Data from: Pathways to fitness: carry-over effects of late hatching and urbanisation on lifetime mating success

Life history theory and most empirical studies assume carry-over effects of larval conditions to shape adult fitness through their impact on metamorphic traits (age and mass at metamorphosis). Yet, very few formal tests of this connection across metamorphosis exist, because this entails longitudinal studies from the egg stage and requires measuring fitness in (semi)natural conditions. In a longitudinal one-year common-garden rearing experiment consisting of an outdoor microcosm part for the larval stage and a large outdoor insectary part for the adult stage, we studied the effects of two factors related to time constraints in the larval stage (egg hatching period and urbanisation) on life history traits and lifetime mating success in the males of the damselfly Coenagrion puella. We reared early- and late-hatched larvae from each of three rural and three urban populations from the egg stage throughout their adult life. Key findings were that both the hatching period and urbanisation shaped adult fitness, yet through different pathways. As expected, the more time-constrained late-hatched individuals accelerated their larval life history and this was associated with a lower lifetime mating success. A path analysis revealed this carry-over effect was mediated by the changes in the two metamorphic traits (reduced age and lower mass at emergence). Notably, urban males had a 50% lower lifetime mating success, which was not mediated by age and mass at emergence, and possibly driven by their shorter lifespan. Our results point to long-term carry-over effects of the usually ignored natural variation in egg hatching dates, and further contribute to the limited evidence showing fitness costs of adjusting to an urban lifestyle.生活史理论（life history theory）与绝大多数实证研究均假设，幼虫环境可通过影响变态性状（metamorphic traits，即变态发生时的年龄与体重）产生携带效应（carry-over effects），进而塑造成虫的适合度。然而，目前针对变态过程中这一关联的正式检验极为匮乏，因为此类研究需从卵期启动纵向追踪，并在半自然（semi-natural）环境中测定适合度。本研究依托一项为期一年的同质园饲养（common-garden rearing）纵向实验开展，该实验分为幼虫阶段的室外微宇宙系统与成虫阶段的大型室外养虫室两部分。我们以豆娘（damselfly）Coenagrion puella的雄性个体为研究对象，探究了幼虫阶段两类与时间限制相关的因素——卵孵化期与城市化水平——对其生活史性状及终生交配成功率的影响。我们从3个乡村种群与3个城市种群中分别获取早孵化与晚孵化的幼虫，从卵期开始饲养直至其成虫生命周期结束。关键研究结果显示：孵化期与城市化水平均会影响成虫适合度，但作用路径各不相同。正如预期，受时间限制更强的晚孵化个体加快了幼虫阶段的生活史进程，且这一现象与更低的终生交配成功率相关。路径分析结果表明，该携带效应是通过两种变态性状的改变（羽化时年龄降低、体重下降）介导产生的。值得注意的是，城市种群雄性的终生交配成功率较乡村种群雄性低50%，这一效应并非由羽化时的年龄与体重所介导，其潜在驱动因素可能是更短的寿命。本研究结果揭示了以往常被忽视的卵孵化日期自然变异所带来的长期携带效应，同时也为“适应城市生活方式会带来适合度代价”这一有限的实证证据链补充了新的支撑。