Slowing of age-specific mortality rates in Drosophila melanogaster

We have studied age-dependent mortality in large cohorts of male and female D. melanogaster from four inbred lines. Average longevity varies substantially between genotypes (broad-sense heritability = 22%). Contrary to the predictions of the Gompertz model, mortality rates tend to decelerate at the most advanced ages. Fitting Gompertz, Weibull, Logistic, and Two-stage Gompertz mortality models to the data, we find that the best fit is obtained with the two-stage model, with exponentially increasing mortality at early ages, and zero or nearly zero increase at older ages. There is little microenvironmental effect from cage to cage. There is a sex-dependent mortality crossover: males and females differ in initial mortality rate and degree of acceleration of mortality rate, but the ordering of the sexes according to mortality parameters depends on genotype. Model fitting can be affected by gaps between deaths in the tail of the survivorship distribution. The observations are inconsistent with the limited life-span paradigm, which predicts sudden and well-defined drops in survivorship and corresponding sharp increases in mortality at advanced ages for large cohorts of genetically identical individuals.