Longevity: Reproduction - The Direct Cost Of Reproduction
The direct cost of reproduction
Reproduction generally reduces survival, more reproduction shortening life span, less reproduction increasing life span. This effect is not absolutely universal, but it is one of the better established patterns in the biology of aging.
The most extreme experiments that demonstrate the cost of reproduction for longevity involve castration. In annual plants, such as soybean, stripping the plant of flowers often prolongs life by months. In animals that reproduce just once, castration before reproduction can increase longevity by years. In Pacific salmon, it was possible to extend the life of a castrated fish by more than a decade. In marsupial mice of the genus Antechinus, castrated males live months longer than intact males. There is some evidence that castrating institutionalized human males increases their life span. The records of the British aristocracy also suggest that females who have fewer children also live longer, although much of this data antedates modern medicine. All these examples indicate that reproduction affects longevity within the lives of single organisms, but they do not indicate how reliable the effect is.
Classic experiments using the fruit fly Drosophila subobscura illustrate the consistency with which reproduction reduces survival. Normal, mated, fruit fly females lay many eggs and die fairly soon, most between five to seven weeks of adulthood. Three kinds of females that lay fewer eggs all live longer on average: (1) flies that lack ovaries because of a genetic mutation, (2) flies that are denied access to males, and (3) flies that have been sterilized but are allowed to mate. The generality of the effect on longevity of reproduction is revealed by the fact that three such different manipulations of reproduction all increased longevity.