Figure 1. Direct calorimeter used by Lavoisier for the measurement of heat loss from a guinea-pig. The guinea-pig's body heat causes ice surrounding its chamber to melt, the rate of melting indicating the amount of heat generated by the animal's metabolism. The same chamber can be used as an indirect calorimeter by measuring the flow rate of air into and out of the chamber, and the change in concentration of oxygen. The difference in quantity of oxygen going into the chamber versus exiting the chamber per unit time indicates the amount consumed by the animal in aerobic metabolism. (Reproduced from Kleiber M (1975) The Fire of Life: An Introduction to Animal Energetics, 2nd edn. Huntington, NY: Krieger Publishing).

Figure 2. Schematic representation of the primary components of intracellular energy metabolism (bottom) and the oxygen and substrate pathways that supply them (top). Hydrolysis of ATP sets a demand for energy to rephosphorylate ADP. Energy is primarily supplied by oxidative phosphorylation in the mitochondria, using energy released from the reactions of the Krebs cycle (KC) inside the mitochondria. The Krebs cycle uses substrate derived from beta-oxidation (OX) of fats (yellow circles) in the mitochondria or breakdown of carbohydrates (CHO, green diamonds) by glycolysis (GS) in the cytosol. Fats and carbohydrate (glycogen) may originate from intracellular (ic) deposits, or may be transported from other cells or organs. They are transported to the cells in arterial (a) blood along with  O₂ (grey circles) taken up in the lungs or gills, and waste products are removed in the venous (v) circulation. (Reproduced from Taylor CR et al. (1996) Design of the oxygen and substrate pathways. I. Model and strategy to test symmorphosis in a network structure. Journal of Experimental Biology 199: 1643–1649).

Figure 3. Total metabolic power as a function of running speed for a racehorse. At any speed, total metabolic power (solid diamonds) is the sum of aerobic power (triangles) and net anaerobic power (circles). At low speeds, nearly all metabolic power is aerobic; at the highest speeds, aerobic power is maximized, and a large amount of energy is derived from anaerobic glycolysis with the build-up of the acid lactate as a waste product. As a result, high speeds can be sustained for shorter periods than lower speeds.

Figure 4. Metabolic rate of a poikilothermic vertebrate as a function of body temperature for different values of Q₁₀. Metabolic rate changes 2- or 3-fold for every 10°C change in temperature from the initial value (10°C) when Q₁₀ is 2 (red line) or 3 (blue line), respectively.

Figure 5. Specific resting metabolic rates (W kg^–1) as a function of body mass for birds and mammals ranging from a 5 g shrew to a 3500 kg elephant. Body mass is shown on a logarithmic scale; metabolic rate is shown on linear (top) and logarithmic (bottom) scales. The shrew's specific resting metabolism is over 10 times higher than those of the largest mammals, and is nearly half as high as the maximum specific metabolic rate of the racehorse (Figure 3). (Modified after Schmidt-Nielsen K (1984) Scaling: Why Is Animal Size So Important? Cambridge: Cambridge University Press.)