Rate of living theory

The rate of living theory proposes that an organism's lifespan is inversely proportional to its mass-specific metabolic rate — the faster energy is consumed, the sooner the organism dies. Max Rubner articulated the idea in 1908 by comparing five domestic mammals (guinea pig, cat, dog, cow, and horse) and showing that lifetime energy expenditure per unit body mass is roughly constant across species; Raymond Pearl extended it in his 1928 book The Rate of Living, coining the term and demonstrating in Drosophila that lower temperatures, which slow metabolic rate, prolong survival. The theory gained mechanistic plausibility when Denham Harman proposed the free radical theory of aging in 1956, linking mitochondrial respiration to cumulative oxidative damage. Complicating evidence followed: birds and bats live substantially longer than non-flying mammals of similar body size despite comparable or higher metabolic rates, as reviewed by Munshi-South and Wilkinson (2010). Hulbert et al. (2007) attributed this partly to stronger mitochondrial antioxidant defenses and lower membrane peroxidizability in longer-lived taxa. Speakman (2005) showed that after controlling for body size, residual daily energy expenditure correlates negatively with maximum lifespan in mammals but not in birds, concluding that interspecific comparisons are confounded by differences in oxidative defense and repair capacity. The simple rate-of-living formulation has been superseded by more mechanistically specific theories, though it remains historically foundational.