The evolution and function of female trait exaggeration in species with traditional sex roles are poorly understood. Competition for social and ecological resources, rather than mating opportunities, may be a key selection pressure in females of socially monogamous species. Moreover, such pre-existing resource competition-selected traits (armaments) may become the object of mating preferences (ornaments); however, the benefits that males may gain in choosing more ornamented females remain unclear. Here, using observational data on free-living female great tits (Parus major), we explore whether tie size, a well-known melanin-pigmented plumage trait, covaries with intrasexual aggressiveness during the breeding season and the investment in current reproduction (fledgling number and mass). We found consistent individual differences (i.e. personality variation) in most of the aggressiveness parameters studied. Moreover, we found support for a signalling role of tie size with respects to how close females approached intruders. Interestingly, this relationship was only evident in older females, not first-year breeding females, suggesting age-related changes in the information content of tie size in the context of same-sex aggressiveness. Additionally, although first-year breeding females, on average, produced fledglings with lower body mass compared to older females, tie size was positively associated with fledgling mass, suggesting that males could potentially gain benefits by choosing females with larger tie size. Experimental studies are now necessary to confirm whether females with larger tie size have a selective advantage in female competition (i.e. armament), as well as whether males base their mate choice on female tie size (i.e. ornament). Why females of many species with traditional sex roles often have exaggerated traits very similar to those found in males remains poorly understood. We explored the functional significance of a well-known melanin-based plumage trait (tie size) in females of a socially monogamous passerine bird, the great tit. Female tie size positively covaried with consistent individual differences in aggressiveness during female-female interactions, but only so in older females and not first-year breeding females. In addition, females with larger tie size produced fledglings with higher body mass, suggesting that it could potentially be used by males in mate choice. Our study emphasizes that greater attention should be paid to female trait exaggeration in socially monogamous species in the context of female-female competition and male mate choice, as well as to potential age-related changes in the information they convey.

中文翻译：

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领带适合女性吗？雌性巨乳（Parus Major）基于黑色素的着色、攻击性人格和生殖投资

对具有传统性别角色的物种中雌性特征夸大的进化和功能知之甚少。社会和生态资源的竞争，而不是交配机会，可能是社会一夫一妻制物种雌性的关键选择压力。而且，这种预先存在的资源竞争选择的特征（武器）可能会成为交配偏好（装饰）的对象；然而，雄性在选择更多装饰的雌性时可能获得的好处仍不清楚。在这里，我们使用关于自由生活的雌性大山雀（Parus major）的观察数据，探索领带大小（一种众所周知的黑色素羽毛特征）是否与繁殖季节期间的性内攻击性以及对当前繁殖（雏鸟数量）的投资发生共变和质量）。我们发现一致的个体差异（即 个性变化）在大多数研究的攻击性参数中。此外，我们发现在女性接近入侵者的距离方面，领带大小的信号作用得到了支持。有趣的是，这种关系只在年长的雌性中明显，而不是在第一年繁殖的雌性中，这表明在同性攻击性的背景下领带大小的信息内容会发生与年龄相关的变化。此外，虽然平均而言，第一年繁殖的雌性与年长的雌性相比产生了体重较低的雏鸟，领带的大小与幼鸟的质量呈正相关，这表明雄性可能通过选择领带尺寸较大的雌性而获得收益。现在有必要进行实验研究来确认领带尺寸较大的女性是否在女性竞争（即武器）中具有选择优势，以及男性是否根据女性领带尺寸（即装饰品）选择配偶。为什么许多具有传统性别角色的物种的雌性通常具有与雄性非常相似的夸张特征仍然知之甚少。我们探索了一种众所周知的基于黑色素的羽毛特征（领带大小）在社会一夫一妻制的雀鸟（大山雀）的雌性中的功能意义。雌性领带大小与雌性相互作用期间攻击性的一致个体差异呈正相关，但仅在年长雌性而非第一年繁殖雌性中如此。此外，领带尺寸较大的雌性会产生体重较高的雏鸟，这表明雄性可能会在配偶选择中使用它。