Data from: Sexual and natural selection in the evolution of extended phenotypes: the use of green nesting material in starlings

Although sexual selection is typically considered the predominant force driving the evolution of ritualized sexual behaviors, natural selection may also play an important and often underappreciated role. The use of green aromatic plants among nesting birds has been interpreted as a component of extended phenotype that evolved either via natural selection due to potential sanitary functions, or via sexual selection as a signal of male attractiveness. Here we compared both hypotheses using comparative methods in starlings, a group where this behavior is widespread. We found that the use of green plants was positively related to male-biased size dimorphism, and that it was most likely to occur among cavity-nesting species. These results suggest that this behavior is likely favored by sexual selection, but also related to its sanitary use in response to higher parasite loads in cavities. We speculate that the use of green plants in starlings may be facilitated by cavity nesting, and was subsequently co-opted as a sexual signal by males. Our results represent an example of how an extended phenotypic component of males becomes sexually selected by females. Thus, both natural and sexual selection are necessary to fully understand the evolution of ritualized behaviors involved in courtship.

尽管性选择（sexual selection）通常被视为驱动仪式化性行为演化的主导力量，自然选择（natural selection）或许也发挥着重要却常被低估的作用。筑巢鸟类对绿色芳香植物的利用，曾被解读为扩展表型（extended phenotype）的组成部分：该特征既可能因潜在的卫生功能经由自然选择演化而来，也可能作为雄性吸引力的信号通过性选择演化。本研究以该行为广泛分布的椋鸟（starlings）类群为研究对象，采用比较分析方法对这两种假说开展验证。研究结果显示，绿色植物的利用程度与雄性体型偏态性呈显著正相关，且该行为在洞巢鸟类中出现概率最高。上述结果表明，该行为大概率受到性选择的青睐，同时也与其应对洞巢内更高寄生虫负荷的卫生功能密切相关。我们推测，椋鸟的绿色植物利用行为或因洞巢习性得以演化，并随后被雄性用作性信号载体。本研究结果展示了雄性的某一扩展表型特征如何被雌性通过性选择所青睐。综上，要全面理解求偶相关仪式化行为的演化过程，自然选择与性选择二者均不可或缺。