Data from: Sexually selected dichromatism in the hihi Notiomystis cincta: multiple colours for multiple receivers

Why do some bird species show dramatic sexual dichromatism in their plumage? Sexual selection is the most common answer to this question. However, other competing explanations mean it is unwise to assume that all sexual dichromatism has evolved by this mechanism. Even if sexual selection is involved, further work is necessary to determine whether dichromatism results from competition amongst rival males, or by female choice for attractive traits, or both. Here we test whether sexually dichromatic hihi (Notiomystis cincta) plumage is currently under sexual selection, with detailed behavioural and genetic analyses of a free-living island population. Bateman gradients measured for males and females reveal the potential for sexual selection, whilst selection gradients, relating reproductive success to specific colourful traits, show there is stabilizing selection on white ear tuft length in males. By correlating colourful male plumage with different components of reproductive success, we show that properties of yellow plumage are most likely a product of male-male competition, whilst properties of the black and white plumage are an outcome of both male-male competition and female choice. Male plumage therefore potentially signals to multiple receivers (rival males and potential mates) and this may explain the multi-coloured appearance of one of the most strikingly dichromatic species in New Zealand.

为何部分鸟类的羽毛会呈现出显著的雌雄羽色二态性（sexual dichromatism）？性选择（sexual selection）是该问题最主流的解释。然而，学界尚存在其他竞争性假说，因此贸然认定所有雌雄羽色二态性均通过该机制演化而来并不妥当。即便性选择参与了演化过程，仍需进一步研究以明确：羽色二态性究竟源于雄性间的竞争，或是雌性对吸引性性状的选择，抑或是两者共同作用的结果。本研究针对自由生活的岛屿种群开展细致的行为学与遗传学分析，检验具雌雄羽色二态性的hihi（Notiomystis cincta）的羽毛是否正经受性选择作用。针对雌雄个体测得的贝特曼梯度（Bateman gradients）揭示了性选择的潜在作用；而将繁殖成功率与特定彩色性状相关联的选择梯度则表明，雄性的白色耳簇长度存在稳定选择（stabilizing selection）。通过将雄性彩色羽毛与繁殖成功率的不同维度进行关联分析，我们发现黄色羽毛性状极有可能源于雄性间的竞争，而黑白羽毛性状则是雄性间竞争与雌性选择共同作用的结果。因此，雄性羽毛可向多重接收者（雄性竞争者与潜在配偶）传递信号，这或可解释新西兰这一最为引人注目的雌雄羽色二态性物种之一的多彩外观。