Data from: Size-dependent ejaculation strategies and reproductive success in the yellow dung fly, Scathophaga stercoraria

Theory predicts that sperm competition will favour the production of larger ejaculates. However, because the benefits of greater reproductive investment are balanced by the costs of spermatogenesis, expenditure should depend on male physiology, mating rate and the relationship between additional investment and fertilization gains. In the yellow dung fly, Scathophagastercoraria, males adopt size-dependent alternative mating tactics that are associated with discrete ecological resources (foraging and oviposition substrates), although males switch between these environments throughout their lives. By copulating on foraging substrate (fruit or flowers), males can bypass intense mate competition that occurs at oviposition sites (cow dung), but as a consequence, must occupy a disfavoured mating role (i.e. face a greater risk that their ejaculate will be displaced from storage prior to fertilization). Small males often mate on foraging substrate, whereas large males mate in this environment only during feeding bouts. Optimal ejaculate expenditure should therefore depend on male size and their current mating role. By measuring copula duration (i.e. ejaculate expenditure) of natural matings and assigning paternity to resulting offspring, we confirmed that copulations on dung sire approximately three times as many offspring as those on foraging substrate. Furthermore, large males reduced copula duration on fruit, as predicted, since this strategy enables greater investment into high-payoff matings on dung. Conversely, small males copulated for shorter durations on dung than on foraging substrate, perhaps to minimize the risk of being displaced from copula by a rival. These patterns of ejaculate expenditure translated into greater offspring production for large males on dung and for small males on fruit. We discuss the possible proximate factors driving these size- and context-dependent patterns of ejaculate allocation by yellow dung fly males. Together, our findings shed light on the allocation strategies and reproductive consequences of alternative mating tactics.理论预测，精子竞争（sperm competition）会驱动雄性产生更大体积的射精量。然而，由于更高繁殖投入（reproductive investment）的收益会被精子发生（spermatogenesis）的成本所抵消，因此雄性的繁殖投入应取决于其生理状态、交配频率，以及额外投入与受精收益之间的关联。在黄粪蝇（Scathophaga stercoraria）中，雄性会采取依赖体型的替代性交配策略（alternative mating tactics），这类策略与两类离散的生态资源——觅食基质（foraging substrate，果实或花朵）与产卵基质（oviposition substrate）——相关联，且雄性在整个生命周期中会在这两类环境间切换。若雄性在觅食基质上交配，便可规避产卵位点（oviposition sites，牛粪）处激烈的配偶竞争（mate competition），但代价是必须承担不利的交配角色（mating role）：即其射精在受精前被从雌性存储器官中置换的风险会显著升高。小型雄性通常在觅食基质上交配，而大型雄性仅在觅食时段才会在此环境中进行交配。因此，最优的射精投入应取决于雄性的体型与当前所处的交配角色。通过测量自然交配的交配时长（copula duration，即射精投入），并对所产后代进行父权分配（paternity assignment），我们证实：在牛粪上交配所产生的后代数量约为觅食基质上交配的三倍。此外，正如理论预测的那样，大型雄性会缩短在果实上的交配时长，这一策略可使其将更多的射精投入转向高回报交配（high-payoff matings）场景。与之相反，小型雄性在牛粪上的交配时长短于在觅食基质上的，这或许是为了尽可能降低被竞争对手从交配状态中置换的风险。这类射精投入模式最终使得大型雄性在牛粪环境中、小型雄性在果实环境中获得了更高的后代产量。我们还讨论了驱动黄粪蝇雄性产生这种依赖体型与环境的射精分配（ejaculate allocation）模式的潜在近因因素（proximate factors）。综上，本研究结果阐明了替代性交配策略的资源分配策略及其繁殖后果。