In egg-laying animals, mothers can influence the developmental environment and thus the phenotype of their offspring by secreting various substances into the egg yolk. In birds, recent studies have demonstrated that different yolk substances can interactively affect offspring phenotype, but the implications of such effects for offspring fitness and phenotype in natural populations have remained unclear. We measured natural variation in the content of 31 yolk components known to shape offspring phenotypes including steroid hormones, antioxidants and fatty acids in eggs of free-living great tits (Parus major) during two breeding seasons. We tested for relationships between yolk component groupings and offspring fitness and phenotypes. Variation in hatchling and fledgling numbers was primarily explained by yolk fatty acids (including saturated, mono- and polyunsaturated fatty acids) - but not by androgen hormones and carotenoids, components previously considered to be major determinants of offspring phenotype. Fatty acids were also better predictors of variation in nestling oxidative status and size than androgens and carotenoids. Our results suggest that fatty acids are important yolk substances that contribute to shaping offspring fitness and phenotype in free-living populations. Since polyunsaturated fatty acids cannot be produced de novo by the mother, but have to be obtained from the diet, these findings highlight potential mechanisms (e.g., weather, habitat quality, foraging ability) through which environmental variation may shape maternal effects and consequences for offspring. Our study represents an important first step towards unraveling interactive effects of multiple yolk substances on offspring fitness and phenotypes in free-living populations. It provides the basis for future experiments that will establish the pathways by which yolk components, singly and/or interactively, mediate maternal effects in natural populations.

中文翻译：

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蛋黄脂肪酸的自然变异，而不是雄激素，预测野生鸟类的后代健康

在产卵动物中，母亲可以通过将各种物质分泌到蛋黄中来影响发育环境，从而影响后代的表型。在鸟类中，最近的研究表明，不同的蛋黄物质可以相互作用地影响后代表型，但这种影响对自然种群中后代健康和表型的影响仍不清楚。我们测量了自由生活的大山雀（Parus Major）在两个繁殖季节的卵中，已知可塑造后代表型的 31 种蛋黄成分含量的自然变化，包括类固醇激素、抗氧化剂和脂肪酸。我们测试了蛋黄成分分组与后代健康和表型之间的关系。孵化和雏鸟数量的变化主要是由蛋黄脂肪酸（包括饱和、单不饱和脂肪酸和多不饱和脂肪酸）——但不受雄激素和类胡萝卜素影响，这些成分以前被认为是后代表型的主要决定因素。与雄激素和类胡萝卜素相比，脂肪酸也是雏鸟氧化状态和体型变化的更好预测因子。我们的研究结果表明，脂肪酸是重要的蛋黄物质，有助于塑造自由生活人群的后代健康和表型。由于多不饱和脂肪酸不能由母亲从头产生，而必须从饮食中获得，这些发现突出了潜在的机制（例如天气、栖息地质量、觅食能力），通过这些机制，环境变化可能会影响母体对后代的影响和后果. 我们的研究代表了解开多种蛋黄物质对自由生活人群后代健康和表型的相互作用影响的重要第一步。它为未来的实验奠定了基础，这些实验将建立蛋黄成分单独和/或相互作用在自然种群中介导母体效应的途径。