Question

The field of allometry assesses how species traits scale with each other. Given the relationship between metabolic rate and body mass, which one of the following statements is true?

• A. Elephants expend more energy per gram per hour than mice do
• B. Mice expend more energy per gram per hour than humans do
• C. Elephants expend less total energy per day than mice do
• D. Humans and elephants expend similar amounts of total energy per day

Hint:

Kleiber's law gives total metabolic rate as roughly \(M^{0.75}\); dividing by mass shows mass-specific (per gram) metabolic rate falls with size, so smaller animals burn more per gram than larger ones.

Answer 1

The Correct Option is B

This question is about the well known scaling rule linking body mass and metabolic rate, sometimes called Kleiber's law, and how it plays out when we compare mice, humans and elephants.

1. Elephants expend more energy per gram per hour than mice do: false. Larger bodies have proportionally lower surface area to volume, so they lose heat more slowly and can afford a slower per gram metabolism; the per gram rate actually drops as size goes up, so an elephant's rate per gram is lower than a mouse's, not higher.
2. Mice expend more energy per gram per hour than humans do: true. Mice are far smaller than humans, and small mammals need a very high per gram metabolic rate just to keep their body temperature up, since a small body loses heat quickly relative to its volume. So gram for gram, a mouse burns much more energy per hour than a human does.
3. Elephants expend less total energy per day than mice do: false. This mixes up total energy use with the per gram rate. Even though an elephant's cells individually work at a lower rate, there are enormously more of them, so the whole animal's daily energy use is far larger than a mouse's total daily use, not smaller.
4. Humans and elephants expend similar amounts of total energy per day: false. Since total metabolic rate rises with mass raised to about the $0.75$ power, and an elephant can outweigh a human by close to a hundredfold, the elephant's total daily energy budget works out much larger than a human's, not similar.

The key idea to hold onto is that total energy use grows with body size, while energy use per gram of tissue actually shrinks with body size. That is why small mice run hot on a per gram basis compared to both humans and elephants.

Let's summarize:

• Total metabolic rate increases with mass, roughly as $M^{0.75}$.
• Mass-specific (per gram) metabolic rate decreases with mass, so smaller animals like mice burn more per gram per hour than larger ones like humans or elephants.

So the true statement is that mice expend more energy per gram per hour than humans do.