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 (Malurus 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, Malurus coronatus, 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 6 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 1) the likelihood of gaining a breeding position; 2) female mate preference (EPP/cuckoldry, divorce); 3) female reproductive investment (breeding timing, clutch size, number of clutches); 4) breeding performance (hatching success, fledging success, fledgling survival, annual reproductive success); and 5) 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，性选择所青睐的性状同样可能发生丢失，尽管这一过程的机制仍未被充分阐明。在细尾鹩莺属（Malurus spp.）鸟类中，雄性提前换得季节性繁殖羽对于获得婚外父权（extra-pair paternity, EPP）至关重要，而这类社会单配制鸣禽的婚外父权比例通常极高。一个显著的例外是紫冠细尾鹩莺（Malurus coronatus）：该物种与同属其他物种一样营合作繁殖，但婚外父权极为罕见。尽管如此，雄性个体仍会在差异极大的时间范围内发育出醒目的季节性繁殖羽。
本研究基于137只个体的6年换羽观测数据，探讨了在（近乎）遗传单配制的背景下，繁殖前换羽时间作为性信号的适应性意义。研究发现，换羽时间存在个体间与个体内的差异，且受年龄与气候因素影响：老年雄性的换羽完成时间更早，且在湿润年份后换羽完成得更早。尽管该换羽时间具备作为雄性质量的性信号的潜力，但早期换羽的适合度收益与代价似乎均较为有限：换羽时间与以下因素均无显著关联：1）获得繁殖地位的概率；2）雌性配偶偏好（婚外父权/配偶不忠、离异）；3）雌性繁殖投入（繁殖时间、窝卵数、繁殖窝数）；4）繁殖表现（孵化成功率、离巢成功率、幼鸟存活率、年度繁殖成功率）；以及5）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成为繁殖者，但繁殖者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，或许意味着早期换羽所承受的性选择压力有所松弛，进而可能导致该性状的消失。