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.

生活史理论与多数实证研究均假定，幼虫期环境的携带效应（carry-over effects）可通过影响变态性状（metamorphic traits，即变态时的年龄与体重）来塑造成虫适合度（fitness）。然而，目前针对变态过程中这一关联的正式检验极少，因为此类研究需要从卵期开始开展纵向追踪，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纵向同质园饲养（common-garden rearing）实验，该实验包含用于幼虫期饲养的室外微宇宙（outdoor microcosm）系统，以及用于成虫期饲养的大型室外昆虫饲养室（insectary）。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期结束。核心研究结果显示，孵化期与城市化（urbanisation）均会影响成虫适合度，但其作用路径各不相同。正如预期，受时间限制更强的晚孵化个体加快了其幼虫期生活史进程，这与其终生交配成功率降低相关。路径分析（path analysis）结果显示，该携带效应由两类变态性状的变化介导——羽化（emergence）时年龄降低与体重下降。值得注意的是，城市种群雄性的终生交配成功率较对照组低50%，该差异并非由羽化时的年龄与体重介导，而可能与其寿命更短有关。本研究结果证实了此前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“适应城市生活方式存在适合度成本”这一观点补充了有限的实证依据。