EXCESS BUFFER. “Once animals reach sexual maturation and raise progeny to independence, the excess physiological capacity, like that engineered into a space vehicle, allows each to continue beyond the vital goal. The further longevity of each is determined by the excess capacity present at the time the goal was reached. In animals that goal, of course, is reproductive success.”
http://protein.bio.msu.ru/biokhimiya/contents/v62/full/62111380.html