Metabolism of Mice and Men

In the 1930’s, Swiss-American animal metabolism pioneer Max Kleiber noticed that the metabolic rate of animals scales as the body mass to the three quarters power.  There is still some controversy over whether the exponent is really three quarters or something else.  Many theories have been proposed for why the exponent  should be three quarters (or two thirds) but I won’t go into that here.  The crucial thing is that it is less than one and that implies that a large animal is more efficient than a small one.  This efficiency with size is not restricted to biological examples.  As Steve Strogatz pointed out in a New York Times column last year, the number of gas stations doesn’t grow linearly with the population of a city but rather grows in proportion to the 0.77 power of the population.  This sublinear scaling also goes for other city infrastructure like the total length of roads and electrical cables. Large cities may in fact be more efficient than small ones.

Now a mouse weighs about 20 to 30 grams so it is about a factor of 3500 times less massive than an average human.  Metabolic rate scales as mass to the three quarters so power density ( e.g. Watts/gram) scales as mass to the minus one quarter.  Hence, a mouse is 3500^{(1/4)} or 7 to 8 times less metabolically efficient than a human. A colony of mice weighing as much as a human would have to eat 7 to 8 times as much food.

However, in terms of total energy utilized, first world humans are much less efficient than mice and perhaps all other organisms.  The metabolic rate of an average person is about 10 megajoules per day or 115 watts but according to Wikipedia, the United States uses about 10,000 watts of power per capita.  This is a factor of 90 over the metabolic rate implying that an average American is a factor of ten less efficient than a mouse.  However, a very low energy use nation like Bangladesh only consumes about twice as much energy per capita as the human metabolic rate and thus an average Bangladeshi is more efficient than a mouse.

6 thoughts on “Metabolism of Mice and Men

  1. I really like this blog, Carson. Nice comparison.

    It would be interesting to look cumulatively at the energy expenditure: the whole human biomass (in terms of total energy utilized) is more or less efficient than the whole murine biomass?

    It’s a slippery question: in other terms, do mice or men contribute more to global warming?


  2. The total world use of power is 1.5\times 10^13 Watts according to Wikipedia. That amounts to 2500 W per person for a population of 6 billion or a factor of twenty more than the metabolic rate. So humans are about twice as less efficient as mice. So if the human biomass is less than half the mice biomass then humans contribute less.


  3. Hi Carson,

    There are other numbers that could be taken into account here. If I remember well, there’s a number called “ecological efficiency” or something like this that quantifies the total biomass of animals according to their position in the food chain. The biomass of the population of the predator is about 10-15% of the biomass of the population of the prey. This seems to be a very well conserved quantity.

    PS: yes, ecological efficiency, that’s it


  4. I am not so sure about this. We are omnivorous so wa have to take into account the biomass of primary products such as fruits and cereals, which is huge.
    However, we may be seen as specific given the fact we cultivate food. Se we should be able to take more profit of the available biomass (since we create it on purpose).
    However again, your recent studies show that we waste a great deal of our food. Thus we may also respond to this 10-15% ratio.


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