The relationship between metabolic rate and lifespan has been debated for over a century. Existing studies rely almost exclusively on indirect calorimetry rather than measuring heat dissipation directly. Here we systematically compile all available direct and microcalorimetry measurements of resting heat production across animal taxa (n = 25 species) and compare them with maximum lifespan records from the AnAge database. We observe a consistent negative association across five taxonomic groups (Spearman ρ = –0.593, p = 0.003; OLS slope on log-log axes = –0.90 ± 0.32): species with higher mass-specific resting heat production tend to have shorter maximum lifespans. This pattern spans approximately four orders of magnitude in both variables. We propose that resting heat dissipation — the fraction of metabolic energy released as thermal waste rather than used for biological work — may be a more mechanistically direct proxy for aging rate than metabolic rate per se. This compilation represents the first systematic survey of this relationship using direct calorimetry data and highlights a critical gap: fewer than 25 species have been measured by direct calorimetry, compared to thousands by indirect methods.