Density and age-specific mortality

Age-specific mortality rates decelerate at older ages in laboratory populations in the Medfly Ceratitis capitata. This has been interpreted by Carey et al. (1992) to reflect a slowing of the aging process, but might also be explained by declining adult density. Here it is argued that the density explanation, as presented by Graves and Mueller (1993), is unpersuasive for several reasons: extrapolations from Drosophila to Medflies are unjustified; the range of densities they studied is 2-120 times higher than that used in other studies; they ignore data on Medflies held in isolation, which rule out density effects; their own data suggest that initial cohort density has no effect on mortality rates at older ages, which is the relevant part of the life cycle; their experiment is too small to provide accurate estimates of mortality; new Medfly experiments executed at multiple densities show decelerating and then declining mortality rates at advanced ages for all densities. When Drosophila survivorship experiments are done on a sufficiently large scale they also show a deceleration of mortality at older ages that is not attributable to density effects. The deceleration of mortality rates is most likely a real facet of aging, and will have to be taken into consideration in any synthesis of the genetics and evolution of aging.