Why do life spans differ? Partitioning mean longevity differences in termsof age-specific mortality parameters

Populations typically differ in mean life spans because of genetic, environ mental, or experimental factors. In this paper. methods are presented that clarify the relationship between differences in the longevity of two popula tions and differences in their underlying age-specific patterns of mortalit y, Data are examined from rodent and fruit fly (Drosophila melanogaster) ex periments that investigated the longevity effects of a variety of environme ntal and genetic manipulations, including temperature, dietary restriction, laboratory selection for increased longevity, and severe inbreeding. Analy ses suggest that longevity differences mediated by temperature and dietary restriction result predominantly from differences in the rate of increase i n mortality with age. Increases in longevity through laboratory selection r esult primarily from a reduction in baseline mortality and not a slowing of the rate of aging. Although the methods are applied primarily in the conte st of simple mathematical models of mortality (e.g., the Gompertz model), t hey are quite general and can be applied to mortality models of arbitrary c omplexity. Mathematica protocols ("notebooks") and computer software have b een developed to perform all the analyses discussed and are available from the first author.