Allometry: Metabolic Rate and Body Mass Scaling

Allometry is the study of how biological traits scale with body size across different species. This question focuses on the relationship between metabolic rate (the rate of energy consumption) and body mass.

Metabolic Rate Scaling Principle

Metabolic rate increases with body mass, but not proportionally. The relationship is generally described by an allometric equation:

$ \text{Metabolic Rate} \propto \text{Body Mass}^{\textit{b}} $

Where $\textit{b}$ is an exponent typically less than 1 (often around 3/4, known as Kleiber's Law). This means that while larger animals have a higher *total* metabolic rate, their metabolic rate *per unit of body mass* is lower.

Mass-Specific Metabolic Rate Explained

The mass-specific metabolic rate is the energy expenditure divided by the organism's mass.

$ \text{Mass-Specific Metabolic Rate} = \frac{\text{Metabolic Rate}}{\text{Body Mass}} $

Because the metabolic rate scales with an exponent less than 1 relative to body mass, smaller animals (like mice) have a higher mass-specific metabolic rate compared to larger animals (like humans or elephants).

Option Analysis

We need to determine which statement accurately reflects this scaling principle regarding energy expenditure per gram per hour (mass-specific rate).

• Mice vs. Humans: Mice are much smaller than humans. Therefore, mice have a higher mass-specific metabolic rate.
• This directly supports the statement: "Mice expend more energy per gram per hour than humans do."

Conclusion: Option 2 correctly applies the principle of allometric scaling of metabolic rate with body mass.