ECOLOGY 761 



food is eaten and the metabolic demands of the body are met from the 

 stores of body fat. Crocodiles, certain frogs and fishes survive periods 

 of high temperature and dryness by undergoing aestivation, a torpid, 

 inactive state comparable to hibernation. 



Birds and mammals have physiologic mechanisms which keep body 

 temperature constant despite wide fluctuations in the environmental 

 temperature (p. 486). These thermostated animals are said to be homoio- 

 thermlc ("warm-blooded" is not quite the proper synonym; they are 

 really "constant temperature-blooded"). Reptiles, amphibia, fish and 

 all invertebrates are poikilothermic; their body temperature fluctuates 

 with that of the environment. "Cold-blooded" is not properly descrip- 

 tive, for a lizard sitting in the sun may have warmer blood than ours. 

 All of the metaboHc processes in poikilotherms are directly influenced 

 by the environmental temperature. Such animals move, feed and grow 

 in warm weather and become inactive in cold weather. Many marine 

 organisms have seasonal north-south migrations to find water with the 

 optimal temperature. 



Light. The amount of Ught is an important factor in determining 

 the distribution and behavior of both plants and animals. Light is, of 

 course, the ultimate source of energy for life on this planet, yet pro- 

 longed direct exposure of protoplasm to light is fatal. The amount of 

 daylight per day, known as the photoperiod, has been found to have a 

 marked influence on the time of flowering of plants, the time of migra- 

 tion of birds, the time of spawning of fish, and the seasonal change of 

 color of certain birds and mammals. The effects of the photoperiod on 

 the vertebrates appear to occur via some hormonal mechanism involving 

 the pituitary. Knowledge of photoperiod phenomena has proven to be 

 of considerable economic importance. Chicken farmers have found that 

 artificial illumination in the hen house, by extending the photoperiod, 

 stimulates the hens to lay more eggs. 



Water. Water is a physiologic necessity for all protoplasm, but is 

 a limiting factor primarily for land organisms. The total amount of 

 rainfall, its seasonal distribution, the humidity, and the ground supply 

 of water are some of the factors limiting distribution of animals and 

 plants. Some lakes and streams, especially in the western and south- 

 western United States, periodically become dry or almost dry and the 

 fish and other aquatic animals are killed. During periods of low water, 

 the water temperature may rise sufficiently to kill off the aquatic forms. 

 Many of the protozoa form thick-walled cysts which enable them to sur- 

 vive the drying of the puddles in which they normally live. Some desert 

 animals have adapted to desert conditions by digging and living in 

 burrows where the temperature is lower and the humidity is higher than 

 at the surface. Measurements have shown that the burrow of a kangaroo 

 rat two feet underground may have^a temperature of only 60 F. when 

 the surface temperature is over 100° F. , u r 



An excess of water is fatal to some animals; earthworms, for ex- 

 ample may be driven from their burrows by heavy rainfall because 

 oxygen is only sparingly soluble in water and they are unable to get 



