Expert: John Locke - 8/3/2008

Question
I was just wondering why mice have such a high metabolism? I have a biology test tomorrow and I think it may have something to do with because mice are so small they dont have much surface area for exhanging materials so they need to be efficient? I'd just like to be 100% sure.
Thanks =)

Answer
This answer may have reached you after your exam, but I hope that you'd be interested in the result nonetheless. For mammals, there exists what's known as a power-law relationship between body mass and metabolism; that is, for a mammal of mass M, its basal metabolic rate is

Rate= C*((M)^(3/4))

where C is a constant value. This relationship is known specifically as Kleiber's Law. From this law, it's apparent that basal metabolic rate (BMR) actually increases with mass: larger animals will have higher metabolic rates. However, the law also tells us that large animals will expend less energy per unit mass than small animals; for example, a dog may be 10 times the mass of a cat, but it will have a BMR only 5.6 times as large, according to Kleiber's Law. This means that the dog will use less energy per unit of body mass than the cat, and the reasoning behind this has everything to do with the physical architecture of smaller animals.

Smaller mammals have a higher percentage of their body mass composed of structural materials (bone, muscle, internal organs, sensory organs, etc.) as opposed to reserve materials (fat, glycogen, and other substances that are not immediately biologically active). Structural materials have higher energy costs associated with them--that is, the body requires more energy to maintain a large amount of muscle than it does a large amount of fat. Larger animals have a higher percentage of their bodies composed of reserve materials, and thus can expend less energy per unit mass. So your answer is essentially correct: because mice are small and have little room for reserve materials, they have to be efficient. They have high metabolisms-per-unit-mass as a result.

Good luck, and I hope that this helped!