Raw data used to analyse the results from Male age alone predicts paternity success under sperm competition when effects of age and past mating effort are experimentally separated

Older males often perform poorly under post-copulatory sexual selection. It is unclear, however, whether reproductive senescence is due to male age itself or the accumulated costs of the higher lifetime mating effort that is usually associated with male age. To date, very few studies have accounted for mating history and sperm storage when testing the effect of male age on sperm traits, and none test how age and past mating history influence paternity success under sperm competition. Here, we experimentally manipulate male mating history to tease apart its effects from that of age on ejaculate traits and paternity in the mosquitofish, Gambusia holbrooki. We found that old, naive males had more sperm than old, experienced males, while the reverse was true for young males. By contrast, neither male age nor mating history affected sperm velocity. Finally, using artificial insemination to experimentally control the number of sperm per male, we found that old males sired significantly more offspring than young males independently of their mating history. Our results highlight that the general pattern of male reproductive senescence described in many taxa may often be affected by two naturally confounding factors, male mating history and sperm age, rather than male age itself.在交配后性选择（post-copulatory sexual selection）情境下，年长雄性通常表现不佳。然而目前尚不清楚，雄性生殖衰老（reproductive senescence）究竟源于雄性自身年龄，还是与雄性年龄通常相关的更高终身交配投入所带来的累积成本。迄今为止，极少有研究在检验雄性年龄对精子性状（sperm traits）的影响时，将交配历史与精子储存情况纳入考量，且尚无研究探讨在精子竞争（sperm competition）情境下，雄性年龄与过往交配经历如何影响其父本成功率（paternity success）。本研究以东部食蚊鱼（Gambusia holbrooki）为实验对象，通过实验操控雄性的交配经历，以区分交配经历与年龄对射精性状（ejaculate traits）及父本成功率的独立影响。我们发现，未交配过的年长雄性精子数量多于有交配经验的年长雄性，而年轻雄性的情况则恰好相反。相较而言，雄性年龄与交配经历均未对精子运动速度（sperm velocity）产生显著影响。最后，本研究通过人工授精（artificial insemination）实验操控每只雄性的精子投放量，结果发现，无论交配经历如何，年长雄性繁育的后代数量均显著多于年轻雄性。我们的研究结果表明，诸多生物类群（taxa）中已被报道的雄性生殖衰老通用模式，往往会受到雄性交配历史与精子年龄这两项天然混淆因素（confounding factors）的影响，而非雄性自身年龄本身。