158 Temperature 



temperatures. Animals with large bodies have surfaces that are rela- 

 tively smaller in relation to their masses than small animals. Smaller 

 extremities expose smaller surfaces (Fig. 5.11). Since heat is lost 

 through the surface, the smaller the area of an animal's skin, the more 

 easily may the animal maintain its temperature in cold weather. 

 Conversely, the development of extremities with large areas aids heat 

 loss and evaporation in hot climates. These simple ecological rela- 

 tionships, in addition to underlying Bergmann's principle and Allen's 

 rule, undoubtedly account in part at least for the fact that no extremely 

 small mammals or birds exist, that is, as small as the majority of insects. 



\ '■ \ ^ 



■\.^ 



Fig. 5.11. Head of (left) arctic fox, Alopex lagopus, {center) red fox, Vulpes 



vulpes, and (right) desert fox, Megalotis zerda, showing gradation in size of ears. 



( Reprinted with permission from Hesse, Alice, and Schmidt, Ecological Animal 



Geography, 1951, John Wiley & Sons, New York.) 



A very specialized influence of temperature on morphology is the 

 apparent control it exerts on the number of vertebrae in certain species 

 of fish— a relationship known as Jordan's rule. Cod hatched off New- 

 foundland where temperature ranges between 4 and 8°C have 56 

 vertebrae, whereas cod hatched east of Nantucket in temperatures 

 averaging 10 to 11°C possess only 54 vertebrae. The relationship 

 is brought about through the control by temperature of metameric 

 segmentation at an early stage of development. Incidentally, the 

 fact is of use to fishery biologists in ascertaining the origin of popula- 

 tions among those species of fish that exhibit this geographical dif- 

 ference in structure. 



The temperature of the environment controls egg type and sex 

 ratio in certain animals. Under moderate heat conditions Cladocera 

 produce parthenogenetic eggs, and these hatch usually into females. 

 Under ordinary circumstances few if any males exist in the popula- 

 tion, but their production and the appearance of "winter" eggs is 

 favored by extremes of temperature. The winter egg of the Clado- 



