The measurement of metabolic rate (MR) provides scientists with an understanding of how an animal expends energy over a unit of time. This measurement represents the total energy cost of all chemical reactions necessary to sustain life, ranging from cellular maintenance to physical activity. Understanding an animal’s metabolic rate offers direct insights into energy budgets and resource requirements in biology and ecology. Determining this rate allows researchers to estimate the minimum food an animal needs to survive and the energy it can allocate toward reproduction or migration. Measuring MR helps establish the physiological limits that govern an animal’s survival in its specific environment.
Quantifying Rate Through Gas Exchange
The most common laboratory approach for determining metabolic rate involves indirect calorimetry. This technique operates on the principle that energy expenditure is directly proportional to the consumption of oxygen (\(\text{O}_2\)) and the production of carbon dioxide (\(\text{CO}_2\)). Indirect calorimetry is used because direct calorimetry, which measures heat output, is rarely used due to its complexity. By quantifying the rates of gas exchange, researchers can calculate the amount of biological fuel being burned by the animal.
Scientists use the respiratory quotient (RQ), which is the ratio of carbon dioxide produced to oxygen consumed (\(\text{VCO}_2 / \text{VO}_2\)), to estimate which macronutrients are fueling the metabolism. For instance, the oxidation of pure carbohydrates yields an RQ near 1.0, while the metabolism of pure fats results in an RQ closer to 0.7. A typical mixed diet produces an RQ value of approximately 0.8, allowing conversion of gas exchange volumes into energy units, such as joules or calories.
Indirect calorimetry is primarily performed using two main experimental setups: open-circuit and closed-circuit respirometry. Open-circuit respirometry involves a steady flow of air through a chamber or mask. Sensitive analyzers continuously monitor the difference in \(\text{O}_2\) and \(\text{CO}_2\) concentrations between the inflowing and outflowing air. This setup is versatile and is widely used for studies ranging from resting measurements to high-intensity exercise tests.
Closed-circuit respirometry seals the animal within a container where the air is recirculated. Oxygen is consumed from the sealed system, and the \(\text{CO}_2\) produced is chemically absorbed. This allows the researcher to measure the decrease in total gas volume. Closed-circuit systems offer high precision for controlled, short-duration measurements, typically favored in small-scale laboratory experiments.
Measuring Long-Term Field Expenditure
When measuring the energy expenditure of a free-ranging animal, researchers employ the Doubly Labeled Water (DLW) technique. This method is the standard for determining the Field Metabolic Rate (FMR) over extended periods without restricting movement. The DLW technique involves injecting the animal with water enriched with two non-radioactive isotopes: Deuterium (\(\text{^2H}\)) for hydrogen and Oxygen-18 (\(\text{^{18}O}\)) for oxygen.
After injection, the isotopic water quickly equilibrates throughout the animal’s total body water pool. The two isotopes are then eliminated from the body at different rates. Deuterium is lost exclusively as water (\(\text{H}_2\text{O}\)), primarily through urine, evaporation, and sweat. Conversely, Oxygen-18 is lost as water and through the production of carbon dioxide (\(\text{CO}_2\)), which exchanges with the body water pool.
By periodically collecting a body fluid sample, such as urine or saliva, scientists measure the elimination rate of each isotope using mass spectrometry over a period of days or weeks. The difference between the faster elimination rate of \(\text{^{18}O}\) and the slower rate of \(\text{^2H}\) is directly proportional to the total \(\text{CO}_2\) produced by the animal. This total \(\text{CO}_2\) production is then converted into the average daily energy expenditure.
The DLW method provides an accurate measure of energy expenditure in a natural setting, capturing the costs of foraging, social interaction, and thermoregulation. However, the technique has practical limitations, including the high cost of the \(\text{^{18}O}\) isotope and the necessity of capturing and recapturing the animal to administer the dose and collect the final samples.
Standardizing and Interpreting Metabolic Rates
The technique used to gather raw gas exchange data is only one part of measuring metabolic rate. The physiological and environmental conditions under which the animal is measured are important for classifying the resulting value. Researchers must standardize these conditions to ensure that metabolic rates are comparable between different individuals and species.
For endotherms, such as mammals and birds, the lowest energy expenditure is defined as the Basal Metabolic Rate (BMR). To qualify as BMR, the animal must be resting, in a post-absorptive state (meaning digestion is complete), and measured within its thermal neutral zone (TNZ). The TNZ is the range of ambient temperatures where the animal does not need to expend extra energy to maintain a stable body temperature.
A closely related but less stringent measurement is the Resting Metabolic Rate (RMR). RMR is measured when the animal is resting and fasting, but not necessarily within its TNZ or post-absorptive. Because RMR conditions are less strictly controlled, the RMR value is higher than the BMR, representing a more practical measure of a non-active state in many experimental settings.
For ectotherms, like reptiles, amphibians, and fish, whose body temperature changes with the environment, the baseline energy use is termed the Standard Metabolic Rate (SMR). SMR is measured while the animal is resting and fasting. It must be reported at a specific, standardized ambient temperature because its metabolism fluctuates with temperature.