Data from: Context-dependence of maternal effects: testing assumptions of optimal egg size, differential- and sex-allocation models

If offspring develop in adverse conditions, the maternal component of their phenotypic variation might increase due to the stronger dependence of offspring traits on parental investment. This should result in increased parental investment to individual offspring, as assumed by the model of optimal egg size. The opposite pattern, i.e., stronger dependence of offspring fitness on parental investment and consequently larger parental investment under good conditions is assumed by both the theory of differential allocation if attractive males provide material benefits, and reproductive compensation if they invest less into paternal care. Another influential idea is the Trivers-Willard model, which assumes sex-specific dependence of offspring fitness on parental investment. Here we tested these ideas by examining the effects of egg size on offspring fitness across many postnatal contexts in the Collared Flycatcher Ficedula albicollis. We employed a cross-fostering design that generated variation in egg size within nests and used brood means of fledgling mass as a functional measure of the quality of rearing conditions. Effects of egg size on three offspring traits, including lifetime reproductive success of recruits, were more pronounced in low-quality broods. These results support the assumption of the model of optimal egg size. Based on female preference for males providing material benefits, this pattern could support differential allocation, if attractive males invest less in paternal care, or reproductive compensation, if they invest more. By comparison, we did not find any evidence for sex specificity of fitness returns that might explain sex monomorphism of egg size in this species. The challenge for future studies will be the integration of components of parental investment and offspring fitness into their global measures and testing how the former affects the latter across gradients of postnatal conditions.

当后代在不利环境中发育时，其表型变异中的母本组分可能会增加，因为后代性状对亲本投入的依赖性更强。这应当会导致亲本对单个后代的投入增加，正如最优卵大小模型（model of optimal egg size）所假设的那样。与之相反的模式，即后代适合度对亲本投入的依赖性更强，因此在良好环境下亲本投入更高，这一假设同时存在于两个理论中：若具有吸引力的雄性提供物质收益，则为差异化分配理论（differential allocation theory）；若雄性对父本抚育的投入更少，则为繁殖补偿理论（reproductive compensation theory）。另一个具有影响力的理论是特里弗斯-威拉德模型（Trivers-Willard model），该模型假设后代适合度对亲本投入的依赖性存在性别特异性。本研究以领姬鹟（*Ficedula albicollis*）为研究对象，通过检测不同产后环境下卵大小对后代适合度的影响，对上述理论进行了检验。我们采用交叉抚育实验设计（cross-fostering design），使同一巢内的卵大小产生变异，并以雏鸟离巢质量的巢均值作为抚育环境质量的功能性衡量指标。在低质量抚育巢中，卵大小对三项后代性状（包括招募个体的终身繁殖成功率）的影响更为显著。这些结果支持了最优卵大小模型的假设。基于雌性对提供物质收益的雄性的偏好，若具有吸引力的雄性对父本抚育的投入更少，则该模式可支持差异化分配理论；若其投入更多，则可支持繁殖补偿理论。相较而言，我们未发现任何能够解释该物种种群卵大小性别单型现象的适合度回报性别特异性证据。未来研究的挑战在于，将亲本投入与后代适合度的组分整合为全局衡量指标，并检验前者在产后环境梯度下对后者的影响机制。