. Elements of ecology. Ecology. 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. ). 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
. Elements of ecology. Ecology. 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. ). 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 \ 'â \ ^ â \.^ Fig. 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 fishâa 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 male
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