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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. |