Data from: Food-limited mothers favor offspring quality over offspring number: a principal components approach

Mothers are expected to balance the tradeoff between the number and quality of offspring, and many theoretical studies describe how the maternal environment might influence the evolution of the number-quality tradeoff. However, few empirical studies attempt to test these theories (and their assumptions) by measuring the fitness consequences of variation in investment per offspring. Part of the problem is that measuring offspring fitness is difficult, which frequently leads experimenters to measure several proxies of offspring fitness in place of a comprehensive fitness assay. This strategy tends to result in multiple univariate analyses that involve different offspring fitness proxies, and these tests can have low power and may produce conflicting conclusions. Here, we demonstrate the benefits of integrating maternal fecundity and proxies of offspring size and fitness into multivariate analyses to elucidate variation in reproductive allocation strategies. In a 2×2 factorial experiment, we manipulated the quality of maternal environment (food availability) throughout early and late adulthood (acute and chronic exposure to the maternal environment) in a field cricket. We developed a multivariate index of reproductive allocation by incorporating maternal fecundity and the performance of offspring in low- and high-food environments into a principle components analysis. This index of reproductive allocation indicated that females decreased fecundity and increased offspring quality after chronic exposure to low-food environments, thereby providing evidence of adaptive plasticity in investment per offspring. In contrast, few treatment effects were observed using univariate analyses. The present study demonstrates that multivariate analysis can increase our ability to assess the adaptive significance of reproductive strategies, particularly in situations when offspring size and fitness are difficult to measure with accuracy. Such an approach might ultimately help assess the adaptive significance of reproductive allocation across a wider range of taxa, thereby providing broader insight into the evolution of reproductive strategies.母体需在后代数量与后代质量之间权衡取舍，诸多理论研究探讨了母体环境如何影响数量-质量权衡的演化进程。然而鲜有实证研究通过测量每单位后代投入的适合度后果，来检验上述理论及其假设前提。究其症结，后代适合度的测量极具挑战性，这使得实验者常转而采用多种后代适合度替代指标，而非开展全面的适合度测定。此类策略往往会催生基于不同后代适合度替代指标的单变量分析，这类检验的统计功效往往偏低，且可能得出相互矛盾的研究结论。本研究旨在阐明将母体繁殖力、后代大小与适合度替代指标整合至多变量分析中的优势，以此揭示繁殖分配策略的变异规律。我们以田野蟋蟀为研究对象，设置2×2析因实验，操控其成年早期与成年晚期的母体环境质量（即食物可获得性），分别设置急性与慢性母体环境暴露处理。我们将母体繁殖力与后代在低、高食物环境下的生长表现整合至主成分分析（Principal Components Analysis）中，构建了繁殖分配的多变量指数。该繁殖分配指数显示，当母体长期暴露于低食物环境时，其繁殖力下降而后代质量提升，由此证明了每单位后代投入的适应性可塑性。与之形成鲜明对比的是，单变量分析几乎未检测到显著的处理效应。本研究表明，多变量分析能够提升我们评估繁殖策略适应性意义的能力，尤其适用于后代大小与适合度难以精准测量的研究场景。此类研究方法未来或可助力评估更广类群中繁殖分配的适应性意义，从而为繁殖策略的演化研究提供更具普适性的理论见解。