Why organisms show late-life mortality plateaus: a null model for comparing patterns of mortality

We show that deceleration in hazard functions (mortality curves) and the resulting mortality plateaus are an intrinsic property of time-to-event traits that are affected by many underlying genetic and environmental factors. We argue that just demonstrating that a mortality curve decelerates with age and reaches a plateau provides little information about the underlying biology associated with lifespan and mortality. To test hypotheses about the rate of deceleration of mortality curves and the level of mortality plateaus, observed curves must be compared with mortality curves expected under a null model – that is, the deceleration of a mortality curve and the mortality plateau generated from a normally distributed trait of the same mean and variance. Comparisons between the observed data and the null models can be achieved with simple statistical tests. The results of these comparisons can be very informative regarding which questions about the shapes of mortality curves will be most meaningful.