630 



COLLEGE ZOOLOGY 



Temperature 



We are well aware of our own sensitivity 

 to temperature and what is true of man is 

 equally true of many other animals. 



Variations in temperature are generally 

 less local than variations in light, and larger 

 areas are therefore affected. The world may 

 be divided into zones largely on the basis 

 of differences in temperature, namely, arctic, 

 temperate, and tropical. Most animals, even 

 the amoeba and paramecium, react to dif- 

 ferences in temperature, and each species 

 has an optimum, which in the paramecium 

 is between 24° C. and 28° C. Temperatures 

 in sunlight and in shade differ and influence 

 animals in their selection of a habitat. Even 

 more notable differences occur in bodies of 

 water between surface layers and deeper 

 strata; animals react to these differences in 

 temperature until they encounter their opti- 

 mum which is the one at which they func- 

 tion best physiologically. An increase in 

 temperature up to a certain point speeds up 

 metabolism within the cells of the body and 

 affects the activities of many animals. How- 

 ever, activity usually ceases before 45° C. is 

 reached. House flies, for example, begin to 

 move at about 6° C, carry on normal ac- 

 tivity at about 17° C, increase their ac- 

 tivity up to about 28° C, cease their activity 

 at about 45° C, and die at about 46.5° C. 



The effects of temperature on the pres- 

 ence or absence of animals in different habi- 

 tats are varied. Certain species will not lay 

 eggs untfl a favorable temperature is reached. 

 Eggs develop more slowly at a low tempera- 

 ture and will not develop at all if the 

 temperature is too low or too high. In some 

 species, the adults die as winter approaches, 

 but the race is maintained by means of 

 "winter" eggs that can withstand the cold 

 weather; other animals escape the cold by 

 hibernation or migration. Some survive at 

 liigh temperatures by estivation; certain ani- 

 mals live normally in hot springs at a tem- 

 perature that would soon kill others accus- 

 tomed to habitats of lower temperatures. 



However, all living things are adapted to a 

 comparatively limited temperature range; 

 each animal has a different range in which 

 life is possible as well as an optimum at 

 which it lives best. 



Chemical factors 



The chemical nature of the medium is 

 another important physical factor that in- 

 fluences the presence or absence of an ani- 

 mal in a particular habitat. As we have 

 seen, protozoans, crayfishes, and many other 

 animals react to chemical changes; these 

 changes are often more conspicuous in an 

 aquatic medium. For the most part, marine 

 animals cannot live in fresh water, nor 

 fresh-water animals in salt water. The pres- 

 ence or absence of certain chemicals may 

 determine whether an animal can live in 

 a habitat or not. For example, carbonate of 

 lime (calcium carbonate) is necessary for 

 building the shells of certain snails which 

 cannot live where there is not a sufficient 

 quantity of this chemical. 



Water 



Animals consist largely of water, and all 

 require water. Since water is very irregularly 

 distributed on the earth's surface, it has an 

 important bearing on the character of the 

 animals that live in various types of habitat. 

 Plants are classified ecologically according 

 to their water requirements. As just noted, 

 the chemical content of the water deter- 

 mines to a considerable extent its availability 

 as a habitat. The amounts of certain gases 

 are also factors, for example, in deep water 

 less oxygen and more carbon dioxide are 

 usually present than in surface water. Or- 

 ganic compounds in water may serve as food 

 for certain animals but prove fatal to others. 

 Animals that live in dry regions usually 

 possess some method of preventing evapora- 

 tion of water; the horned lizard ("toad"), 

 by means of a thick covering, can live for 

 about 4 months in a desiccator; and the 

 camel, by storing water in the reticulum of 

 its stomach, can live for a week on dry 



