Constant Lifetime Heartbeat Number in Mammals: The Membrane Lipid Peroxidation Fatigue Limit of the Heart

Background: Among non-primate eutherian mammals, the lifetime number of heartbeats is approximately constant at around one billion, despite wide variation in body size, heart rate, and lifespan. Primates, including humans and the small gray mouse lemur, generate about three times this number. This regularity and its primate-specific deviation lack a clear physical explanation. Hypothesis: We propose that lifetime heartbeat number reflects a material fatigue limit set by membrane lipid peroxidation. Each cardiac cycle causes oxidative membrane damage; the heart fails when cumulative damage exceeds a fixed tolerance threshold. The total number of cycles tolerated is therefore determined by the peroxidation resistance of membrane lipids. Evidence: Four lines of evidence support this model. First, membrane peroxidation index correlates negatively with maximum lifespan across species, with lifespan doubling associated with a 19–24% lower peroxidation susceptibility. Second, all primates studied share an elevated lifetime heartbeat number of about three billion, indicating a primate-specific trait. Third, primate membranes have low peroxidation indices and low in vivo lipid peroxidation rates. Fourth, antioxidant interventions that do not alter membrane composition do not extend lifespan. Conclusion: The constant lifetime heartbeat number is the physical expression of the heart’s material fatigue limit under membrane lipid peroxidation. The primate deviation is explained by uniquely peroxidation-resistant membranes. The hypothesis generates testable predictions.