Changes in mate availability and sperm competition should generate selection to adjust investments into different pre- and post-copulatory traits so that the product of mating and fertilization success maximize net male reproductive success. Given costly sperm production and the risk of sperm depletion, males should invest strategically in ejaculates. Here, we use the eastern mosquitofish (Gambusia holbrooki), where males have a single coercive mating tactic, to test whether the number of cues indicative of female availability affects the rate of sperm available for mating (so-called sperm priming). We also tested whether plasticity in sperm production varies with male body size. We created four socio-sexual treatments that differ in the number of female-derived cues: none, chemical, chemical and visual, and full access to a female. We used ablation surgery, removing the tip of the male gonopodium (intromittent organ), to prevent males from mating with a female in the treatment where they interacted with females. We hypothesized that elevated sperm priming would be associated with more cues about female availability, and be more apparent in smaller, subordinate males due to their lower baseline sperm count (higher risk of sperm depletion) and their potential disadvantage during premating competition (leading to higher marginal benefits from sperm investment). There was, however, no evidence for sperm priming. The rate of sperm availability for mating and the baseline sperm reserves were, however, dependent on male body size. We discuss possible reasons for our findings. We also note that our study provides novel insights into the proximate mechanisms associated with sperm release in Poeciliids. Our confirmation of the fact that removing the gonopodium tip prevents a male from releasing sperm when housed with a female has many potential applications (e.g., in the study of effects of ejaculate investment and mating effort on male mating success and longevity). When ejaculates are energetically costly, males should strategically adjust sperm production in response to relevant social cues such as female availability. Using four socio-sexual scenarios, we demonstrate that male mosquitofish G. holbrooki do not produce less sperm when housed alone, compared to being exposed only to chemical cues from females, or to chemical and visual cues, or even when allowed full access to a female. The absence of plasticity in sperm production, when compared to that reported in other Poeciliid fishes, suggests that the relationship between sperm number and mating success might depend upon the mating system. If increased sperm number has a small effect on male reproductive success, there might not be selection for plastic shifts in sperm production. We also showed that ablation surgery on the male’s gonopodium allows us to let males interact with females without ejaculating.

中文翻译：

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新的消融技术显示雄性东方蚊鱼对雌性可用性线索没有精子启动反应

配偶可用性和精子竞争的变化应该产生选择，以调整对不同的交配前和交配后性状的投资，以便交配和受精成功的产物最大限度地提高净雄性繁殖成功率。鉴于精子生产成本高昂和精子耗尽的风险，男性应战略性地投资于精液。在这里，我们使用东部蚊鱼 (Gambusia holbrooki)，其中雄性具有单一的强制交配策略，以测试指示雌性可用性的线索数量是否影响可用于交配的精子率（所谓的精子启动）。我们还测试了精子产生的可塑性是否随男性体型而变化。我们创建了四种社会性治疗，它们在女性衍生线索的数量上有所不同：无、化学、化学和视觉，以及完全接触女性。我们使用消融手术去除雄性生殖器（插入器官）的尖端，以防止雄性在与雌性互动的治疗中与雌性交配。我们假设提高精子启动将与更多关于女性可用性的线索相关，并且在较小的、从属的男性中更加明显，因为它们的基线精子数量较低（精子消耗的风险较高）以及它们在预交比赛中的潜在劣势（导致更高的精子投资的边际收益）。然而，没有证据表明精子启动。然而，用于交配的精子可用率和基线精子储备量取决于男性的体型。我们讨论了我们发现的可能原因。我们还注意到，我们的研究为与 Poeciliids 中精子释放相关的近似机制提供了新的见解。我们确认去除生殖器尖端会阻止雄性在与雌性同住时释放精子有许多潜在的应用（例如，在研究射精投资和交配努力对雄性交配成功和寿命的影响）。当射精的能量消耗很大时，男性应该根据相关的社会线索（例如女性的可用性）战略性地调整精子的产生。使用四种社会性别场景，我们证明雄性蚊鱼 G. holbrooki 在单独饲养时不会产生更少的精子，与仅暴露于来自雌性的化学线索或化学和视觉线索相比，甚至在允许完全访问女性。与其他 Poeciliid 鱼类报道的相比，精子产生缺乏可塑性，这表明精子数量与交配成功率之间的关系可能取决于交配系统。如果增加的精子数量对男性生殖成功的影响很小，则可能无法选择精子产生的可塑性变化。我们还表明，雄性生殖器的消融手术使我们能够让雄性与雌性互动而不射精。