Abstract

Maternal allocation of androgens to the egg yolk allows mothers to adaptively manipulate offspring phenotype. Increases of egg androgen levels have often been shown to induce sex-specific effects. Some previous studies suggest that females specifically may suffer a fitness reduction after early exposure to high androgen levels. In this study, we explored whether female birds that developed exposed to high yolk androgen concentrations would reduce their maternal investment later in life. We explored two different aspects of female reproduction in spotless starlings (Sturnus unicolor): egg coloration (a post mating sexual signal in this species) and egg size. We manipulated androgen yolk levels in a large sample of clutches of this species. We monitored the reproduction of the females that hatched from these eggs, measuring the size and colour of the eggs they laid throughout their life (from 1 to 7 years of age). We found no overall difference in egg colour in relation to treatment, clutch number or age. However, females exposed to high egg androgen levels showed a steeper decrease in egg size along the laying order than controls. This pattern likely results in a more unbalanced distribution of resources within the clutch, possibly favouring brood size reductions in experimental females. In addition, control and experimental females differed in how egg volume changed with age. These results show that some egg characteristics may be affected by the female’s exposure to yolk androgens during her own embryonic development. Our research calls for further long-term research on the influence of prenatal androgens on the fitness mechanisms regulating reproductive investment and its potential signalling role in a perspective of sexual selection.

Significance statement

In birds, mothers prepare the offspring to the environment they will encounter at hatching by depositing a mixture of compounds into the eggs. Androgens specifically exert a variety of effects during embryo development and chick growth, but some of these may be negative in adult life. In particular, early androgen may be detrimental for females in the long term, decreasing maternal investment and fitness. Using the spotless starling as study species, we tested whether an experimental increase in yolk androgens negatively affects breeding investment, measured by egg colour (a sexual signal) and egg size. We found no effects of the treatment on egg colour. However, females exposed to high androgen levels during their development reduced egg size along the laying order more strongly than controls. This may affect nestling survival, and indicates that prenatal yolk androgens may influence some egg characteristics with known relevance for offspring viability.

中文翻译：

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产前对蛋黄雄激素水平的控制会影响蛋的大小，但不会影响鸣禽中的蛋颜色

摘要

母体将雄激素分配给蛋黄使母亲能够适应性地操纵后代表型。鸡蛋雄激素水平的升高通常已被证明可诱发特定性别的影响。先前的一些研究表明，女性在早期暴露于高雄激素水平下可能会导致健康降低。在这项研究中，我们探讨了暴露于高蛋黄雄激素浓度下发育的雌鸟是否会在以后的生活中减少母本投资。我们探索了一尘不染的八哥（Sturnus unicolor）中女性生殖的两个不同方面）：卵的颜色（该物种的交配后性信号）和卵的大小。我们在大量此类离合器的样品中操纵了雄激素的卵黄水平。我们监测了从这些卵中孵出的雌性的繁殖情况，测量了它们一生（1至7岁）产下的卵的大小和颜色。我们发现蛋色与治疗方法，离合器数量或年龄没有总体差异。但是，暴露于高鸡蛋雄激素水平的雌性在产蛋顺序上显示出比对照大得多的卵大小下降。这种模式可能导致离合器内的资源分配更加不平衡，可能有利于减少实验雌性的育雏尺寸。此外，对照和实验雌性在卵量随年龄变化的方式上也有所不同。这些结果表明，某些卵的特性可能受雌性自身胚胎发育过程中暴露于卵黄雄激素的影响。我们的研究要求就性别选择的观点，对产前雄激素对调节生殖投资的适应机制及其潜在信号作用的影响进行进一步的长期研究。

重要性声明

在鸟类中，母亲通过将化合物的混合物沉积到卵中来为后代在孵化中遇到的环境做好准备。雄激素特别在胚胎发育和雏鸡生长过程中发挥多种作用，但其中一些对成年生活可能是不利的。尤其是，长期而言，早期雄激素可能会对女性有害，从而降低孕产妇的投资和健康状况。我们使用一尘不染的star鸟作为研究物种，测试了卵黄雄激素的实验性增加是否会对繁殖投资产生负面影响（以蛋的颜色（性信号）和蛋的大小衡量）。我们发现该处理对蛋色没有影响。但是，在发育过程中暴露于高雄激素水平的雌性比对照组在产蛋顺序上更能减小卵的大小。这可能会影响雏鸟的生存，