allometry

allometry, in biology, the change in organisms in relation to proportional changes in body size. An example of allometry can be seen in mammals. Ranging from the mouse to the elephant, as the body gets larger, in general hearts beat more slowly, brains get bigger, bones get proportionally shorter and thinner, and life spans lengthen. Even ecologically flexible characteristics, such as population density and the size of home ranges, scale in a predictive way with body size. The study of allometry stems from work in the late 19th century by the Scottish zoologist D’arcy Thompson and in the early 20th century by the English biologist Julian Huxley, the latter of whom coined the term for this field of study.

Scaling is often considered to be one of the few laws in biology. Allometric equations take the general form Y = aM^b, where Y is some biological variable, M is a measure of body size, and b is some scaling exponent. In allometry, equations are often presented in logarithmic form so that a diverse range of body sizes can be plotted on a single graph.

The most common example of allometry is geometric scaling, in which surface area is a function of body mass. In general, for organisms that preserve their basic shape as they vary in size, the organism’s linear dimensions vary as the 1/3 and their surface area as the 2/3 powers of their body mass. The relationship of energy consumption (or metabolic rate) and body mass in mammals is another well-known example of scaling (Kleiber’s law): metabolic rate scales as the 3/4 power of body mass.

Biologists have studied scaling within individual organisms, among different individual organisms, and across groups of many individuals or species. Studies of allometry take two basic forms. One approach emphasizes determination of the exponents, or invariant properties across organisms, as in Kleiber’s law. The other approach concerns how and why organisms change relative to size—for example, why deer that have large antlers for their size tend to use them more for fighting and aggressive behaviour.

One mechanism proposed to account for scaling states that biological organisms are limited by the rates at which energy and materials can be distributed between surfaces where they are physiologically exchanged and the tissues are used. Thus, allometric relations may be ultimately related to anatomical and physiological features of energy usage.