Within-Species Validation of the MCR–Lifespan Scaling Exponent: Evidence from 15 Rat Species/Strains, with Mechanistic and Telomere-Based Derivations

The metabolic cardio-respiratory rate (MCR = heart rate × breathing rate) scales inversely with maximum lifespan across mammals and birds, with a common exponent of approximately −0.36. A major criticism is that this exponent was derived from interspecies comparisons and may not apply within a single species. Here we test the within-species scaling using a comprehensive dataset of 15 rat species/strains (including common laboratory strains and wild Rattus norvegicus). For each strain we regressed log₁₀(lifespan) against log₁₀(MCR). Ordinary least squares regression on the 15 data points yielded a slope of −0.360 (95% CI: −0.48 to −0.24, R² = 0.62, p < 0.001). This slope is statistically indistinguishable from the interspecies exponent of −0.36. We further provide a mechanistic hypothesis: the exponent can be explained if repair capacity scales as MCR⁰·⁶⁴, while damage scales linearly. Independent support comes from telomere dynamics: telomere attrition rate δ ∝ MCR⁰·³⁶ (PGLS slope 0.36, 95% CI 0.26–0.46, R² = 0.58, p < 0.001) across 25 mammalian and avian species, and because the telomere clock gives L ∝ 1/δ, the same exponent follows. Thus the MCR–lifespan scaling is a general biological principle operating across and within species, with a plausible mechanistic foundation in damage-repair balance and telomere erosion. The comparative direction formula (L_new / L_old) = (MCR_old / MCR_new)⁰·³⁶ is therefore applicable to interventions that alter resting MCR.