The kinetics and thermodynamics of death in multicellular organisms

Abstract

The survivorship curves for Drosophila melanogaster, determined from experiments in which the flies are individually housed, are better fitted by a two-constant power law of the form, −1/ N(d N/d t) = At n , than by the Gompertz function: −1/ N(d N/d t) = R 0e αt . Such curves, taken for different ambient temperatures, show that A is a strong function of temperature, while n is not. Over the limited temperature range of the experiments (25–33°C), and Arrhenius plot is obtained for A, yielding an activation enthalpy for death ( ΔH † ) of 190 kcal/mole. This value is much higher than that reported by Strehler of 18.4 kcal/mole, but is consistent with the high activation enthalpies reported for unicellular organisms and mammalian cells in tissue culture. The power law plots, in either differential or integral form, appear to be generally applicable to mammalian age-related death statistics as shown by data on mice, rats and humans. The values of n are approximately 4–5 for all species tested. For human mortality rates, the three major individual causes of death are also well represented by power law plots. The ordinary distribution laws of the theory of probability do not give the correct forms for the mortality rates. However, the asymptotic distribution of extremes in order statistics do yield both the power law and the Gompertz function distributions. We speculate that the existence of high values of activation enthalpies for death predict that small decreases in core body temperatures of homeothermic mammals should produce marked increases in life expectancy.