Barn Swallows in East Asia show strong tolerance against nest mites

ABSTRACT Hosts defend against parasites through two main strategies: resistance and tolerance. These two strategies have different influences on the parasite load of the host and so have variable effects on the fitness of parasites. In this study, we investigated the strategy used by the Barn Swallow (Hirundo rustica) to defend against a hematophagous mite (Dermanyssus hirundinis) in East Asia. For the first time, we found Barn Swallows were parasitized by D. hirundinis in Asia and, in northeastern China, over 60% of broods were parasitized. We examined whether swallows with better body condition or the males with more extravagant sexually selected traits (longer tail streamers or redder ventral plumage) showed stronger resistance or tolerance to these parasites. The results showed that males with more extravagant sexual ornaments had weaker resistance (heavier brood mite load) than males with less extravagant traits; however, the heavier mite load was likely to be a byproduct of larger brood size. Moreover, the Barn Swallow population showed strong tolerance against the parasite (i.e. the brood mite load did not affect brood size or nestling body mass). It is worth noting that male swallows with redder breast plumage showed weaker tolerance (lower seasonal reproductive success) than other males. Further, we conducted experimental ectoparasite manipulations in 46 Barn Swallow nests to verify the potential effect of these parasites on the body condition of nestlings. Neither the body mass nor the hemoglobin concentration differed between the designed groups; however, the hemoglobin concentration of nestlings decreased with mite load. We concluded that the Barn Swallow population in East Asia used the tolerance strategy, rather than resistance to defend against nest mites, and the ectoparasitic mite did not appear to be a strong selective factor for reproductive success owing to the strong tolerance of the studied Barn Swallow population. LAY SUMMARY To defend against parasites, hosts either reduce parasite burden (resistance) or compensate for parasite damage (tolerance). In this study, we are the first to report the existence of a bloodsucking parasite species Dermanyssus hirundinis on Barn Swallow (Hirundo rustica) nestlings in Asia, and we investigated the swallows defense against this ectoparasite. Different from previous studies which showed that Barn Swallow males with longer tails or redder plumage coloring had stronger resistance against nest mites, our results showed that such males had weaker resistance to ectoparasites in northeastern China, which was likely to be a byproduct of a larger brood size. As heavier mite load did not appear to impair their reproductive success, the studied Barn Swallow population showed strong overall tolerance to these ectoparasites; however, the nest mite could still affect swallow nestling hemoglobin concentration. 摘要 宿主主要通过两种策略防御寄生虫：抵抗力和忍耐力。 采取这两种策略对于宿主的寄生虫负载量有不同的影响，从而对于寄生虫的适合度产生不同 影响。 在本项研究中，我们探讨了位于亚洲东部的一个家燕（Hirundo rustica）种群防御某种吸血型螨虫（燕皮刺螨，Dermanyssus hirundinis）所采取 的策略。 我们首次报道了亚洲家燕被燕皮刺螨所寄生：在中国东北地区，有超过60%的家燕巢被这种螨虫寄生。 我们检验了具有更好身体状况或更 明显性选择特征（更长的尾羽飘带或更红的腹侧羽色）的家燕雄性个体是否对燕皮刺螨表现出更强的抵抗力或忍耐力。 研究结果显示，具有更明显性 选择特征的雄性个体较其他雄性个体具有更弱的抵抗力（巢内螨虫负载量较高）；而较高的巢螨负载量似乎是巢内雏鸟更多所带来的副产品。 此外， 家燕种群整体对于燕皮刺螨表现出较强的忍耐力，即巢内的螨虫负载量并不影响家燕窝雏数或雏鸟体重。 需要注意的是，具有更红胸部羽色的雄性 家燕较其他个体表现出更弱的忍耐力（年度繁殖成效较低）。 进一步地，我们通过在46个家燕巢内人为操控寄生虫数量的实验来证实燕皮刺螨对于 雏鸟身体状况的潜在影响。 不同实验组别间家燕雏鸟的体重或血红蛋白含量并无显著差异，但是雏鸟的血红蛋白含量整体上随螨虫负载量的升高而 降低。 我们认为，东亚的家燕种群采取了忍耐力策略而非抵抗力策略来对螨虫进行防御；所研究的家燕种群对于燕皮刺螨较强的忍耐力使这种体表 寄生虫不足以成为作用于家燕繁殖成效的较强选择压力。 关键词：燕皮刺螨，家燕，体表寄生虫，繁殖成效，抵抗力，忍耐力，性选择