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.

若后代在不利环境中发育，其表型变异中的母系成分可能会增强，因为后代性状对亲本投入的依赖性更强。这将导致亲本对单个后代的投入增加，正如最优卵大小模型（optimal egg size model）的假设所言。与之相反的模式——即后代适合度对亲本投入的依赖性更强，且在优良环境下亲本投入更高——则同时被两个理论所采用：若有吸引力的雄性提供物质收益，则为差异化分配（differential allocation）理论；若雄性对父本抚育投入更少，则为生殖补偿（reproductive compensation）理论。另一个颇具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，通过检测卵大小对多个产后环境下后代适合度的影响，对上述假说进行了验证。我们采用了交叉抚育实验设计（cross-fostering design），在窝巢内制造卵大小的变异，并以离巢幼鸟体重的窝群均值作为抚育环境质量的功能性衡量指标。卵大小对三项后代性状（包括招募个体的终生繁殖成功率）的影响，在低质量窝群中更为显著。上述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，若有吸引力的雄性对父本抚育投入更少，则该结果可支持差异化分配理论；若投入更多，则可支持生殖补偿理论。相比之下，我们未发现任何能够解释该物种种卵大小性别单态性的适合度回报性别特异性证据。未来研究的核心挑战在于，将亲本投入与后代适合度的各组分整合为综合衡量指标，并探究产后环境梯度下前者对后者的调控效应。