Egg size variation in a long-lived polyandrous shorebird in the context of senescence and breeding phenology

Abstract

Abstract Anisogamy is a central component of sex role evolution, however, the effect of female-female mating competition on egg size variation in polyandrous species is unclear. Moreover, egg size may also be shaped by age-dependent trade-offs between reproductive investments and somatic maintenance that are responsible for senescence. Here we investigate how mating behaviour and senescence are associated with egg size variation in female snowy plovers (Charadrius nivosus). Snowy plovers are long-lived shorebirds (longevity record: 20 years) that often produce several nests each year, with females either sequentially changing partners between breeding attempts or remaining monogamous between attempts. We examined how age, seasonality, body size, and mating behaviour relate to within- and between-female variation in egg volume using repeated measures collected over a 15-year period. We found no evidence of reproductive senescence in egg volume in snowy plover females. Rather, egg volume, polyandry, and re-nesting were strongly linked to breeding phenology: early breeding females had a higher likelihood of being polyandrous or replacing failed clutches, yet these individuals laid smaller eggs likely due to physiological limitations associated with the early season. Older individuals and local recruits secured the earliest breeding opportunities in the season suggesting that prior experience could give an edge in the female-female competition for mates. Larger females laid the largest eggs, as expected, but there was no relationship between body size and lay date – implying that size may not provide an advantage in female-female competition. Our findings highlight the existence of several direct and indirect constraints on female reproductive investment that likely shape individual variation in lifetime reproductive success. Future research investigating reproductive senescence of wild populations should consider mating system dynamics when examining variation in reproductive investment. Impact Summary The divergent gamete sizes of males and females (anisogamy) is a key principle for sex role evolution, however, it remains unclear whether and how egg size variation is shaped by female-female competition in species with reversed sex roles. Furthermore, polygamy is typically associated with age such that egg size variation may also be shaped by age-dependent processes including senescence. Here, we use a 15-year longitudinal mark-recapture dataset of a wild subtropical population of snowy plovers (Charadrius nivosus) breeding in western Mexico to investigate how mating behaviour and senescence are associated with egg size variation. The snowy plover is a long-lived shorebird characterized by a flexible polyandrous mating system. This rare breeding behaviour represents a unique background for investigating senescence in light of individual variation in reproductive investment. We found no evidence of reproductive senescence in egg volume or polyandry. Instead, egg volume and polyandry were strongly linked to the seasonal timing of breeding: early nesting females had a higher likelihood of being polyandrous but laid smaller eggs. Early nesters also had a higher probability of laying a replacement clutch following breeding failure. Taken together, this suggests that females are driven to initiate laying as early as possible despite the cost this has on egg size of their early season clutches. Furthermore, older individuals and local recruits secured the earliest breeding opportunities in the season indicating that prior experience gave individuals an advantage in the competition for mates. Larger females laid the largest eggs, as expected, but there was no relationship between body size and lay date – implying that size is not an important factor in female-female competition. We conclude that individual female reproductive performance is regulated by flexible mating behaviour (i.e., monogamy or polyandry), age- and season-dependent effects, and prior local experience.