Summary: Foster et al. (2012)

Postmenopausal life span in humans has encouraged studies investigating the evolution prolonged post-reproductive life span in other animals. Two mechanisms are proposed: 1) increased longevity - evolutionary benefits accumulate only during the reproductive phase; 2) evolved adaptation - the post-reproductive life span increases individual "genetic" survival, resulting in increased inclusive fitness. Foster et al. (2012) used a Cox proportional hazards model (6) to study the influence of a mother killer whale’s (Orcinus orca) death on offspring survival. Neither sex disperses from the maternal group, so old mothers can maximise their inclusive fitness by increasing offspring survival and reproductive success. In addition, mothers should only help sons, as mating occurs outside the matriline. Foster et al. (2012) showed that old females assist male offspring. As reproductive success increases with male age, increased survival of sons increases the inclusive fitness of mothers. Mothers can assist with foraging or will provide support during agonistic encounters. Foster et al.'s (2012) results support the premise that a long post-reproductive life span is an evolved adaptation.