Chapter 8: Aerobic and Anaerobic Forms of Metabolism

The authors introduce direct and indirect calorimetry as the two principal methods of measurement. Direct calorimetry assesses heat output from an animal, while indirect calorimetry estimates metabolic rate from oxygen consumption or carbon dioxide production. The respiratory quotient (RQ) is discussed in detail, providing insight into which substrates (carbohydrates, lipids, or proteins) are being metabolized. The chapter explains the significance of standard metabolic rate (SMR) in ectotherms and basal metabolic rate (BMR) in endotherms, with attention to the strict conditions under which these must be measured—resting, fasting, and thermoneutral. Field metabolic rate (FMR) is introduced to reflect energy expenditure in natural settings. Real-world applications include the doubly labeled water method and the use of telemetry and respirometry to study free-living animals. The authors then examine how body size and temperature influence metabolic rate. Metabolic scaling is described using the allometric equation, showing that metabolic rate increases with body mass but less than proportionally. The chapter also introduces the concept of mass-specific metabolic rate and the temperature coefficient Q₁₀ to express how sensitive metabolic rate is to temperature change. Finally, the authors address how activity level, reproductive state, and environmental stressors can dynamically alter energy demands. This chapter establishes a crucial quantitative framework for interpreting energy budgets and ecological performance across animal taxa.