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Tiger in River

Dealing with Exposure to Sunlight

Life on earth is dependent on the sun, yet the sun can be harmful to creatures on land.

Problems include:

Dehydration 

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 about this!

frog

 Sunburn

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.

sea Lizard

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.

Temperature Fluctuations

 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.

green lizard
High temperatures 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 Endothermy

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 : for viewing only, not for downloading.  
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Copyright © 1999.   Elizabeth Anne Viau and her licensors.  All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at t eviau@earthlink.net