The problems of survival of animals on land are very different from those of survival of
animals in aquatic environment. Describe four problems associated with animal survival in
terrestrial environments but not in aquatic environments. For each problem, explain a
physiological of structural solution.
Four problems faced by animals on land are breathing (respiration), water conservation
in excretions, successful reproduction, and the producing an egg which can survive
outside of the water.
All animals need to respire, but I have no idea why. Maybe you would like to answer
that? Aquatic animals use gills, which are outgrowths from the body which increase
surface area over which gas exchange can occur. Inside the gills of aquatic animals, the
circulatory system removes oxygen, and delivers waste carbon dioxide. Land vertebrates
have developed a different approach to the problem of gas exchange, as water is not
present in all of the terrestrial environment. Terrestrial vertebrates have developed
lungs to solve this problem. Air enters through the nasal passages, or the mouth, passes
through the trachea, then branches off at the two bronchi, and goes through many
branching passages called bronchioles, which end in alveoli. Alveoli are sack-like
structures where the circulatory system meets the respiratory system.
Since terrestrial vertebrates do not live in water, they need to develop a means of
conserving water. One way we do this is through our excretions. Nitrogen forms a major
waste product in animals. When amino acids and nucleic acids are broken down, they
release toxic ammonia (NH3). To rid the body of this toxin, several mechanisms have
evolved, each appropriate to the habitat or survival of the animal. Aquatic animals
secrete NH3 directly into the surrounding water. Land animals cannot do this because of
the toxicity of NH3. Instead, NH3 is converted into urea in our livers. Urea is
significantly less toxic than NH3, and thus requires less water to excrete in the urine.
The reason we need the water to excrete this is because the water is needed to dilute the
urea (or NH3 if we did excrete it in that form), in order to make it less toxic. Birds
excrete ammonia in the form of uric acid, that's what they're always dropping on our
heads. Those mangy little rats with wings... have you ever wondered why we let those
little pests run free in the cities, but we wont let dogs and cats free, even though
most people consider the birds more of a nuisance? I didn't think so, anyways:
A third adaptation to terrestrial life is internal fertilization. In aquatic animals,
many eggs are laid, usually allowing the water, and chance to fertilize the eggs. We
can't do this on land, because the eggs and sperm would dry out, and would stay in the
same place, unless they could walk (he he he). To solve this problem, we have developed a
system of internal fertilization. The sperm are released directly inside the female,
providing an increased chance of fertilization.
The amniotic egg of birds and reptiles represents a transition to terrestrial life. The
egg provides conditions similar in some ways to the aquatic environment. In the aquatic
environment, eggs have soft, usually permeable shells, which do not have to worry about
losing water. The amniotic cavity formed by the amnion is fluid-filled, protecting the
embryo. The egg case often leathery in reptiles, and calcified in birds protects the
contents, while permitting gas to be exchanged with the surroundings. This egg also
prevents the evaporation of water from the embryo, since the egg cannot walk to the store
and buy some Evian, it needs all to water it has.
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