Allometry: revealing evolution's engineering principles.

An animal's body size impacts many aspects of its structure and function (Y); insights that only become apparent when viewed over several orders of magnitude of body mass (M) and expressed allometrically in the form of power law equations (Y=aMb). The resulting relationships are apparent clusters of similar exponents (b) revealing emergent 'patterns of design' that shed light on the universal principles of structure and function. Basic physical principles of surface area, volume and heat exchange apply to all objects, including animals, and many consequences must be attributed to these fundamental properties. Starting with Galileo's description of the shapes of bones in the 16th century and extending to 19th century explanations of heat production and loss by Sarrus and Rameaux, allometric patterns have provided numerous biological insights. Here, we examine several of these insights and explore how the selective pressures and scaling may differ when comparing animals in a vegetative (basal) state and those utilizing their maximum metabolic capacities. It seems that the selective pressures under those two conditions differ. We caution that allometric patterns invite explanations that lack supporting data or may be dismissed because there is hesitation among biologists to make comparisons lacking phylogenetic support. We argue that emergent allometric patterns have inherent value and continue to be the fodder for many fruitful hypotheses.