Why does every mammal get 1 billion heartbeats in their life?

Most mammals live for roughly one billion heartbeats because their metabolic rate (energy burn) and lifespan are inversely linked and scale in a balanced way. As animals get larger, their metabolism slows down per unit of mass, but their lifespan extends proportionally to compensate, keeping the total number of lifetime heartbeats nearly constant across species.

• Metabolic scaling — Metabolic rates follow a three-quarters power law, meaning as an animal increases in mass, it burns energy more efficiently than a linear increase would predict [08:52].
• Thermal regulation — Large animals cannot scale energy use linearly with mass because their body volume would generate more heat than their surface area could shed, leading to overheating [05:19].
• WBE Theory — Biological transport networks, like circulatory systems, use self-similar fractal branching to distribute nutrients to all cells with minimal energy loss [16:02].
  • Fractal efficiency — This geometry effectively increases the surface area for resource exchange, allowing large organisms to sustain life with lower metabolic rates per gram [17:27].
• Inverse relationship — Lifespan acts as the inverse of metabolic rate; as an organism burns energy slower—characterized by a slower heart rate—its life expectancy increases [21:16].
• Heartbeat constant — Because heart rate decreases and lifespan increases at inverse rates as size increases, the mathematical product remains near one billion for most mammals [22:23].
• Human outliers — Humans far exceed this biological average, reaching nearly three billion heartbeats, primarily because sanitation and medicine have artificially extended our survival beyond natural limits [24:03].
• City infrastructure — Scaling rules apply to non-biological systems, where urban utilities like roads and electrical grids require less energy per person as a population increases [27:15].
  • Urban growth — Unlike infrastructure, socioeconomic outputs like wealth and crime scale superlinearly, meaning these factors grow faster than the population size [27:54].

How does the "surface law" differ from Kleiber's Law in predicting metabolic rates? Why does the amount of infrastructure required per person decrease as cities grow?