Most animals reproduce until they die, but in humans, females can survive long after ceasing reproduction1,2. In theory, a prolonged post-reproductive lifespan will evolve when females can gain greater fitness by increasing the success of their offspring than by continuing to breed themselves3,4,5,6. Although reproductive success is known to decline in old age1,2,3,4,5,6, it is unknown whether women gain fitness by prolonging lifespan post-reproduction. Using complete multi-generational demographic records, we show that women with a prolonged post-reproductive lifespan have more grandchildren, and hence greater fitness, in pre-modern populations of both Finns and Canadians. This fitness benefit arises because post-reproductive mothers enhance the lifetime reproductive success of their offspring by allowing them to breed earlier, more frequently and more successfully. Finally, the fitness benefits of prolonged lifespan diminish as the reproductive output of offspring declines. This suggests that in female humans, selection for deferred ageing should wane when one's own offspring become post-reproductive and, correspondingly, we show that rates of female mortality accelerate as their offspring terminate reproduction.
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We are grateful to T. Clutton-Brock, N. Davies, B. Hatchwell, E. Haukioja, J. Jokela and R. Kilner for comments, T. Coulson for help with calculating individual λ values, and K. Pokkinen, A. Siitonen and T. Verho for collecting the Finnish demographic data. We also thank the Royal Society, UK (V.L. and A.F.R.), Academy of Finland (V.L.), Wihuri Foundation (M.L.), Emil Aaltonen Foundation (S.H.) and Social Sciences and Humanities Research Council of Canada (M.T.) for funding.
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