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，经性选择塑造的性状也可能发生丢失，尽管这一过程的内在机制仍未被充分阐明。在细尾鹩莺属（Malurus spp.）鸟类中，提前换羽形成季节性繁殖羽是获取婚外父权（extra-pair paternity，EPP）的关键环节——这类社会单配制鸣禽的婚外父权比例普遍极高。紫冠细尾鹩莺（Malurus coronatus）是个显著例外：尽管它与同属物种一样采取合作繁殖策略，但该物种的婚外父权极为罕见。即便如此，该物种的雄性个体仍会在时间差异极大的节点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，探究了在近乎遗传单配制的背景下，繁殖前换羽时间作为雄性性信号的适应性意义。换羽时间在个体间与个体内均存在显著差异，且受年龄与气候因素调控：老年雄性的换羽完成时间更早，湿润年份后的换羽进程也普遍提前。尽管换羽时间具备作为雄性质量性信号的潜力，但提前换羽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）个体获得繁殖地位的概率；（ii）雌性配偶偏好（婚外父权/配偶不忠、离婚事件）；（iii）雌性繁殖投入（繁殖启动时间、窝卵数、繁殖窝数）；（iv）繁殖表现（孵化成功率、雏鸟出飞成功率、幼鸟存活率、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者，但繁殖个体的换羽时间整体早于从属个体。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于松弛，这一过程可能最终导致该性状的消失。