Contrasting responses of pre- and post-copulatory traits to variation in mating competition

1. Original sperm competition theory assumed that males trade expenditure on searching for mates for expenditure on the ejaculate, and predicted that males should increase their expenditure on the ejaculate in response to increased risk of competition. A recent extension of this theory has modeled pre-copulatory expenditure in terms of direct contest competition, and predicts that when the gains from marginal investment in weaponry are large, males might be expected to allocate resources to armaments even at the expense of the ejaculate. 2. Here we examine socially cued plasticity in allocation to pre- (body condition) and post-copulatory (testes mass) traits in a male dimorphic beetle, Onthophagus taurus, where major males fight for access to females and minor males obtain reproductive success via sperm competition. Both male morphs were either reared in social isolation or exposed to rivals during the period of sexual maturation following adult emergence. 3. Testes mass was found to b...1. 经典精子竞争理论（sperm competition theory）假定雄性会在寻找配偶的能量投入与射精相关的能量投入之间进行权衡，并预测雄性会随竞争风险升高而增加射精相关的资源投入。近期该理论的拓展模型将交配前投入纳入直接争斗竞争的分析框架，提出当争斗武器的边际投资收益较高时，雄性即便可能牺牲射精相关的资源投入，也会将资源分配至争斗武器。 2. 本研究以雄性二态性甲虫塔蜣螂（Onthophagus taurus）为实验对象，探究其在交配前（身体状态）与交配后（睾丸质量）性状上的资源分配如何响应社会线索诱导的表型可塑性：其中大体型雄性通过争斗获取配偶，小体型雄性则通过精子竞争获得繁殖成功。所有供试的两种雄性形态个体均被分为两组，一组置于社会隔离环境中饲养，另一组在成虫羽化后的性成熟阶段暴露于竞争者环境中。 3. 研究发现睾丸质量……