Data from: Females mate with males with diminished pheomelanin-based coloration in the Eurasian nuthatch Sitta europaea

Sexual selection can drive the evolution of phenotypic traits because of female preferences for exaggerated trait expression in males. Sexual selection can also lead to the evolutionary loss of traits, a process to which female preferences for diminished male trait expression are hypothesized to contribute. However, empirical evidence of female preferences for diminished male traits is virtually lacking. Eurasian nuthatches Sitta europaea provide an opportunity to test this possibility, as a chestnut flank patch produced by the pigment pheomelanin is present since the first plumage of these birds and its color is more intense in nestlings in poor condition in our study population. It has been proposed that developing birds in poor condition may increase their production of pheomelanin as a detoxifying strategy. Female nuthatches may thus prefer mating with males showing flank feathers of diminished color, as this could indicate that males experienced good conditions early in development, which can positively affect the fitness of future generations. Here we show results according with this prediction in a wild population of Eurasian nuthatches, as adult males with lighter chestnut feathers paired earlier in the season, while chestnut coloration had no effect on female mating success. Chestnut color expression was not affected by the body condition of birds, suggesting that females obtain information on the body condition in early life of their potential mates and not on their current body condition. This constitutes one of the few examples of females mating with males showing diminished traits and provides the only explanation so far by which this process can occur.

性选择（Sexual selection）可推动表型性状（phenotypic traits）的演化，核心驱动因素为雌性对雄性夸张性状表达的偏好。性选择亦可引发性状的演化丢失，该过程被假说认为由雌性对雄性弱化性状表达的偏好所促成。然而，目前几乎缺乏雌性偏好弱化雄性性状的实证证据。欧亚䴓（Eurasian nuthatches, Sitta europaea）为检验这一假说提供了理想的研究模型：该物种雏鸟的首次羽衣即带有褐黑素（pheomelanin）沉积形成的栗色胁斑，且在我们的研究种群中，身体状况较差的雏鸟的该胁斑颜色更深。有研究提出，身体状况欠佳的发育中鸟类可通过增加褐黑素的产生作为解毒策略。因此，欧亚䴓雌性可能更倾向于与胁羽颜色更浅（即弱化的性状）的雄性交配，因为这可表明该雄性在发育早期曾处于良好的生长环境，进而对后代的适合度产生积极影响。本研究在一个野生欧亚䴓种群中验证了这一预测，结果显示：胁羽呈浅栗色的成年雄性在繁殖季更早完成配对，而栗色着色程度对雌性的交配成功率无显著影响。成年个体当前的身体状况并未影响其胁部栗色的表达，这表明雌性获取的是潜在配偶发育早期的身体状况信息，而非其当前的身体状况。本研究是少数发现雌性偏好与携带弱化性状雄性交配的案例之一，同时也是目前首个对该性状演化丢失过程作出合理解释的研究。