Kleiber's Law (1932): Metabolic rate B = a·M
β with β ≈ 0.75 across 27 orders of magnitude of body mass.
B = a · M3/4 (watts, kg)
West-Brown-Enquist (1997): β = 3/4 emerges from fractal vascular networks minimizing delivery cost.
Specific metabolic rate B/M ∝ M
−1/4: larger animals have slower cell-level metabolism.
Heart rate ∝ M
−1/4, lifespan ∝ M
1/4 → heartbeats per lifetime ≈ constant ~1.5 billion.
Current: B = — W, specific = — W/kg