Biodiversity

Sunday, April 30, 2006

LORDS OF THE AIR

“One of the earliest birds was a toothy, long-tailed creature the size of a crow, with feathers evolved from scales”. (Reader’s Digest Editors. 1974, p.250) This was Archaeopteryx, first discovered in a limestone quarry near Solnhofen, Germany in 1861. Since then birds have adapted greatly, perfecting their anatomy for flying and their physiology for providing the energy to fly.

Although Archaeopteryx most probably could fly for short distances, its body was heavy and not well adapted for it. Birds had to greatly reduce their weight to make them able flyers. Today, birds lost the heavy reptilian tail and they have replaced a heavy jaw and teeth with a light but strong beak. Many of their bones are hollow and almost paper-thin, but they are reinforced by internal struts for extra strength. The lower vertebra of their back and the hip girdle are fused together for strength and act as an anchor for the muscles that control their tail feathers. They all have a keeled breastbone for attachment of the strong pectoral muscles that produce the downstroke in flight. The small breast muscles raise the wings by a “rope and pulley” attachment at the top of the arm bone. The collar bones are fused (wishbone) to give stability at their shoulders. (1, 2)

The position of their organs is also optimal for flight, with the lungs being high up in their trunk and the heavier organs like the heart, digestive organs and the pectoral muscles being positioned ventrally. This arrangement also places the centre of gravity in an ideal position, preventing the bird from flipping over in flight. (2) Even the reproductive organs have been modified by shrinking to a fraction of their functional weight out of breeding season. (1)

Because flight can demand high levels of energy (for the flight muscles can consume huge amounts of oxygen), a bird’s heart is big relative to its body. Its respiratory system is also unique. Birds have in addition to their lungs special air sacs that even extend into the bones. When they inhale, the air flows through the lungs and into the air sacs and when they exhale the air passed back through the lungs, giving the bloodstream another chance to take up still more oxygen and get rid of carbon dioxide. These special air sacs also function as a cooling system in times of intense exertion. (1)

Birds also need to take in high-calorie foods to provide them with the energy they need. They mostly eat foods like seeds, insects, fruit, nectar or small mammals and avoid food that is high in bulk and low in calories. Their metabolism is high, due to their relative high body temperature, which they maintain through their excellent insulating properties of their feathers. Their high metabolism rate can also be seen as a weight-saving advantage, as food is quickly digested and waste products soon excreted. (1)

Ultimately, it is the feather that has made the birds lords of the air. Their feathers can weigh twice as much as their skeleton, but individually they are “as light as a feather”. Apart from the downy underlayers, feathers have some 600 barbs on either side of the main shaft (quill), which forms the feather’s vanes. Each barb again has a main stem with about 800 barbules on either side. The barbules pointing towards the tip of the feather have little hooks, while those pointing to the base have a little ridge at the top onto which the hooks fit. This forms an almost air tight surface, which is essential for flying. It also has the advantage that a “torn” vane can be re-zipped by being stroked lengthwise several times. This is what actually happens when birds are preening themselves. They are repairing their feathers, keeping them in good condition. (Preening also entails waterproofing the feathers by spreading oil from a special oil gland just above their tail.) (1)

A bird’s wing shape differs according to their specific function and thus the bird’s lifestyle. Large seabirds have long narrow wings, which allow them to glide effortlessly which is very energy efficient. Soaring birds, like raptors have long broad wings with prominent primary feathers for manoeuvrability and stability in turbulent air. They make use of thermals to conserve energy. Ground feeding birds have short broad wings with arched bones that allow them to take off fast when threatened. Swept-backed wings that taper to slender tips make swifts very agile for catching flying insects, without having to flap their wings. A hummingbird has a unique wing structure. They are able to hover best of all birds and can even fly backwards. Their “arms” are very short, but their “hands” long with ten long and powerful primary feathers. Their elbow and wrist joints are rigid, but their shoulder joints are so mobile, that their wings can practically move in any direction, giving them their extraordinary flying abilities. But it comes at a cost, as it consumes huge amounts of energy. (1)

The birds are the most widely travelled and species rich of all vertebrates. Through flight they have mastered the air, but it is the uniqueness of their feathers that has given them this edge.

References:

1. Reader’s Digest Editors. 1974. Animal Families. Marvels and mysteries of animal behaviour. Pages 250-261 in Animals in Action. Hong Kong: Reader’s Digest Association Far East Limited.
2. Wikipedia contributors. Bird skeleton [Internet]. Wikipedia, The Free Encyclopedia; 2006 Mar 10, 23:12 UTC [cited 2006 Apr 30]. Available from: http://en.wikipedia.org/w/index.php?title=Bird_skeleton&oldid=43210309.

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/

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/