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 be insensitive to social cues of future mating competition for both major and minor males. Major males allocated more to body condition when exposed to rivals, a response expected for a species in which the outcome of dyadic contests strongly affect male reproductive success. In contrast, minor male allocation to condition was insensitive to social cues. 4. Our data illustrate how socially cued plasticity in pre- and post-copulatory traits can depend on the relative importance of these episodes of selection for individual male fitness. In O. taurus dung beetles, males strategically adjusted the amount of resources they allocated to winning pre-copulatory contests over access to females. Strategic allocation to pre-copulatory contest competition did not come at a cost to male investment in sperm competition, suggesting that males may trade investment into contest competition against some other life history trait, such as longevity. The lack of plasticity in testes size suggests that selection from sperm competition may be a relatively constant feature of this species mating system.

1. 经典精子竞争理论（sperm competition theory）假定雄性会将寻找配偶的能量投入置换为射精相关的能量投入，并预测当竞争风险升高时，雄性会增加其在射精相关性状上的投入。近期该理论的延伸研究以直接争斗竞争为框架对交配前（pre-copulatory）能量投入进行了建模，该模型预测，当武器装备的边际投入收益较高时，雄性可能会将资源分配给争斗武器，即便这会以牺牲射精相关投入为代价。2. 本研究针对雄性二态性甲虫塔蜣螂（Onthophagus taurus）展开，该物种中大型雄性通过争斗争夺交配机会，小型雄性则通过精子竞争获得繁殖成功；我们在此检验了其在交配前（身体状况）与交配后（post-copulatory）性状上的资源分配如何响应社会线索诱导的表型可塑性。所有雄性个体均在成虫羽化后的性成熟阶段，被分为两组：一组饲养于社会隔离环境，另一组则暴露于竞争者环境中。3. 研究发现，无论大型还是小型雄性，其睾丸质量均对预示未来交配竞争的社会线索不敏感。当暴露于竞争者环境时，大型雄性会增加对身体状况的资源投入，这一响应符合该物种的演化预期：因为成对争斗的结果对雄性繁殖成功具有决定性影响。与之形成鲜明对比的是，小型雄性的身体状况资源分配不受社会线索的调控。4. 我们的数据表明，交配前与交配后性状的社会线索诱导表型可塑性，取决于这两类选择事件对雄性个体适合度的相对重要性。在塔蜣螂中，雄性会策略性地调整其资源投入量，以赢得争夺交配机会的交配前争斗。对交配前争斗竞争的策略性资源投入并未以雄性在精子竞争上的投入为代价，这表明雄性可能将争斗竞争的投入与其他生活史性状（如寿命）进行了资源置换。睾丸大小缺乏表型可塑性这一结果表明，精子竞争带来的选择压力可能是该物种交配系统中相对稳定的选择压力。