1. Section of Ecology, Department of Biology, University of Turku, FIN-20014 Turku, Finland
2. Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK
3. Department of Zoology, University of Cambridge, Cambridge CB2 3EJ, UK
4. Interdisciplinary Research Group on Demography and Genetic Epidemiology, University of Quebec, Chicoutimi, Quebec G7H 2B1, Canada

Correspondence to: Mirkka Lahdenperä¹ Email: mipayl@utu.fi

Abstract

Most animals reproduce until they die, but in humans, females can survive long after ceasing reproduction^{1, 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 themselves^{3, 4, 5, 6}. Although reproductive success is known to decline in old age^{1, 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.

1. Section of Ecology, Department of Biology, University of Turku, FIN-20014 Turku, Finland
2. Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK
3. Department of Zoology, University of Cambridge, Cambridge CB2 3EJ, UK
4. Interdisciplinary Research Group on Demography and Genetic Epidemiology, University of Quebec, Chicoutimi, Quebec G7H 2B1, Canada

Correspondence to: Mirkka Lahdenperä¹ Email: mipayl@utu.fi

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