Of mice and elephants: food for thought about metabolic rates
A nice point about metabolic rates is that elephants have about 1% of the metabolic rate of mice, per gram of tissue.
This is because of the principles of scaling in thermodynamics (https://en.wikipedia.org/wiki/Metabolic_theory_of_ecology).
So, imagine a gram of tissue (averaged among organs) of elephants, and a gram of tissue (similarly derived) of mice. And imagine the rate at which these entities consume sugar, in the process of respiration.
(In respiration, cells convert sugar and oxygen to carbon dioxide and water, getting the energy of life out of the reactions.)
These rates of consumption differ 100-fold (and generate heat and carbon dioxide correspondingly).
Now, assume that an individual elephant weighs about 4000 grams and an individual mouse weighs about 100 grams. If so, the body masses differ by a factor of 40 thousand.
What this would mean is that body mass differs 40,000-fold, but metabolic rate differs only 100-fold.
This, in turn, would mean that metabolic rate 'lags' behind body mass, and this is a 400-fold 'lag'.
If the elephant were 3,000,000 g and the mouse only 50 g, then this 'lag' factor would be 60,000-fold.
So, it seems likely that elephants weigh about fifty thousand times more than mice, yet use energy about one hundred times more slowly, per unit of body mass.
Comments
Please note that the comparison of elephants and mice depends on the fact that both are 'endothermic' (https://en.wikipedia.org/wiki/Endotherm).
The difference between endothermic and ectothermic (https://en.wikipedia.org/wiki/Ectotherm) vertebrates is illustrated by the following example.
Rattus norvegicus (https://en.wikipedia.org/wiki/Brown_rat) weighs about the same as the lizard Tiliqua rugosa (https://en.wikipedia.org/wiki/Tiliqua_rugosa).
However, the metabolic rate of the former is about 7-fold that of the latter.
This means that the former needs to eat about7-fold as much food as the latter, per day.
Add a Comment