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Historically, biologists have been particularly impressed with mammalian and human homeostasis. Lower vertebrates have received less attention. However, while internal physiology may vary more in a frog than in a human, there are mechanisms which assure the survival of frogs. For instance, when the ambient temperature drops significantly in the autumn in northern latitudes, leopard frogs move into lakes or rivers which do not freeze. Moving into lakes and rivers is a behavioral response to a change in the external environment which results in internal temperature stability. The metabolism and structure of the frog is inadequate to protect the frog from freezing, but the specific heat of the water is such that freezing does not occur except at the surface of the overwintering lake or river. Even though life at the bottom of a lake with an ice cover moves at a slower pace than during the warm summer months, a functioning circulatory system is essential for survival. In general, frog blood (not unlike crankcase oil prior to the era of multiviscosity oil) increases in viscosity with as temperature decreases. Frog blood, however, decreases in viscosity with the prolonged autumnal and winter cold temperatures, thus assuring adequate circulation during the long nights under an ice cover. This is another control mechanism that assures the survival of frogs by maintaining a relatively stable internal environment during the harsh winter. With a return of a warm external environment, northern leopard frogs leave cold water to warm up under the spring sun. Warm temperature causes frog blood viscosity to increase to summer levels. It may be that the behavioral and physiological changes do not prevent oscillations that would be unsuitable for warm blooded animals but, in the frog, the fluctuations do not interfere with survival, and in biology, that is all that is essential.

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