[Physics] Why do small animals appear to move faster than larger ones

Question

I am keen to understand why smaller creatures move relatively faster than larger ones. Not only do they move faster, but their metabolism runs at a faster rate, they seem to process information faster (try swatting a fly!) – their entire lives seem to burn more rapidly living shorter lifespans. I’ve read about Kleibers scaling law, but this explanation alone does not seem to offer a fully satisfactory answer.

Answer 1

OK, I think that I can interpret this question in this way

"Why do smaller animals seem more active than larger animals (like a rat seeming more active than a dog)?"

Yes, the point that you have noted is quite correct, their metabolic rates are quite fast- (I disagree to the point that this is a question for Biology SE). Assume that you have a small mouse and a larger animal like an elephant. Now the ratio of Surface Area to the body mass of the mouse will be much larger than that of the elephant, which is fairly obvious

Mouse-$0.875 cm^2g^{-1}$

Elephant (Asian)-$0.0425 cm^2g^{-1}$

(These are estimates I made from data off a few papers, the original values may deviate, but not appreciably)

Naturally, a larger surface area leads to greater loss of heat to the surrounding. $$\frac{\Delta Q}{\Delta T} \propto A(T_b-T_s)$$ You can see from the above equation, that the heat lost depends on the difference in temperature (known as Newton's law of cooling in physics), and on the surface area of the organism. We take the ratio rather than the total surface area here because a small change in heat will produce a larger change in the body temperature of smaller organisms.

So, in order to maintain their body temperature constant, there is a need for active and constant metabolism. And constant metabolism naturally implies that they must be more active (which means that that they seem to move relatively faster) and need a continuous supply of food to compensate for this process.

But this applies mainly for regulating organisms (i.e. organisms that maintain a constant internal temperature) such as mammals and small birds. You might find a small mammal in the tropics (where $T_b-Ts$ is less) but not in Antarctica, (where $T_b-Ts$ is large).

So, I feel that the point about swatting a fly (or processing information faster, for that matter) is purely an evolutionary incident as opposed to metabolic rates. Lower organisms have a reflexive body i.e. they rely more on reflex signals and the apparent inability to hit a fly is a reflex action- in any case, it is too much to expect information processing in a primitive brain.