Chap. 5 



ANIMALS AND THEIR ENVIRONMENTS 



83 



60,000 Ft 



THE ATMOSPHERIC HAYSTACK 



Fig. 5.16. Atmospheric pressure illustrated by stacked hay showing the weight 

 it would carry at various heights to 60,000 feet. The proportions of the gases in 

 the atmosphere do not change at different heights but their total amount does. 

 This is why the air is thin in high places. 



brought to the surface, just as gas expands when a bottle of compressed fluid 

 pops. When deep-sea divers rise to the surface rapidly the pressure on the 

 nitrogen in the blood is released too quickly; it gathers in bubbles in their 

 muscles and joints producing a condition known as the bends (Fig. 5.18). 



Temperature. Except for those that live in hot springs, plants and animals 

 can live only within a narrow range of temperature and can endure relatively 

 low temperatures better than high ones. Many tropical animals cannot bear 

 extreme exposure to the sun's heat. In zoological gardens ostriches, croco- 

 diles, and snakes have often been killed by heat. Birds have the advantage in 

 their cooling devices of air sacs and mammals of panting and sweating. 



Wherever there are severe winters, animals resort to various ways of avoid- 

 ing or meeting them. Birds go to warmer regions or remain in the cold and 

 depend on heavy feeding to keep up their metabolism; many mammals, rab- 

 bits, foxes, and others are active but must have abundant food; other mam- 

 mals hibernate, put on layers of fat in the fall, and live at a kind of physio- 



FiG. 5.15. Chemical cycles. The carbon cycle. Respiration of plants and animals 

 returns most of the carbon to the air as carbon dioxide. The storage of carbon in 

 coal and oil is an important exception to the general rule that the carbon used by 

 the green plant in photosynthesis returns to the air. Coal is largely carbon derived 

 from the cellulose of the trees about 250 or more million years ago. Carbon is 

 also captured in the calcium carbonate (CaCO^) of clams, crabs and others. 



The nitrogen cycle is much more complex than that of carbon. The main reason 

 is that many organisms do not release nitrogen in a form that can be immediately 

 used by green plants. They can use it when it appears as certain inorganic salts, 

 particularly nitrates. 



