In short, these older gals tend to be dumped or ignored in favor of younger women, who in theory have better genes and can, because of their age and energy, possibly reproduce more, keeping the male's lineage going.
Women, of course, don't just drop dead after menopause. The forces of natural selection seek only the survival of the species through individual fitness. They protect fertility in women while they are most likely to reproduce, but after that period, they cease to quell the genetic mutations that ultimately bring on menopause and a possible host of other potential health problems, such as increased risk for heart attack and stroke.
Human, and killer whale, females, who also go through menopause, often live long lives after their baby-producing days are done. They even continue to increase the propagation of their genes during later life.
"They do this by helping to increase the survival of their older sons, which in turn increases the number of grandchildren fathered by their sons," Darren Croft, a senior lecturer in animal behavior at the University of Exeter, told Discovery News. "Through this process, evolution favors females that live longer after their menopause. This explains why female killer whales have evolved the longest lifespan after menopause of all non-human animals."