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, <i>Gambusia holbrooki</i>. 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.

雄性个体在交配后性选择情境下的繁殖表现通常会随年龄增长而劣化。然而目前尚不清楚，这种繁殖衰老现象究竟是源于雄性自身的年龄，还是与雄性年龄常伴随的终生更高交配投入所累积的生理代价。迄今为止，鲜有研究在检验雄性年龄对精子性状的影响时考虑交配史与精子储存情况，且尚无研究探讨在精子竞争情境下，雄性年龄与过往交配史如何影响其父本成功率。本研究以食蚊鱼（<i>Gambusia holbrooki</i>）为实验对象，通过实验操控雄性的交配史，以区分其与年龄分别对射精性状及父本成功率的影响。我们发现，未经历过交配的老年雄性个体的精子数量多于有交配经验的老年雄性，而年轻雄性的情况则恰好相反。与之相对，雄性年龄与交配史均未对精子运动速度产生显著影响。最后，本研究通过人工授精实验操控每只雄性的精子投放量，结果发现，无论交配史如何，老年雄性的后代繁育量均显著高于年轻雄性。本研究结果表明，诸多类群中已被报道的雄性繁殖衰老通用模式，通常可能受到两种天然混淆因素的影响——雄性交配史与精子年龄，而非雄性自身的年龄。