Sex-specific life table aging rates in large medfly cohorts

Life table aging rate (LAR) is defined as the age-specific rate of mortality change with age and corresponds to the first derivative or slope of the mortality schedule at each age. We computed the sex-specific LARs for 167 Medfly cohorts, containing a total of approximately 600,000 individuals of each sex. We found that: (a) the LAR is not constant for either sex; at the youngest ages, mortality changes by approximately 1.4-fold daily, whereas at the older ages (>30 days), mortality levels off and decreases; (b) female LAR is higher than male LAR from 0 to 12 days, at which time the rates cross over and male LAR is higher than female LAR; however, at about 30 days, both male and female LARs are less than or equal to unity; (c) mortality in each of the 167 cohorts leveled off for both sexes. The average age for this leveling off was around 23 days for females and 31 days for males; and (d) the mortality rate at the age of leveling off was approximately 1.4-fold higher in males than in females--0.13/day for females and 0.19/day for males. Implications of these findings are briefly discusses, including the use of summary measures of senescence, the arbitrariness of selecting the age of senescence onset, and test of hypotheses concerned with explanations of mortality patterns such as leveling off at older ages.