Kleiber's law: metabolic rate scales as M^(3/4) across 27 orders of magnitude of body mass
West-Brown-Enquist (1997) fractal network theory: hierarchical branching vascular networks optimize transport under space-filling constraints, deriving α=3/4. Remarkably, heart rate ∝ M^(-1/4) and lifespan ∝ M^(1/4), so lifetime heartbeats ≈ constant ~1.5 billion.