Ingestion rate is the largest term in the balanced growth equation and sets an upper limit to all other variables. Allometric relations describing the components of the energy budget can seem reasonable and coherent only if ingestion is greater than any other single rate and equal to the sum of all other rates. Such checks on coherence are particularly important, since all allometric relations have a large residual variation and always contain unevaluated sources of error.

Ingestion rate is also the basis for any calculation of the efficiency with which an animal converts food to new tissue or metabolic power. Such efficiencies, calculated as the ratios of the energy used in growth or respiration to that eaten, normalize differences in absolute rates among animals of different size or metabolic grouping. In this and in subsequent chapters, energetic efficiencies highlight important differences and similarities within the animal kingdom. For example, such calculations will show if homeothermy has resulted in more efficient use of food for growth or if this strategy has simply increased heat loss through respiration.

Ingestion is far more than a tool for ecological bookkeepers. It also represents predation, certainly the most apparent and probably the most significant interaction between an animal and its community. As a measure of a predator's demands, ingestion rate estimates the impact of a given animal on its ecosystem.