29. How do within-species lifespan interventions differ from cross-species longevity differences?

Within a species, lifespan extension often exploits regulatory plasticity: slowing damage production, improving maintenance, shifting allocation, or increasing reserve. These interventions act on an organismal design that is already locally optimized and therefore often carry tradeoffs. Across species, longevity reflects evolutionary co-optimization of many traits at once; therefore, correlates of lifespan across species need not match mechanisms that extend lifespan within a species. Body temperature illustrates this point: lowering temperature within a species can extend lifespan, whereas long-lived birds and other long-lived, high-temperature species evolved stronger protection against higher damage flux. This distinction helps explain why an approach that is effective in a laboratory strain may not extrapolate directly to humans or to naturally long-lived species.