Is Accelerated Senescence a Cost of Reproduction?

Abstract

Senescence is the innate deterioration of the body, leading to a decline in adult fitness components such as life expectancy and fertility with advancing age. Not all age-related decline in performance is attributable to senescence. Injuries and the damaging effects of earlier diseases may show a higher incidence in older age groups but these insults are basically environmental in origin. However, if increasing age per se leads to a higher incidence of accidents or infection, then senescence is implicated. Detecting senescence in wild populations is problematic. Part of the reason is the purely practical one that longitudinal studies on marked individuals are required and numbers in the older age classes are often very low. Furthermore, the older individuals may well not be a random sample of the original cohort because their longevity may be attributable to individual characteristics that also lead to, for example, high fertility. In addition, senescence occurs against a background of other adaptive age-related changes in demographic characteristics. If growth continues during adult life, then both survival probability and fertility may show an increase during at least part of the lifespan (Charlesworth, 1980). Under these circumstances senescence may merely reduce the extent of these increases, rather than produce a net decline. Moreover, most wild animals and plants do not die of old age; many survival curves approximate to the negative exponential expected if death rate is constant with respect to age (Medawar, 1952), although others do show an age-related decline in survival probability or fertility (Charlesworth, 1980). The existence of senescence is often most clearly revealed when natural hazards are removed in captivity (Medawar, 1952).