Title : Egg size variation in a long-lived polyandrous shorebird in the context 2 of senescence and breeding phenology 3

279 words): 29 Anisogamy is a central component of sex role evolution, however, the effect of female30 female mating competition on egg size variation in polyandrous species is unclear. Moreover, 31 egg size may also be shaped by age-dependent trade-offs between reproductive investments 32 and somatic maintenance that are responsible for senescence. Here we investigate how 33 mating behaviour and senescence are associated with egg size variation in female snowy 34 plovers (Charadrius nivosus). Snowy plovers are long-lived shorebirds (longevity record: 20 35 years) that often produce several nests each year, with females either sequentially changing 36 partners between breeding attempts or remaining monogamous between attempts. We 37 examined how age, seasonality, body size, and mating behaviour relate to withinand 38 between-female variation in egg volume using repeated measures collected over a 15-year 39 period. We found no evidence of reproductive senescence in egg volume in snowy plover 40 females. Rather, egg volume, polyandry, and re-nesting were strongly linked to breeding 41 phenology: early breeding females had a higher likelihood of being polyandrous or replacing 42 failed clutches, yet these individuals laid smaller eggs likely due to physiological limitations 43 associated with the early season. Older individuals and local recruits secured the earliest 44 breeding opportunities in the season suggesting that prior experience could give an edge in 45 the female-female competition for mates. Larger females laid the largest eggs, as expected, 46 but there was no relationship between body size and lay date – implying that size may not 47 provide an advantage in female-female competition. Our findings highlight the existence of 48 several direct and indirect constraints on female reproductive investment that likely shape 49 individual variation in lifetime reproductive success. Future research investigating 50 reproductive senescence of wild populations should consider mating system dynamics when 51 examining variation in reproductive investment. 52 . CC-BY-NC-ND 4.0 International license available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprint this version posted August 2, 2021. ; https://doi.org/10.1101/2020.08.07.240150 doi: bioRxiv preprint