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Fundamentals of Biology

NOTE: What follows is a paper written for this class by a junior high student. This is the type of thing I am looking for in a major paper. Bear in mind, however, that the current class is a high school level class, so the overall standards will be a little higher.

    Note also that the subject of this paper is not quite what we have in mind. It is a fine paper about the animals of the desert, but does not discuss the biome itself and all the interactions of plants and animals with the environment as much as we would like.

Desert Animals

                                                                                                                                     by Daniel D. Green

    The desert, covering up to one third of the earth's surface, is home to a surprising variety of animals who are remarkable in their ability to survive and even thrive in such a barren climate.  The common picture of an uninhabited wasteland is misleading -- most deserts (those with an annual precipitation above 50 millimeters) have abundant diversity of plant an animal life.  In fact, all of the major deserts in the world are also inhabited by people.  In this paper we will explore a few of the animal life forms from the insect, reptile, bird, and mammal families which live in the desert.

    The lack of water and blistering heat are the two most formidable natural challenges to life in the desert.  Therefore, animals have adapted to cope with these conditions.  Most animal life is resting during the intense heat of mid-day.  [An interesting exception to this is the silver ant of the Sahara Desert.  When the desert temperature reaches 115 they dash out of their nests and scurry around in search of corpses.  They are the fastest insects for their size.  Even they can stand the intense heat for no more than a half an hour, but apparently they appreciate the heat for protecting them from their chief predator, the lizard.]  All animals avoid the hot surface of the sandy terrain.  Insects burrow into the sand, to a level considerably cooler than the surface, as do reptiles.  Mammals such as kangaroo rats, jerboas, badgers -- even coyotes -- dig beneath the surface.  Birds maintain the most activity during the day, but when they rest they are most likely to be found above the ground perching in high places.  In flight, the airstream they create helps to cool them, and the feathers act as great insulators against the sun.  Since the surface is considerably hotter animals try to be as far away from it as possible.  They will even go to the extent, in the case of insects and lizards, to hold their body as high up as possible and run on their toes.  Another interesting fact to take notice of is that a lot of animals have large body surface to volume ratio.  Some mammals can pant to cool of and some lick their fur so that when the water evaporates they are cooled a little.  Desert birds also engage in gular fluttering, which is sort of their equivalent to panting.  They open their mouths and vibrate their throats, drawing and expelling air in rapidly.  This evaporates moisture off their lungs.

    Due to the lack of water most animals are amazing in their ability to use as little water as possible and get the most of what little there is.  For example, the darkling beetle, when there is a fog, will stand with its hind legs on the crest of a dune and its front legs on the slant and let water condense on its back and roll towards its mouth.  The sidewinder, a snake, will also do the same.  Only coldblooded animals can do this though.  Some animals, like the kangaroo rat, are so adapted to the lack of water that they don't need to drink at all.  One reason a lot animals don't need to drink is because they produce metabolic water.  This water is produced when food is digested.  The sugar in food is broken down and releases carbon dioxide, hydrogen, and oxygen.  Then the hydrogen and oxygen combine and form minute amounts of metabolic water.

    One of the most feared animals of the desert is the scorpion.  However, only a few have a lethal venom.  There are two kinds of scorpion venom:  one causes only local swelling and inflammation, similar to a bee sting; the other rapidly disperses into body tissues, producing respiratory and cardiac difficulties, sometimes resulting in death.  Scorpions are somewhat orphans in the biological tree.  The fact that they have eight legs means that they are classified as arachnids, though they are not spiders, nor are they insects.  Although they have anywhere from two to twelve eyes, they only perceive light and dark and must rely mostly on other senses.  Delicate hairs on the pincers are very sensitive to air movement, and comblike appendages on the underside detect odors and help it to feel its way.  Scorpions prey on anything small that crawls.  In turn they are preyed on by humans, who collect them for milking of their venom (used by biomedical laboratories).  One of the easiest ways to find them is to go out at night with a black light, because many of them glow when UV lights hit them.

    Spiders have also adapted to the desert, in spite of the abrasiveness of the sand.  The white lady spider injects sand grains into her web and then drapes the web over her burrow.  Unsuspecting prey can't tell the web apart from the sand, and walking over it, become trapped.  The back flip spider does almost the same thing, except it pulls the web over itself and lies on its back.

    Insects have a twofold purpose in the life of the desert.  First of all, they are responsible for cross-pollinating many desert plants.  Because so many insects are night animals, certain flowers abound that open their petals at night to take advantage of insect pollination, such as the night-blooming cereus, the evening primrose, and the jimson weed (these night-blooming plants are usually white and very fragrant).  Secondly, without insects, there would be no bats or nighthawks, as these animals are dependent on insects for food.  A bat may eat more than half of its weight in insects in a single night.  This kind of diet keeps the insect population under control.

    Lizards, snakes, and skinks contribute their presence to the desert scene.  Most lizards feed on insects or other lizards.  The spiny-tailed lizard is a notable exception: it feeds on plant material.  The lizards are colorful, highly textured, and  have become inspiration for art, ritual, and mythology in many indigenous societies.  Skinks are the largest family of Australian lizards.  Some swim through the sand like eels, and some have transparent discs that cover their eyes so they can see under the sand.  Lizards have adapted to loose sand by growing fringed toes and shovel noses.  The fringed toes give better traction; the shovel nose and an underhung jaw enable burrowing.  Some have frog-like webbing for better traction.

    The Western brown snake is one of the most common desert snakes.  Like the cobra, it flattens its neck to appear larger when confronting an opponent.  We have to say something about the sidewinder.  On normal firm ground, a snake moves forward by flexing the muscles of its body against the ground.  The slipperiness of sand means that the sidewinder must throw an entire loop of its body sideways while setting the full length of its side against the sand (and at a right angle to its path of movement), thus creating enough friction to move sideways rapidly.  It catches prey by burying all but the top of its head in the sand (the eyes are placed on the top of the head) and is therefore largely invisible to unsuspecting prey.  Three of the most commonly seen snakes of North American deserts are not poisonous: the bull snake (aggressive when first met, but becomes docile when stroked), the western king snake, and the coachwhip, or prairie runner (a fast nervous snake that lacerates the skin when biting, rather than puncturing).  The diamondback rattlesnake is one of the few poisonous snakes in North America.  The front fangs of the rattler retract and lie flat against the gums when the mouth is closed, but raise and point forward when the animal strikes.

    The most notorious bird of the (North American) desert is the roadrunner.  It spends most of its time on the ground, and utters a series of low cooing noises.  It is a member of the cuckoo family.  It eats rodents that are difficult to swallow, especially because of their bones.  So it takes the dead animals in its beak and beats it vigorously against a rock maybe one hundred times to break its bones and soften it.  One of its startling abilities is the way it kills rattlesnakes, by nimbly sidestepping or jumping up when the rattlesnake strikes (and spreading its wings because the wings are just bones and feathers) and then stabbing the snake's head with a rapid thrust of its beak.  A young roadrunner (nestling) eats almost entirely reptiles.  As it grows, the diet will include mostly insects (a help to farmers), although it will also eat other birds, spiders, bats, scorpions, and a few plants.  The roadrunner can really run -- up to twenty miles an hour.

    Another well-known bird is the Gila woodpecker.  The Gila is red on top, with black and white striped wings.  It flies with a swooping motion, shrieking all the time.  It likes to gouge holes in the saguaro cactus, looking for insect larvae.  These giant cacti are also the preferred site for Gila nests.  Other birds often move into abandoned Gila nests, because sap forms a hardened shell around the inside (to prevent water loss from the plant) which makes them unusually durable.  The woodpecker's diet is mainly insects, with occasional cactus fruits or lizards.  Unlike many seed-eating birds, the Gila generally consumes foods that are moist and so has little need for drinking water.  Even seed-eating birds, though, have managed to cope with the hardships of the desert.  For instance, the sandgrouse must have fresh water daily, and will fly 50 to 75 kilometers to get it.  They have an unusual way of watering their young: their breast feathers are so constructed that they absorb water when immersed; the adults then fly to the nest, where the waiting young strip the water out of the soaked feathers.

    The mammal kingdom has its representatives in the desert as well.  Some animals that can be found in the desert are not native to the desert.  For instance, wild cats and wild pigs are more closely associated with tropical forests, but have been versatile enough to find means for survival in the desert as well.  Jaguars, ocelots, bobcats, and mountain lions, although not found in large numbers, are present in the desert.  Their coloring is usually lighter, and they are thinner, in the desert, and they have to do more stalking of their prey than they would do in the tropical forests where they spring from large trees upon unwary animals.  The peccary is the only wild pig native to North America.  It survives through its ability to eat almost anything, including spiny cactuses -- which supply it with moisture during times of drought.  They rest during the day and then roam the desert at night in bands of a dozen or more.

    In a desert, where food sources are rare and unpredictable, some animals survive by becoming fiercely territorial and living solitary.  The kangaroo rat is one such example.  It is called a kangaroo rat because it hops on its hind legs (this means of locomotion is described as "saltatorial").  When another rat tries to steal some of its food, it will attack him viciously.  They leap at each other and try to kick each other with their hind feet.  To evade attacks, they will jump as high as two feet straight up in the air, though sometimes they will use leaping just for fun.  When something is chasing it, the kangaroo rat uses its tail as a decoy.  It has a dark tuft, and the kangaroo rat moves forward by jumping sideways at 45 degree angles.

    The kangaroo rat is crucial to the desert habitat, because it is a primary food source for many predators.  If there weren't so many kangaroo rats, these other animals would famish.  And if the kangaroo rat weren't so well adjusted to the rigors of desert life, there wouldn't be so many of them.  It's not that they cope well with the heat of the desert.  In fact, they need a burrow in the daytime considerably cooler than the sand surface.  But they do survive incredibly well with the lack of water because of the efficiency of their metabolism.  They lose little water through evaporation from skin or breathing (they do not sweat or pant), they have a kidney five times as powerful as man (they produce highly concentrated urine), and they lose much less water than other animals in their droppings, which are hard and dry (and they eat part of their droppings for essential vitamins and to reabsorb some moisture.)  The main way they manage to cope without drinking water is using metabolic water.

    When water, in the form of rain, comes to the desert, many animals are quick to take advantage of it.  In some areas, the mating and nesting of birds takes place during times of rain, rather than automatically in springtime.  Even the Spadefoot toads (so-called because their spade-like hind feet are good for digging in sand) appear after a rain and lay their eggs in the pools that remain.  Within two or three days tadpoles appear -- a rate much faster than in areas where water is abundant.

    Who would ever expect to find fish in the desert?  And yet there are a few kinds.  One is the pupfish, a small fish less than one inch in length.  The entire world's population of one species of pupfish lives in a small pool called Devil's Hole, near Death Valley.  These fish have become adapted to life in highly salted water.

    Thus we see that representatives of all the animal phyla are present and thriving (or at least surviving) in the world's deserts.  They have been  designed to adapt to the unique conditions of the desert.  They are part of a complex network of plants and animals that create the desert biome as we know it now -- with variety, fertility, and beauty beyond expectation.

Bibliography

Conley, Robert A. M.. "Locusts: 'Teeth of the Wind'." National Geographic, August 1969, pp.202 -226.
Graves, William, ed. "Desert Ants Venture Out in the Heat of the Day." National Geographic, February, 1993, vi.
Sutton, Ann and Myron.  The Life of the Desert.  New York: McGraw-Hill Book Company, 1966.
Wagner, Frederic. Wildlife of the Desert.  New York: Harry Abrams, Inc., 1980.
Whitson, Martha A..  "The Roadrunner: Clown of the Desert." National Geographic, May, 1983, pp. 694 - 702.
Zahl, Paul.  "Scorpions: Living Fossils of the Sands."  National Geographic, March, 1968, pp. 436 - 442.

 

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