ENVIRONMENTAL RELATIONSHIPS 



temperature effect. Similarly, the ability of the atmosphere to hold water 

 vapor in suspension changes as the temperature changes. In general, a warm 

 atmosphere is less easily saturated with water vapor than a cold one; that is, 

 at a given relative humidity, expressed as a percentage of complete saturation, 

 a warm atmosphere holds more moisture than a colder one. Thus, an ap- 

 parent direct effect of temperature on the survival of a terrestrial animal 

 might actually reflect the action of temperature in altering the atmospheric 

 humidity. It is clear, therefore, that experiments to test the effects of temper- 

 ature on organisms must be planned, and the results interpreted, with atten- 

 tion to the indirect effects of temperature acting through other environmental 

 factors. 



Oxygen. The availability of oxygen is of obvious significance in de- 

 termining the survival and distribution of animals. In an overwhelming 

 majority of species cellular metabolism and energy release depend on a series 

 of reactions, the ultimate step of which involves the combination of carbon 

 with oxygen to form carbon dioxide. In the absence of adequate amounts 

 of free oxygen, the aerobic phase of cellular metabolism is impossible. The 

 external environment is the source of oxygen for the animal, and we have 

 previously noted (pp. 516 517) the variety of structural and functional 

 adaptations perfected in animals for the abstraction of oxygen from the 

 environment, in the process of gas exchange. Only a relatively few species 

 of animals, notably saprozoic free-living protozoans and a number of 

 intestinal parasites among the Metazoa, can live by anaerobic metabolism 

 in the absence of free oxygen. Metabolism in these forms is a process com- 

 parable with the anaerobic phase of metabolism in aerobic organisms. Many 

 species appear to be "obligate anaerobes" which cannot survive in the 

 presence of free oxygen; others utilize such small amounts of oxygen as may 

 be present but are not dependent on it. 



Some natural waters, such as the bottom layers of deep lakes, and waters 

 rich in decaying organic matter, contain no free oxygen and are thus suitable 

 only for anaerobic organisms. In other aquatic situations the percentage of 



Fig. 19.2. Comparison of 

 the heads of fo.xes from 

 different climatic regions. 



A, arctic fox, Cams lagopus; 



B, red fox, Cams vulpes, of 

 temperate regions; C, desert 

 fox. Cams zerda. (Redrawn 

 from R. Hesse, W. C. Allee, 

 and K. P. Schmidt, Ecological 

 Ammal Geography, second edi- 

 tion, copyright 1951 by John 

 Wiley and Sons, Inc., printed 

 by permission.) 



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