| Summary: | Nearly all animals show senescent deterioration in physical condition with increasing age but the social and ecological context of how senescence is maintained by natural selection in wild populations remains unclear. I investigated human female reproductive ageing in a multi-generational dataset of pre-industrial Finns, which reconstructed family-lines from dates of birth, marriage and death and also contained information on family economicstatus. Family-building often required many births because not all offspring survived to reproduce themselves, however, high numbers of breeding offspring in landless economicstatus families limited total grand-offspring number (Chapter 1). The likelihood a female was married and thus had an opportunity to reproduce peaked at ages 30-40 years before declining due to female death or widowhood and low re-marriage probabilities for older widows with dependent offspring (Chapter 2). High birth order offspring had lower survival and reproductive success than their elder siblings, especially in landowning families where land inheritance favoured firstborns (Chapter 3). Female reproductive capability improved after the first birth but declined after many previous births, resulting in reduced infant survival and providing evidence for reproductive senescence (Chapter 4). These findings indicate that senescence could be maintained by two processes: declines in the exposure of genetic variation to natural selection at the oldest fertile ages; and because maternal reproductive investment in earlier born offspring has a greater effect on their reproductive success due to the advantage of earlier born offspring in sibling competition. There was also evidence that total lifetime reproduction could be limited by family resource availability. This illustrates how both ecological and social factors can constrain reproductive success while simultaneously maintaining senescence in the female life history.
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