The members of a number of mammal species, ourselves included, live long past their reproductive years. The question would be why this postreproductive longevity has evolved: what advantage does it confer? For humans, the grandmother hypothesis suggests that it has something to do with enhancing the survival of grandchildren, but this is debated. Here, researchers look at killer whales to argue that the advantage lies in enhanced survival of the male children of long-lived mothers: "Prolonged life after reproduction is difficult to explain evolutionarily unless it arises as a physiological side effect of increased longevity or it benefits related individuals (i.e., increases inclusive fitness). There is little evidence that postreproductive life spans are adaptive in nonhuman animals. By using multigenerational records for two killer whale (Orcinus orca) populations in which females can live for decades after their final parturition, we show that postreproductive mothers increase the survival of offspring, particularly their older male offspring. This finding may explain why female killer whales have evolved the longest postreproductive life span of all nonhuman animals." Male mammals are capable of siring offspring far later in life than females, so if a longer-lived mother can increase the number of years in which a male child continues to mate, that would constitute an advantage even if the mother can no longer reproduce.

Link: http://www.ncbi.nlm.nih.gov/pubmed/22984064