Table of Contents
Metabolic rate is a measure of how much energy an organism uses per unit time. It translates the abstract concept of “energy conversion in metabolism” into something we can quantify and compare between individuals, species, or physiological states.
In this chapter, the focus is on:
- What exactly is meant by “metabolic rate”
- How it is expressed and measured in practice
- Which factors influence metabolic rate
- How metabolic rate scales with body size
- Why different types of organisms show different patterns of metabolic rate
(Details of direct and indirect calorimetry, and definitions like basal metabolic rate, are handled in their own chapters.)
What Is Metabolic Rate?
Metabolic rate is the rate at which the body converts chemical energy from nutrients into other energy forms (such as heat, mechanical work, and chemical work) and into stored energy.
Formally, metabolic rate is the energy turnover per unit time:
$$
\text{Metabolic rate} = \frac{\text{Energy used}}{\text{Time}}
$$
Common units are:
- Joules per second (J/s), which is 1 watt (W)
- Kilojoules per day (kJ/day)
- Kilocalories per day (kcal/day)
Because body size has a huge impact, metabolic rate is often standardized:
- Per body mass: e.g. kJ·kg\(^{-1}\)·day\(^{-1}\)
- Per body surface area: e.g. kcal·m\(^{-2}\)·h\(^{-1}\)
Two main aspects are distinguished:
- Total metabolic rate (sometimes called field metabolic rate): actual energy expenditure of an organism under its normal daily conditions.
- Basal or standard metabolic rate: a defined reference state under strictly controlled conditions (treated in a separate chapter).
Components of Metabolic Rate
Total metabolic rate is not a single homogeneous process. It is composed of several major energetic “costs”:
- Maintenance metabolism
- Energy required to keep the organism alive at rest:
- Maintaining ion gradients (e.g. Na\(^+\)/K\(^+\)-ATPase)
- Turnover of proteins and other macromolecules
- Baseline activity of organs (heart, brain, liver, kidneys)
- This is present at all times and dominates basal metabolic rate.
- Thermoregulation (in thermoregulating organisms)
- In endotherms, additional energy is used to keep body temperature within a narrow range.
- Includes:
- Heat production (e.g. shivering, non-shivering thermogenesis)
- Heat conservation (posture, vasoconstriction) indirectly affecting expenditure.
- Activity metabolism
- Energy for movement and work:
- Locomotion (walking, flying, swimming)
- Feeding, territorial defense, courtship, escape from predators
- Can increase energy expenditure several-fold above basal levels.
- Specific dynamic action (SDA) or diet-induced thermogenesis
- Extra energy required to digest, absorb, and process food.
- Peaks after a meal and then declines.
- Growth and reproduction
- Growth requires synthesis of new tissue (high anabolic costs).
- Reproduction adds costs for:
- Gamete production
- Gestation or egg production
- Parental care
At any moment, total metabolic rate is the sum of these components; their relative importance varies with life stage, environment, and behavior.
Ways to Express Metabolic Rate
To compare metabolic rate across species or individuals, several normalized measures are used:
- Absolute metabolic rate
- Total power output of the whole organism.
- Example: a resting adult human may have ≈80–100 W.
- Mass-specific metabolic rate
- Metabolic rate per unit body mass:
$$
\text{Mass-specific MR} = \frac{\text{Metabolic rate}}{\text{Body mass}}
$$ - Useful for comparing animals of different sizes.
- Often declines with increasing body size: small animals use more energy per gram of tissue than large animals.
- Area-specific metabolic rate
- Metabolic rate per unit body surface area.
- Used particularly in human physiology because heat exchange with the environment depends strongly on surface area.
- Field metabolic rate (FMR)
- Average metabolic rate of free-living animals under natural conditions over a period (e.g. days).
- Includes all components: activity, thermoregulation, etc.
Factors Influencing Metabolic Rate
Metabolic rate is highly variable and responds to internal and external factors. Important influences include:
1. Body Size
- Across animals, larger species generally have:
- Higher absolute metabolic rate (they use more energy in total).
- Lower mass-specific metabolic rate (each gram of tissue uses less energy).
- This size effect is often described mathematically (see “Allometric Scaling” below).
2. Body Temperature
- Ectotherms (e.g. reptiles, many invertebrates):
- Metabolic rate changes strongly with environmental temperature.
- A common measure is the temperature coefficient Q\(_{10}\), which indicates how much a rate changes with a 10 °C increase.
- For many biochemical reactions, Q\(_{10}\) ≈ 2–3 (rate roughly doubles or triples).
- Endotherms (e.g. birds, mammals):
- Maintain near-constant internal temperature.
- Metabolic rate increases when environmental temperature:
- Drops below the lower critical temperature (to produce more heat).
- Rises above the upper critical temperature (to power cooling mechanisms).
3. Activity Level
- Physical activity is one of the most variable components of metabolic rate.
- Factors include:
- Type of locomotion (e.g. flying vs. walking)
- Speed and duration of movement
- Terrain or medium (air, water, land)
- Energetic cost of transport (energy per distance moved) can be calculated to compare efficiency of different modes of movement.
4. Nutritional State
- After feeding, metabolic rate increases due to digestion and assimilation (SDA).
- In fasting or starvation:
- Metabolic rate usually decreases.
- Energy is drawn from stored reserves (glycogen, fat, and eventually proteins).
5. Age and Developmental Stage
- Many organisms have higher mass-specific metabolic rates during:
- Growth phases (larvae, juveniles)
- Reproductive phases (especially in females during gestation or egg production)
- In many species, metabolic rate declines with advanced age.
6. Hormonal and Physiological Status
- Hormones such as thyroid hormones, catecholamines, and others can:
- Increase or decrease metabolic rate.
- In some situations (e.g. hibernation, torpor, estivation):
- Metabolic rate is dramatically reduced to conserve energy.
7. Environmental Conditions (Non-thermal)
- Oxygen availability:
- Low oxygen can limit aerobic metabolism, affecting metabolic rate and forcing shifts to less efficient pathways.
- Water availability and salinity:
- Osmoregulation itself consumes energy and can elevate metabolic costs.
- Stress (predators, pollutants):
- Can increase metabolic rate through stress hormone responses.
Metabolic Rate and Allometric Scaling
Metabolic rate does not increase in direct proportion to body mass. Instead, it generally follows an allometric relationship.
A common empirical relationship for many groups of animals is:
$$
\text{Metabolic rate} = a \cdot M^{b}
$$
where:
- $M$ = body mass
- $a$ = scaling constant (depends on taxon and conditions)
- $b$ = scaling exponent
Typical values:
- For many endothermic animals, $b$ is often near 0.75.
- For ectotherms, exponents can vary but are usually between 0.6 and 0.9.
Consequences:
- Absolute metabolic rate increases with size, but less than proportionally.
- Mass-specific metabolic rate decreases with size, because:
$$
\frac{\text{Metabolic rate}}{M} = a \cdot M^{b-1}
$$
With $b < 1$, the exponent $b - 1$ is negative.
This explains observations such as:
- A mouse uses much more energy per gram of body mass than an elephant.
- Small animals must eat relatively more, breathe faster, and have higher heart rates to meet their energetic needs.
Metabolic Rate and Lifestyle
Different ecological and physiological strategies are reflected in characteristic patterns of metabolic rate:
- Endotherms vs. ectotherms
- Endotherms typically have higher basal/standard metabolic rates than ectotherms of the same size.
- This supports active lifestyles and stable internal temperatures, at the cost of higher food requirements.
- Active vs. sedentary species
- Species with high daily activity and high levels of sustained exercise (e.g. many birds, predators) tend to have higher metabolic rates than more sedentary species of similar body mass.
- Seasonal strategies
- Some animals reduce metabolic rate seasonally:
- Hibernation (winter dormancy in cold climates)
- Daily torpor (short-term reduction in small mammals and birds)
- Others increase metabolic rate seasonally during:
- Migration
- Breeding periods
Measuring and Interpreting Metabolic Rate
Metabolic rate is not directly “seen” but is inferred from measurable quantities:
- Heat production (direct calorimetry)
- Gas exchange, particularly O\(_2\) consumption and CO\(_2\) production (indirect calorimetry, respirometry)
In practice, once one has the rate of oxygen consumption $\dot{V}_{\mathrm{O_2}}$ and knows the energetic equivalent (depending on the substrate mixture), metabolic rate can be estimated:
$$
\text{Metabolic rate} \approx \dot{V}_{\mathrm{O_2}} \times \text{Energy per liter O}_2
$$
The exact methods and their limitations are covered in other chapters, but the key point here is that metabolic rate is a derived quantity and must always be interpreted together with:
- The physiological state (resting vs. exercising vs. sleeping)
- Environmental conditions (temperature, availability of food)
- Time scale of measurement (minutes, hours, days)
Biological Significance of Metabolic Rate
Understanding metabolic rate is essential because it links:
- Cellular biochemistry (ATP production, metabolic pathways)
to - Whole-organism physiology and ecology (feeding behavior, growth, reproduction, survival)
Some important implications:
- Energy budgets: Organisms must balance energy intake and expenditure. Persistent imbalance affects growth, reproduction, and survival.
- Life-history strategies:
- Species with high metabolic rates often have faster growth, earlier reproduction, and shorter lifespans (“fast” life histories).
- Species with lower metabolic rates tend to grow and reproduce more slowly but may live longer (“slow” life histories).
- Environmental adaptation: Differences in metabolic rate reflect adaptation to habitats (e.g. cold vs. warm environments, food-rich vs. food-poor ecosystems).
In summary, metabolic rate is a central quantitative descriptor of how living systems convert and use energy over time. It integrates molecular processes within cells with the behavior, ecology, and evolution of whole organisms.