Consistent between-individual differences in behaviour have been documented across the animal kingdom. Such variation between individuals has been shown to be the basis for selection and to act as a pacemaker for evolutionary change. Recently, equivocal evidence suggests that such consistent between-individual variation is also present in hormones. This observation has sparked interest in understanding the mechanisms shaping individual differences, temporal consistency and heritability of hormonal phenotypes and to understand, if and to what extent hormonal mechanisms are involved in mediating consistent variation in behaviour between individuals. Here, we used zebra finches of the fourth generation of bi-directionally selected lines for three independent behaviours: aggression, exploration and fearlessness. We investigated how these behaviours responded to artificial selection and tested their repeatability. We further tested for repeatability of corticosterone and testosterone across and within lines. Moreover, we are presenting the decomposed variance components for within-individual variance (i.e. flexibility) and between-individual variance (i.e. more or less pronounced differences between individuals) and investigate their contribution to repeatability estimates. Both hormones as well as the exploration and fearlessness but not aggressiveness, were repeatable. However, variance components and hence repeatability differed between lines and were often lower than in unselected control animals, mainly because of a reduction in between-individual variance. Our data show that artificial selection (including active selection and genetic drift) can affect the mean and variance of traits. We stress the importance for understanding how variable a trait is both between and within individuals to assess the selective value of a trait.

中文翻译：

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为个性而双向选择的斑马雀在皮质酮和睾丸激素的重复性上有所不同。

整个动物界都有一致的个体差异。个体之间的这种差异已被证明是选择的基础，并且是进化变化的起搏器。最近，模棱两可的证据表明，激素之间也存在这种一致的个体间差异。这项发现激发了人们对了解形成个体差异，荷尔蒙表型的时间一致性和遗传力的机制，以及了解荷尔蒙机制是否以及在何种程度上介导个体之间行为一致变化的兴趣。在这里，我们将第四代双向选择线的斑马雀用于三种独立的行为：侵略，探索和无畏。我们调查了这些行为如何响应人工选择并测试了其可重复性。我们进一步测试了跨行和跨行的皮质类固醇和睾丸激素的可重复性。此外，我们提出了个体内部方差（即灵活性）和个体间方差（即个体之间或多或少明显的差异）的分解方差成分，并研究了它们对可重复性估计的贡献。这两种激素以及探索和无畏而不是侵略性都是可以重复的。但是，品系之间的差异成分以及因此的可重复性不同，并且通常低于未选择的对照动物，这主要是由于个体间差异的减少。我们的数据表明，人工选择（包括主动选择和遗传漂移）会影响性状的均值和方差。我们强调了解个体之间和个体内部特征的可变性以评估特征的选择性价值的重要性。