Adding fuel to the "fire of life": energy budgets across levels of variation in ectotherms and endotherms

Energy metabolism - the "fire of life" - is the sum of the processes by which animals acquire energy, channel energy into useful functions, and dissipate energy from their bodies. Acquisition and allocation processes deal with energy mainly as a quantity, but energy is absorbed or expended over a period of time; therefore, the most relevant way to conceptualize energy budgets is with the use of rate functions. A large number of recent studies have examined co-variation in metabolic and behavioral traits among individuals. A general finding stemming from this body of work is that there is no single cause-and-effect mechanism driving these relationships, but that the direction of the effects is dynamic and shifts in different contexts and environments. However, the remarkable energetic differences between endotherms and ectotherms are an unexploited basis for achieving a further understanding of such relationships.