| Roughly, surface area, A, of an animal scales as its length, L, squared |
| A = L2 |
| and the mass, M, scales as L cubed |
| M = L3 |
| Heat is dissipated across the surface, and metabolic rate is proportional to heat dissipation, so |
| metabolic rate ≈ L2 = M2/3 |
| and |
| (metabolic rate)/(unit mass) ≈ M-1/3 |
| Seems reasonable. Pulse rate is related to metabolic rate per unit mass, and we know smaller animals have faster heartbeats. |
| However, extensive and careful measurements by Kleiber showed that metabolic rate per unit mass scales as M-1/4, instead of M-1/3. Why should this be? |
| More heat is lost through the lungs than across the skin surface, so the fractal structure of the lungs may account for this difference. |
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