Data from: No fitness benefits of early molt in a fairy-wren: relaxed sexual selection under genetic monogamy?

The evolution of male ornamentation has long been the focus of sexual selection studies. However, evidence is accumulating that sexually selected traits can also be lost, although the process is ill-understood. In male fairy-wrens (<i>Malurus</i> spp.), early molt into the seasonal breeding plumage is critical for obtaining extra-pair paternity (EPP), which reaches very high levels in these socially monogamous songbirds. A notable exception is the purple-crowned fairy-wren, <i>Malurus coronatus</i>, which, like its congeners, breeds cooperatively, but where EPP is very rare. Nevertheless, males develop a conspicuous seasonal breeding plumage at highly variable times. Based on six years of molt data collected for 137 individuals, we investigated the adaptive significance of pre-breeding molt timing as a sexual signal under (near) genetic monogamy. Molt timing varied between and within individuals with age and climate: molt was completed earlier in older males and after wetter years. Despite its potential to act as a sexual signal of male quality, fitness benefits and costs of early molt appear limited: molt timing did not correlate with (i) the likelihood of gaining a breeding position; (ii) female mate preference (EPP/cuckoldry, divorce); (iii) female reproductive investment (breeding timing, clutch size, number of clutches); (iv) breeding performance (hatching success, fledging success, fledgling survival, annual reproductive success); and (v) male survival. However, although molt timing did not predict which subordinates would become breeders, breeders molted earlier than subordinates. The lack of EPP in this species might imply relaxed sexual selection on early molt with potential to lead to trait disappearance.

雄性饰羽的演化长期以来都是性选择研究的核心议题。然而，越来越多的证据表明，性选择青睐的性状也可能发生丢失，尽管这一过程的机制仍未被充分阐明。在细尾鹩莺属（Malurus spp.）中，雄性提前换羽形成季节性繁殖羽是获得婚外父权（extra-pair paternity, EPP）的关键条件，这类社会单配制鸣禽的婚外父权比例普遍极高。但紫冠细尾鹩莺（Malurus coronatus）是个显著例外：尽管它与同属物种一样存在合作繁殖行为，但其婚外父权事件却极为罕见。尽管如此，雄性个体形成醒目季节性繁殖羽的时间却存在极高的个体间与个体内变异性。本研究基于对137只个体长达6年的换羽观测数据，探讨了在（近乎）遗传单配制的情境下，繁殖前换羽时间作为性选择信号的适应性意义。换羽时间随个体年龄与气候条件呈现个体间与个体内差异：老年雄性的换羽完成时间更早，且湿润年份后换羽完成得更早。尽管提前换羽具备作为雄性质量的性选择信号的潜力，但其相关的适合度收益与成本似乎十分有限：换羽时间并未与以下指标存在相关性：（i）获得繁殖地位的概率；（ii）雌性配偶偏好（婚外父权/配偶不忠、离婚）；（iii）雌性繁殖投入（繁殖时间、窝卵数、繁殖窝数）；（iv）繁殖成效（孵化成功率、离巢成功率、幼鸟离巢后存活率、年度繁殖成功率）；以及（v）雄性存活率。不过，尽管换羽时间无法预测哪些从属个体能够晋升为繁殖个体，但繁殖个体的换羽时间确实早于从属个体。该物种缺乏婚外父权的现象，或许意味着针对提前换羽的性选择压力已趋松弛，进而可能引发该性状的消失。