Question

There are several hypotheses to explain the relationship between body size and metabolic rate.  Explain two of them:

Answer 1

Metabolic rate refers to the rate at which the body of an organism converts food into energy. There are several factors affecting the metabolic rate, one of which is body size. There are several hypothesis explaining the relation between the two. Two of them are:

1. One hypothesis says that smaller the size of an organism, greater the metabolic rate. This is because smaller organisms have greater surface area to volume ratio and have greater heat loss than larger animals who have lower surface area to volume ratio. This however is true for endotherms who try to maintain a constant body temperature. Hence greater heat loss means greater energy requirement to maintain body temperature and hence greater metabolic rate.

2. Another hypothesis says greater the size of the organism greater its weight and complexity and hence greater is its energy requirement. Larger animals have more complex structure and physiology. For eg, due to larger size and weight the food requirement of an elephant is greater than a mouse and hence greater energy is required by the elephant to digest its food than the mouse. Similarly all physiological processes although similar in both require more energy in case of the larger animal compared to the smaller one. Thus according to this hypothesis greater the size greater is the metabolic rate.