Identifying age-dependent trade-offs between reproductive effort and survival in wild organisms

284 words): 32 Identifying age-dependent trade-offs between reproductive effort and survival in wild organisms 33 is central for understanding the evolutionary mechanisms of senescence. According to the 34 disposable soma theory, early-life energy investments in reproduction compromise late-life 35 investments in somatic maintenance – leading to senescence. Once thought to only be detectable 36 in captive populations, senescence has recently been documented by several longitudinal studies 37 of wild organisms. However, some reproductive traits that are used to quantify senescence may 38 also be shaped by other age-dependent processes such as mating tactics. For instance, polygamy, 39 which is often positively associated with age, may lead to reduced gamete size due to trade-offs 40 between gamete quantity and quality. Here we investigate how mating behaviour and senescence 41 are associated with reproductive trade-offs of female snowy plovers (Charadrius nivosus). Snowy 42 plovers are long-lived shorebirds (longevity record: 19 years) that produce several nests each year, 43 with females either being polyandrous and sequentially changing partners between breeding 44 attempts or remaining monogamous between attempts. We examined how age, seasonality, and 45 mating tactics affect within-female variation in egg volume using repeated measures collected over 46 a 14-year period. Our results provide clear evidence for senescence in snowy plover females 47 starting at three years of age. Furthermore, females laid smaller eggs in years when they were 48 polyandrous compared to years when they were monogamous, with earlyand late-season clutches 49 having the smallest eggs. We suggest that individual female reproductive performance is regulated 50 by flexible mating tactics and ageand season-dependent effects. Our findings highlight the 51 existence of multiple trade-offs for female reproductive investments that likely shape individual 52 variation in lifetime reproductive success. We encourage future research investigating 53 reproductive senescence of wild populations to incorporate age-dependent reproductive 54 investments in light of mating system dynamics. 55 56 Impact Summary (297 words): 57 Why do organisms senesce at older ages? Life-history theory predicts that early life investments 58 in reproduction compromise future investment opportunities for somatic repair, which leads to 59 senescence. Earlier works assumed that senescence was only observable in captive populations 60 due to the high degree of extrinsic mortality experienced by wild organisms. However, with the 61 expansion of longitudinal studies collecting repeated measures from wild individuals, recent works 62 have identified key insights into the selective processes driving patterns of senescence in nature. 63 Here, we use a 14-year longitudinal mark-recapture dataset of snowy plovers (Charadrius nivosus) 64 breeding in a wild subtropical population in western Mexico to investigate age-dependent trade65 offs between female reproductive effort and performance. The snowy plover is a long-lived 66 shorebird characterized by a flexible polyandrous mating system. This rare breeding behaviour 67 represents a unique background for investigating senescence in light of between and within68 individual variation in reproductive investment. Our study focused on age-dependent dynamics of 69 egg volume – a convenient measure of a female’s intrinsic reproductive investment, particularly 70 in birds with precocial chicks. We explored age-related correlates of within-female egg volume 71 dynamics while controlling for among-female variation and the confounding effect of trait72 dependent (dis)appearance of young (or old) females in the population. Our results show that egg 73 volume expresses an early-life increase followed by a late-life decrease, indicative of reproductive 74 senescence. Furthermore, females tended to lay smaller eggs in years during which they were 75 polyandrous than in years when they were monogamous. Notably, polyandry was not associated 76 with age. As expected, egg volume was highly repeatable within females and clutches. We 77 conclude that age-dependent reproductive effort is an important component driving betweenand 78 within-individual variation in reproductive performance of facultatively polygamous species. We 79 encourage future works studying reproductive senescence of wild populations to incorporate age80 dependent reproductive investment and mating tactics. 81 Main Text: 82 INTRODUCTION (1254 words) 83 The costs of survival and reproduction are considered to be central for the evolution of life history 84 strategies. Life-history theory predicts that fitness trade-offs between current and future 85 reproduction shape inter-specific variation in reproductive strategies observed among iteroparous 86 organisms (Partridge, 1989). To maximize the transfer of their genes to the next generation, 87 individuals adjust their current reproductive investment in light of future reproductive prospects 88 (Jankowiak et al., 2018). The compromise between current and future reproduction enforces trade89 offs between investments in reproduction and other competing functions, such as self-maintenance 90 (Stearns, 1977, 1992; Christe et al., 2012). 91