On measuring the rate of ageing

Ageing, or senescence, is a decline in state at later ages that is manifested through a reduction in survival and fecundity. Ageing means that reproductive prospects and hence the life history options (trade-offs) open to the organism decline. Evolutionary theories of ageing suggest that it evolves in response to the level of externally imposed mortality and insults to fertility, either as part of life history optimization or as a result of mutation pressure. Several recent empirical and theoretical studies have produced apparently anomalous results. Some have suggested that the rate of ageing can decline at later ages, others that demographic evidence for ageing can appear in parallel with an improvement in state. All of these studies used measures of ageing that would not be expected to give an accurate reflection of changes in the state of the organism with age. We propose that Fisher's `reproductive value' is a natural measure of state at each age, which includes prospects for both survival and reproduction. If this measure is used, the apparently anomalous findings are not at variance with evolutionary theories of ageing.