On Size of Animal and Metabolism: Bergmann's Rule

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On Size of Animal and Metabolism: Bergmann's Rule

May 21 2011 6:47:13 pm EST

Topics: Morphology,

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Heat loss in an organism is proportional to the surface area, or the square of its linear dimensions; the larger the surface area, the greater the heat loss. If the tissues of all homeotherms were to generate heat uniformly, the overall generation of heat would be proportional to the mass of the organism, or the cube of its linear dimensions. In other words, the larger the organism, the more tissue to generate heat, and the more heat generated overall.

However, this is not the case. Smaller mammals must overcome the effects of a larger surface area by generating more heat per unit of mass. Consider the following passage in On Growth and Form:

The tissues of one mammal are much like those of another. We can hardly imagine the muscles of a small mammal to produce more heat (caeteris paribus) than those of a large ; and we begin to wonder whether it be not nervous excitation, rather than quality of muscular tissue, which determines the rate of oxidation and the output of heat. It is evident in certain cases, and may be a general rule, that the smaller animals have the bigger brains; “plus I’animal est petit,” says M. Charles Richet, “plus il a des échanges chimiques actifs, et plus son cerveau est volumineux.” That the smaller animal needs more food is certain and obvious. The amount of food and oxygen consumed by a small flying insect is enormous; and bees and flies and hawkmoths and humming-birds live on nectar, the richest and most concentrated of foods. Man consumes a fiftieth part of his own weight of food daily, but a mouse will eat half its own weight in a day; its rate of living is faster, it breeds faster, and old age comes to it much sooner than to man. A warm-blooded animal much smaller than a mouse becomes an impossibility; it could neither obtain nor yet digest the food required to maintain its constant temperature, and hence no mammals and no birds are as small as the smallest frogs or fishes. The disadvantage of small size is all the greater when loss of heat is accelerated by conduction as in the Arctic, or by convection as in the sea. The far north is a home of large birds but not of small; bears but not mice live through an Arctic winter; the least of the dolphins live in warm waters, and there are no small mammals in the sea. This principle is sometimes spoken of as Bergmann’s Law.

Dates:

2011

2010