The body-size dependent relationships of mortality and longevity are examined for birds and eutherian mammals. Differences between mass exponents for maximum recorded longevity and survival times for fractions of original adult populations confirm the age-dependence of mortality in both classes and a size-dependency of population-age distribution. The potential number of offspring produced by a surviving fraction of a mammalian population appears to be a size-independent ecological constant. Social structure would be more likely in larger animals since greater continuity would be provided when a higher proportion of the population consisted of senior, experienced animals, as described by the ratio of time for survival of 1 in 1000 to maximum potential lifespan: t0.001/tmax = 0.91 m0.32/2.94 m0.20 = 0.31 m0.12, that is, the expected lifespan approaches the maximum as size increases.