The metabolic rate of an organism is the amount of energy it needs per unit time. It is often estimated by measuring the rate at which oxygen is consumed. The most important factor affecting the metabolic rate of an individual is its size (mass).

The study of the relationship of body size to shape, anatomy and physiology is called allometry. It can be defined broadly as ‘the study of size and its consequences.’ Metabolic rate varies with body mass. However, rates are not directly proportional to body mass. The metabolic rate per unit of body mass in very small organisms is immensely higher than the metabolic rate per unit body mass of large organisms. The relationship between resting metabolic rates and body mass can be expressed by the allometric relationship.

Metabolic rates = a (Body mass)^{b}

In this equation, a is a constant that is characteristic of the kind of organism, b referred to as the scaling exponent (called allometric constant) that relates metabolic rate to body mass. When b is 1, metabolic rate is directly proportional to body mass. If logarithms of both side of this equation are taken, it can be rewritten as:

log [metabolic rate] = log [a] + b (log [body mass])

The value of the exponent, b, relating metabolic rate to body mass, tend to lie close to 0.75, irrespective of the units used for the measurements either of metabolic rate or of body mass. Because b is less than 1.0, larger organisms need less energy per day, relative to their body mass, than do smaller organisms. This means that small organisms have a high metabolic rate relative to their body size. Remember, the metabolic rate per unit body mass, not the total metabolism of the individual, decreases with increasing mass or size. Thus, an adult human being requires more total food than a small child, but less food per kilogram of body mass. For many years, there is controversy on the value of the exponent, b – whether the value of b is closer to 0.66 or 0.75. Recent analyses of metabolic rates, body mass data have argued that there is no single exponent b that applies to a phylogenetic class. It has also been argued that there are different exponents for different phylogenetic groups.

#### FAQ

### FAQs on Metabolic rate and size of individuals

#### 1. Explain how is metabolic rate measured?

The metabolic rate of an organism is the amount of energy it needs per unit time. It is often estimated by measuring the rate at which oxygen is consumed.

#### 2. Define metabolic rate and the body size?

The metabolic rate per unit of body mass in very small organisms is immensely higher than the metabolic rate per unit body mass of large organisms.

#### 3. Metabolic rate and body size relationship

The metabolic rate per unit of body mass in very small organisms is immensely higher than the metabolic rate per unit body mass of large organisms.