Data from: Dressed to impress: breeding plumage as a reliable signal of innate immunity

Animal signals involved in sexual selection are often indicators of individual quality. The assumption that sexual characters such as breeding plumage may indicate immune state has rarely been tested in free-living migratory birds, particularly in relation to innate immunity. If sexual characters indeed reflect immune condition, then these could be used to evaluate individual quality. Melanin is a common pigment used in animal communication that mitigates the effects of oxidative stress and has positive effects on energy homeostasis, important functions during the strenuous activity of long-distance flights. However, melanin is also immunosuppressive, and the melanised patches of breeding plumage may to some extent compromise immune responsiveness. We studied melanin-based secondary sexual characters (SSC) in a long-distance migratory wader, the Black-tailed godwit (Limosa limosa), and found that breeding plumage features of male and female godwits are linked to components of innate immunity. Males with a larger colour extension had higher circulating levels of haptoglobin and hemolysis activity, while they also presented a lower body condition; whereas females presented a negative trend between colour and bar extension and hemolysis activity, and a positive trend for natural antibodies. The association between signal, immune state and physical condition in males suggests a cost for signal production and immune condition during prenuptial migration. Sex differences in how signals relate with immune capacity are a likely consequence of sex-specific signalling roles and energy demands. Our results indicate that male godwit breeding plumage reflects innate immunity state, and is therefore a likely signal for females to use during mate choice as an honest indicator of male’s capacity to allocate energy/resources to both expensive traits during periods of energetic constraint.

参与性选择（sexual selection）过程的动物信号，通常是个体品质的指示性指标。此前针对野生迁徙鸟类，鲜有研究检验“繁殖羽等性征可反映免疫状态”这一假设，尤其缺乏针对先天免疫（innate immunity）的相关验证。若性征确实能够反映免疫状况，则可借此评估个体品质。黑色素（melanin）是动物通讯中常见的色素，它能够减轻氧化应激（oxidative stress）的负面影响，并对能量稳态（energy homeostasis）产生积极作用，而能量稳态在长途飞行的剧烈活动中发挥着关键功能。不过，黑色素同时具备免疫抑制（immunosuppressive）效应，繁殖羽中的黑色素沉着区域可能在一定程度上削弱免疫应答能力。我们以长途迁徙涉禽黑尾塍鹬（Black-tailed godwit, Limosa limosa）为研究对象，对基于黑色素的次级性征（secondary sexual characters, SSC）展开了研究，结果发现雌雄黑尾塍鹬的繁殖羽特征与先天免疫组分存在显著关联。体色延展范围更大的雄性个体，其循环结合珠蛋白（haptoglobin）水平与溶血活性（hemolysis activity）更高，但体况（body condition）却更低；而雌性个体的体色与斑纹延展范围和溶血活性呈负相关趋势，与天然抗体（natural antibodies）则呈正相关趋势。雄性个体的信号表达、免疫状态与身体状况之间的关联，提示其在繁殖前迁徙阶段，信号产生与免疫维持存在适配代价。信号与免疫能力之间的关联存在性别差异，这大概率源于性别特异性的信号功能与能量需求差异。我们的研究结果表明，黑尾塍鹬雄性的繁殖羽能够反映其先天免疫状态，因此这一特征可能是雌性在配偶选择（mate choice）过程中用以评估雄性的诚实信号（honest indicator），体现雄性在能量受限时期为两类高成本性状分配能量/资源的能力。