Rate of living theory

The rate of living theory proposes that an animal's lifespan is inversely tied to its mass-specific metabolic rate. In short: the faster it burns energy, the sooner it dies. Max Rubner laid out the idea in 1908. He compared five domestic mammals (guinea pig, cat, dog, cow, and horse) and showed that lifetime energy use per unit of body mass is roughly constant across them. Raymond Pearl extended it in his 1928 book The Rate of Living, coining the term. He showed in fruit flies (Drosophila) that cooler temperatures, which slow metabolism, lengthen survival. The idea gained a mechanism when Denham Harman proposed the free radical theory of aging in 1956, linking respiration to building oxidative damage. Then came complications. Birds and bats live far longer than non-flying mammals of similar size, despite equal or higher metabolic rates, as reviewed by Munshi-South and Wilkinson (2010). Hulbert et al. (2007) tied this partly to stronger mitochondrial antioxidant defenses and membranes that resist oxidation in longer-lived animals. Speakman (2005) showed that, once you adjust for body size, leftover daily energy use correlates negatively with maximum lifespan in mammals but not in birds. He concluded that comparisons between species are confounded by differences in oxidative defense and repair. The simple version has been replaced by more specific theories, though it remains historically foundational.

Last reviewed: June 22, 2026

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