Chap 5 ANIMALS AND THEIR ENVIRONMENTS 81 



never remains in them. Whether they are pine trees or cattle, living organisms 

 take in relatively large amounts of water that gradually filters completely 

 through their bodies. 



Physical Environment 



The chief physical influences upon plants and animals are gravity, pressure, 

 temperature, and light. 



Gravity. Its weight, actually the earth's pull, greatly affects an animal. The 

 bridge-type of four-legged animal is a four-cornered support of the body 

 against the pull of the earth (Fig. 9.11). Birds are the master adjusters to the 

 force of gravity. No other animals approach them in lightness and strength, 

 due to the air-filled outpocketings of their lungs that extend into the bones, 

 their rapid elimination of waste products, and the lightness of feathers (Chap. 

 36). 



Pressure. The medium in which animals live presses upon them continually 

 from every point, upon their forms, actions, and the amount of gases which 

 they hold. 



The atmosphere of the earth is like a haystack (Fig. 5.16). At the bottom 

 or sea level its content is closely packed; the atoms of oxygen are near to- 

 gether. At sea level an animal, like every other object, carries 14.7 pounds of 

 atmospheric pressure on each square inch of the surface of its body, and this 

 pressure so evenly permeates its body that none is felt. At 20,000 feet (300 

 feet lower than Mt. McKinley) the same animal would be exposed to pressure 

 less than half that of sea level. In spite of their high oxygen demand in breath- 

 ing, birds fly through air of low oxygen content probably securing an adequate 

 supply because of the speed with which they drive into it. At 18,000 feet mules 

 in South America carry riders without great difficulty, and this is said to be 

 due to their frequent stops during which oxygen accumulates in their blood. 

 Anyone acquainted with them knows that mules have the same sagacity at sea 

 level where they also make frequent stops. 



Water is about 775 times more dense than air and consequently heavier. It 

 is peculiar in that it becomes denser and heavier as it cools to a temperature 

 of 39.2° F. (4° C). When colder than that it is less dense and lighter, finally 

 floating as ice. Because of this the pond is covered with a blanket of ice below 

 which fishes can disport themselves in safety (Fig. 5.17). 



The pressure upon an animal in water is the weight of a column of water 

 extending above a given area of its body plus the atmospheric pressure above. 

 The pressure on a fish in Lake Tahoe in California, over 6,000 feet above sea 

 level, is far less than that on a codfish in the Atlantic Ocean. At great depths 

 of the ocean the pressure is several tons per square inch. It does not crush the 

 animal because the fluids in its body are under the same pressure as the water 

 surrounding it. Pressure compresses gas which expands when deep-sea fish are 



