Other Effects of Temperature 157 



blooded hosts by a turning reaction to the heat of their bodies. Rat- 

 tlesnakes, copperheads, and other "pit vipers" can detect the presence 

 of mammals or birds that may be only a few degrees warmer than 

 the surroundings. Oriented by the heat radiation, these snakes can 

 strike accurately at their prey even in the dark. Other behavior 

 reactions are influenced by heat conditions. When the temperature 

 drops below 5°C, the leopard frog is stimulated to burrow into the 

 mud bottom of its pond. At the same temperature sunfish crowd 

 together in aggregations but, if the water is warmed to 8°C, the fish 

 tend to swim about separately. Snakes similarly gather in aggrega- 

 tions and form balls in their retreats among the rocks when cold 

 weather arrives in the autumn. 



Temperature, as well as moisture, light, and other factors, has long 

 been known to affect the coloration of some animals. In warm, humid 

 climates many mammals, birds, and insects, tend to be more melanic, 

 that is, darker in color than races of the same species living in cool, 

 dry climates. This generality, already referred to as Gloger's rule, is 

 well recognized but many exceptions are encountered, and the relative 

 influences of heat and moisture are not known. In some instances 

 coloration seems more affected by genetic selection than by the direct 

 action of climate. The possibility of hereditary transmission of heat- 

 color relationships is demonstrated by experiments showing that the 

 temperature to which pupae are exposed will affect the wing color of 

 butterflies even to the second generation. This whole subject is con- 

 troversial since observations on various species in different areas do 

 not agree. Coloration is evidently controlled by interactions between 

 environmental and hereditary factors, but a more exact understanding 

 must await further study. 



The temperature factor is also known to affect the absolute size 

 of many animals as well as the relative proportions of certain parts. 

 The general fact that among birds and mammals the same species 

 attains a greater body size in cold regions than in warm regions and 

 that among closely related species the larger ones inhabit the colder 

 climates is known as the Bergmann principle. Poikilothermous ani- 

 mals, as exemplified particularly by reptiles and amphibians, exhibit 

 the reverse relationship since they tend to be smaller in colder cli- 

 mates. 



The related observation that extremities, such as the tail, ears, and 

 legs, of mammals are shorter in colder climates has been delineated 

 as Allen's rule. Since both these generalities apply to warm-blooded 

 forms, they are probably related to the difficulty of retaining heat at 

 low temperatures and the desirability of losing it at excessively high 



