Rubrics for Lesson 9
Rubrics for Lesson 9
Dealing with Exposure to Sunlight
Life on earth is dependent on
the sun, yet the sun can be harmful to creatures on land.
I would guess that the first intertidal (and then land living)
animals were probably very tiny organisms buried under damp seaweed,
and little crabs and other animals with exoskeletons to protect
them from drying out. Dehydration is a big problem for animals
living in air. People still die of dehydration today. Keeping
enough water in the body requires a protective skin and also
a new awareness of a sense of thirst. Do fish feel thirsty? Think
The sun provides energy that keeps life on earth going --
but it can also kill organisms, by drying them out and burning
delicate tissues. Shade plants can have their leaves sunburned,
and delicate animals, like little frogs, seek refuge in the water
when the sunlight gets too strong. The earliest land animals
had to adapt to sunlight, and only the best-suited animals survived.
An important development was water-proof skin., like that
covering this reptile.
Even today land organisms protect themselves by
growing fur or feathers, seeking shelter in the shade, and blocking
the sunlight by storing opaque pigments in their eyes and skin.
This reptile has skin pigment to protect its body from sunburn.
In the ocean, temperatures change
slowly and do not vary very much. Animals in the ocean are exotherms,
"cold-blooded" animals whose bodies are the same temperature
as the water. (This does not apply to land-living animals who
returned to the water, like whales and dolphins.) They don't
have to do much adapting to temperatures, and can generally choose
a depth that is comfortable for them.
On land, temperatures vary widely,
and can change from hot to cold in a couple of hours. This poses
big challenges to land-living creatures.
can kill, because the proteins in our bodies start to break down
at temperatures over about 108 degrees Fahrenheit. (This is why
people die of high fevers.) Animals must not get too hot. However,
heat can generally be avoided by finding shade, taking shelter
in a burrow, or wading into water. The animals also drink a lot
of water and do as little as possible during the hot hours.
Choosing Exothermy or
Physical processes are chemical, and
chemical reactions take place more quickly at higher temperatures.
Animals that have chosen exothermy (having the same temperature
as the environment) respond to low temperatures by slowing down
-- they move more slowly and metabolize more slowly. This makes
them vulnerable to warm blooded endotherms, who maintain
the same body temperature no matter what happens in the environment.
However, being warm-blooded is expensive -- a warm blooded endotherm
must eat about ten times more KiloCalories than an exotherm of
the same weight. We have both endotherms and exotherms on earth,
so both ways of being work here. We see warm blooded endotherms
all over the planet, but exotherms tend to do better in biomes
where there are no long periods of cold weather.
Photograph from a Corel CD-ROM
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