC [cited 2006 Apr 30]. Available from: http://en.wikipedia.org/w/index.php?title=Sea_snake&oldid=49492696.
5. Wikipedia contributors. Hemipenis [Internet]. Wikipedia, The Free Encyclopedia; 2006 Jan 16, 21:53 UTC [cited 2006 Apr 30]. Available from: http://en.wikipedia.org/w/index.php?title=Hemipenis&oldid=35453339.
6. Wikipedia contributors. Reptile [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 15, 20:44 UTC [cited 2006 May 16]. Available from: http://en.wikipedia.org/w/index.php?title=Reptile&oldid=53382896.

Karen Marais
BCB Hons NISL student
University of the Western Cape
Private Bag X17
Bellville

E-mail 2657211@uwc.ac.za

Web http://brit-journal.com/karen2006bcbnisl/

3 Comments:

  • Hi Karen

    Good work karen. may you please add more about flying snakes. I am interested about this.

    Thanx

    By Blogger lufuno, at May 03, 2006 10:23 AM  

  • hi Karen dont you know anithing about lizard without legs

    By Blogger lethabo, at May 15, 2006 10:31 AM  

  • Your reference numbers got a bit confused this time. It is also usual to start the reference list with the reference that appears first in the text.

    Are you telling me that the radiation of snakes in the Paleocene is common knowledge?

    As usual, a good essay though there were a few typo's.

    By Blogger Gwen, at May 23, 2006 4:59 PM  

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