Data from: Modification of reproductive schedule in response to pathogen exposure in a wild insect: support for the terminal investment hypothesis

Trade-offs in the time and energy allocated to different functions, such as reproductive activities, can be driven by alterations in condition which reduce resources, often in response to extrinsic factors such as pathogens or parasites. When individuals are challenged by a pathogen they may either reduce reproduction as a cost of increasing defence mechanisms, or alternatively, modify reproductive activities so as to increase fecundity thereby minimizing the fitness costs of earlier death, a behaviour consistent with the terminal investment hypothesis (TIH). The TIH predicts that individuals with decreased likelihood of future reproduction will maximize current reproductive effort, which may include shifts in reproductive timing. We examined how wild, adult female click beetles (Agriotes obscurus) responded after exposure to the fungal pathogenMetarhizium brunneum. Field-collected beetles exposed to a high concentration of M. brunneum died earlier and in greater numbers than those exposed to a low concentration. Using a multi-variate approach we examined the impact of pathogen challenge on lifespan and a suite of reproductive traits. Stepdown regression analysis showed that only female lifespan differed among the fungal treatments. Fungal-induced reductions in lifespan drove changes in the reproductive schedule, characterized by a decrease in preoviposition period. Moving the start of egg laying forward allowed the females to offset the costs of a shortened lifespan. These changes suggest that there is a threshold for terminal investment which is dependent on strength of the survival threat. From an applied perspective, our findings imply that exposing adult click beetles to M. brunneum to reduce their population densitymight not succeed and is an approach that needs further investigation.

针对繁殖活动等不同生命史功能的时间与能量分配权衡，可由个体生存状况改变所驱动——这类改变会削减可用资源，且通常响应于病原体、寄生虫等外在胁迫因素。当个体受到病原体侵染时，它们要么以降低繁殖投入作为增强防御机制的代价，或是调整繁殖活动以提升繁殖力，从而尽可能降低早亡带来的适合度成本，这类行为符合终端投资假说（Terminal Investment Hypothesis, TIH）。该假说预测，未来繁殖可能性降低的个体将最大化当前繁殖投入，这可能包括繁殖时间的调整。我们针对野生成年雌性暗色叩甲（Agriotes obscurus）暴露于布氏绿僵菌（Metarhizium brunneum）后的响应展开了实验研究。野外采集的甲虫中，暴露于高浓度布氏绿僵菌的个体比暴露于低浓度的个体更早死亡且死亡率更高。我们采用多变量方法分析了病原体侵染对寿命及一系列繁殖性状的影响。逐步回归分析显示，仅雌性寿命在不同真菌处理组间存在显著差异。真菌诱导的寿命缩短驱动了繁殖节律的改变，具体表现为产卵前期缩短。提前启动产卵使得雌性个体能够弥补寿命缩短带来的适合度代价。这类变化表明，终端投资存在阈值，且该阈值取决于生存威胁的强度。从应用视角来看，我们的研究结果表明，通过向成年叩甲施用布氏绿僵菌以降低其种群密度的方法可能无法达到预期效果，该策略仍需进一步研究验证。