Transgenerational effects of parental age on fitness.

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Why do organisms age? This is one of the big, unsolved questions in biology, because it is unclear how a process that ultimately decreases fitness can persist evolutionarily, and why species do not evolve to live longer. Many theories have been put forward to explain from an evolutionary perspective why organisms change with age, but it is likely that many factors contribute to the phenomenon. One factor that can strongly affect selection pressure for longevity has, however, not received much attention: the transgenerational effects of senescence. Epigenetic effects can strongly affect the predicted outcome of certain evolutionary models. One such epigenetic and trans-generational factor that affects fitness is maternal age. It has been known for nearly a century that a mother's age can affect her offspring's, and potentially even her grand-offspring's, fitness. However, all the relevant studies have been done in the laboratory. Therefore, it is unknown whether this effect of maternal age operates in wild populations, and whether it can contribute to our understanding of the evolution of longevity.We propose to study the effects of parental and grand-parental age on their offspring's lifespan, reproductive success, physiology and genetics in a wild population of house sparrows. We have data available from a uniquely suited, long-term study of house sparrows in which to examine these topics in detail. Only in a closed population such as the one that we have is it possible to accurately estimate lifespan, fitness and parentage, which are all prerequisites for the longitudinal study of parental age on individual fitness. In addition, we will use focused experiments with captive birds to cement our hypotheses, and individual-based simulation models to explore quantitatively the implications of maternal age effects on the evolution of longevity.We expect the epigenetic effects of maternal age on offspring fitness to have important consequences on the demography of a population, and in turn also on the balance of the classic trade-off between reproduction and survival. Ultimately, these effects will strongly affect the selection pressures acting on longevity itself, and should lead to novel theories of the evolution of longevity.