Gompertz growth in number dead confirms medflies and nematodes show excess oldster survival

A recent report (Easton, 1995) showed that, at least for Mediterranean fruit flies, a Gompertz growth equation based on the increase in number of individuals that die is a better predictor of survival data than is the classical Gompertz survivorship model based on the decrease in number that survive (analysis of medfly data of Carey et al., 1992). In the growth model, the rate of increase of the number dead (i.e., the death rate) decreases exponentially with age. The poor fit of the classical model predicts "excess survival" of older members, but, when the scale of the better-fitting growth model is increased 2400x, such excess is now also evident as a small but distinctly separate cohort of the medfly subjects. The smaller population appears to be about 0.01% of the larger, and the death rate decreases about one-fourth as fast. Survival of the nematode C. elegans (Brooks et al., 1994) is also better predicted by the growth model, which also shows excess survival of the worms at great age.