為何大杜鵑的蛋總能這麼像其他鳥蛋？
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| 大杜鵑 (Cuculus canorus) photo: Planet of Birds |
標題：A shared chemical basis of avian host−parasite egg colour mimicry
摘要
Avian brood parasites lay their eggs in other birds’ nests and impose considerable fitness costs on their hosts. Historically and scientifically, the best studied example of circumventing host defences is the mimicry of host eggshell colour by the common cuckoo (Cuculus canorus). Yet the chemical basis of eggshell colour similarity, which impacts hosts’ tolerance towards parasitic eggs, remains unknown. We tested the alternative scenarios that (i) cuckoos replicate host egg pigment chemistry, or (ii) cuckoos use alternative mechanisms to produce a similar perceptual effect to mimic host egg appearance. In parallel with patterns of similarity in avian-perceived colour mimicry, the concentrations of the two key eggshell pigments, biliverdin and protoporphyrin, were most similar between the cuckoo host-races and their respective hosts. Thus, the chemical basis of avian host–parasite egg colour mimicry is evolutionarily conserved, but also intraspecifically flexible. These analyses of pigment composition reveal a novel proximate dimension of coevolutionary interactions between avian brood parasites and hosts, and imply that alternative phenotypes may arise by the modifications of already existing biochemical and physiological mechanisms and pathways.
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| Fig. 1 大杜鵑在匈牙利、芬蘭與捷克的蛋與當地寄主的相似度 |
大杜鵑 (Cuculus canorus) 是一種從內而外，從頭到尾都模仿其他物種的鳥類，成體的外觀類似某些雀鷹，牠們的鳥蛋能夠藉著擬態各種不同其他的鳥類的蛋，下在其他鳥類的窩裡，利用其他鳥類的資源來培養牠們的後代，其生活史幾乎可以說是寄生在其他鳥類的資源上，這個現象自然引起科學家對杜鵑濃厚的興趣，認為這無論是研究行為或演化，都是十分適合的生物模型。
這篇文章的作者群想討論一個問題：杜鵑的鳥蛋可以這麼相似，變化度又這麼大，是透過合成鳥蛋的化學機制控制，或是有什麼其他的機制控制呢？作者群取樣數種分佈在匈牙利、芬蘭與捷克的鳥種，其中有共域杜鵑的主要寄主、次級寄主與非寄主的鳥類，分析其鳥蛋表面的反射光譜與色素組成的化學結構，從光學與化學的角度推測杜鵑鳥蛋擬態的機制來源。
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| Fig. 2 光譜分析結果 |
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| Fig. 3 化學成分分析結果 |
結果發現杜鵑的蛋殼與當地寄主的蛋殼光譜有部分重疊，而杜鵑的蛋殼更廣泛的涵蓋到其他非寄主的蛋殼光譜。色素化學的分析結果顯示兩種主要形成蛋殼色素的化學物質，biliverdin與protoporphyrin，與當地寄主的蛋殼的濃度類似，顯示顏色的相似度可能來自色素化學物質濃度的趨同所導致。
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