JOB DONE. “Thus, the forces of natural selection diminish after animals reach reproductive success because survival beyond that event has diminished value for the survival of the species. Energy is better spent on guaranteeing reproductive success than it is for increasing individual longevity. Thus, after reproductive success the forces of natural selection do not favor increased longevity. However, after reproductive success an animal has the potential to survive for a period of time determined by the level of excess physiological capacity reached at sexual maturation.”
http://protein.bio.msu.ru/biokhimiya/contents/v62/full/62111380.html