What is the relationship between body size and metabolic rate in animals?

The relationship between body size and metabolic rate in animals is characterized by an inverse relationship; as body size decreases, metabolic rate tends to increase when measured per unit of body mass. Smaller animals generally have higher metabolic rates because they lose heat more quickly relative to their size and thus need to consume more energy to maintain their body temperature and support their higher activity levels.

This concept is often illustrated by the scaling of metabolic rates relative to body mass, which is described by Kleiber's law. According to this law, metabolic rate per gram of body weight decreases as body size increases. Therefore, smaller animals, like mice or hummingbirds, have much higher metabolic rates compared to larger animals, such as elephants or whales, when measured on a per gram basis.

In contrast, many larger animals have a slower metabolic rate per unit of weight due to their lower surface area-to-volume ratio, making it easier for them to retain heat and requiring less energy per gram to maintain physiological functions.

The other options suggest incorrect interpretations of this relationship or introduce unrelated factors, such as age, which does not directly correlate with size and metabolic rate in the same way. Understanding this dynamic helps clarify the energy dynamics across different species in relation to their physical characteristics.