Data and code for: Optimal clutch size revisited: separating individual quality from the costs of reproduction

Life-history theory, central to our understanding of diversity in
morphology, behaviour and senescence, describes how traits evolve through
the optimisation of trade-offs in investment. Despite considerable study,
there is only minimal support for trade-offs within species between the
two traits most closely linked to fitness – reproduction and survival –
questioning the theory’s general validity. We used a meta-analysis to
separate quality effects from the costs of reproduction using studies of
parental investment and survival in birds. Experimental enlargement of
broods caused reduced parental survival. However, the effect of
experimental enlargements was small and opposite to the effect of
phenotypic quality, where individuals that naturally produced larger
clutches also survived better. The opposite effects on survival in
experimental and observational studies of parental care provide the first
meta-analytic evidence for theory suggesting that quality differences mask
trade-offs. Fitness projections using the overall effect size revealed
that reproduction presented negligible costs, except when reproductive
effort was forced beyond the level observed within species, to that seen
between species. We conclude that there is little support for the most
fundamental life-history trade-off, between reproduction and longevity,
operating within a population. We suggest that within species, the fitness
landscape of the reproduction–survival trade-off is flat until it reaches
the boundaries of the between-species fast-slow life-history continuum.
Our interpretation explains why the costs of reproduction are not apparent
and why variation in reproductive output persists within species.

生活史理论（Life-history theory）是理解生物形态、行为与衰老多样性的核心框架，该理论阐述了生命性状如何通过优化资源投入的权衡关系演化而来。尽管已有大量相关研究，但在物种内部，与适合度（fitness）关联最紧密的两个性状——繁殖与存活——之间的权衡关系仅得到极少的实证支持，这引发了学界对该理论普适性的质疑。我们针对鸟类亲本投入与存活的相关研究开展了元分析（meta-analysis），以区分繁殖成本与个体质量效应。对窝卵数进行实验性扩增会导致亲本存活率下降。然而，实验扩增带来的效应规模极小，且与表型质量（phenotypic quality）的效应方向相反：自然产大窝卵的个体存活表现反而更优。亲本抚育相关的实验研究与观察研究在存活效应上呈现相反结果，这为“个体质量差异会掩盖权衡关系”的理论提供了首项元分析证据。基于整体效应量的适合度预测显示，繁殖几乎不存在显著成本，唯有当繁殖投入被强制提升至超出物种内观测水平、达到物种间观测水平时，成本才会显现。我们的研究结论表明，种群内部最核心的生活史权衡——繁殖与寿命之间的权衡——几乎没有实证支撑。我们提出，在物种内部，繁殖-存活权衡的适合度景观处于平坦状态，直至触及物种间“快-慢”生活史连续体的边界。该解释阐明了为何繁殖成本难以被观测到，以及为何物种内繁殖产出的变异始终存在。