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，经性选择所青睐的性状同样可能发生丢失，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.）的雄性个体中，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极高。一个显著的例外是紫冠细尾鹩莺（Malurus coronatus）：尽管其与同属物种一样实行合作繁殖，但该物种的婚外父权现象极为罕见。即便如此，雄性个体仍会在高度可变的时间节点发育出醒目的季节性繁殖羽。本研究基于针对137只个体收集的6年换羽数据，探究了在近乎遗传单配制的体系下，繁殖前换羽时机作为性信号的适应性意义。换羽时机在个体间及个体内均随年龄与气候条件存在差异：年长雄性的换羽完成更早，且湿润年份后的换羽进程也会提前。尽管提前换羽具备作为雄性质量的性信号的潜力，但其适合度收益与成本似乎均较为有限：换羽时机与以下5项因素均无显著关联：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、离婚）；3）雌性繁殖投入（繁殖时机、窝卵数、窝次数量）；4）繁殖表现（孵化成功率、离巢成功率、雏鸟存活率、年度繁殖成功率）；5）雄性存活率。不过，尽管换羽时机无法预测哪些从属个体将成为繁殖者，但繁殖个体的换羽时间确实早于从属个体。本物种中婚外父权的缺失，或许意味着针对提前换羽的性选择压力已趋松弛，进而可能导致该性状的消失。