Polyandry and senescence shape egg size variation in a precocial bird

Identifying age-dependent trade-offs between reproductive effort and survival in wild organisms is central for understanding the evolutionary mechanisms of senescence. According to the disposable soma theory, early-life energy investments in reproduction compromise late-life investments in somatic maintenance – leading to senescence. Once thought to only be detectable in captive populations, senescence has recently been documented by several longitudinal studies of wild organisms. However, some reproductive traits that are used to quantify senescence may also be shaped by other age-dependent processes such as mating tactics. For instance, polygamy, which is often positively associated with age, may lead to reduced gamete size due to trade-offs between gamete quantity and quality. Here we investigate how mating behaviour and senescence are associated with reproductive trade-offs of female snowy plovers (). Snowy plovers are long-lived shorebirds (longevity record: 19 years) that produce several nests each year, with females either being polyandrous and sequentially changing partners between breeding attempts or remaining monogamous between attempts. We examined how age, seasonality, and mating tactics affect within-female variation in egg volume using repeated measures collected over a 14-year period. Our results provide clear evidence for senescence in snowy plover females starting at three years of age. Furthermore, females laid smaller eggs in years when they were polyandrous compared to years when they were monogamous, with early- and late-season clutches having the smallest eggs. We suggest that individual female reproductive performance is regulated by flexible mating tactics and age- and season-dependent effects. Our findings highlight the existence of multiple trade-offs for female reproductive investments that likely shape individual variation in lifetime reproductive success. We encourage future research investigating reproductive senescence of wild populations to incorporate age-dependent reproductive investments in light of mating system dynamics.