Abstract: Warm-blooded vertebrates accumulate approximately conserved numbers of physiological cycles over a natural lifetime: of order 109 heartbeats and 108–3 × 108 breaths. These regularities are not exact constants, but their persistence across orders-of-magnitude variation in body mass, metabolic power, physiological frequency, and lifespan suggests that biological time is not measured by chronological duration alone. We develop the Principle of Biological Time Equivalence