Combined measures of mimetic fidelity explain imperfect mimicry in a brood parasite–host system

The persistence of imperfect mimicry in nature presents a challenge to mimicry theory. Some hypotheses for the existence of imperfect mimicry make differing predictions depending on how mimetic fidelity is measured. Here, we measure mimetic fidelity in a brood parasite–host system using both trait-based and response-based measures of mimetic fidelity. Cuckoo finches Anomalospiza imberbis lay imperfectly mimetic eggs that lack the fine scribbling characteristic of eggs of the tawny-flanked prinia Prinia subflava, a common host species. A trait-based discriminant analysis based on Minkowski functionals—that use geometric and topological morphometric methods related to egg pattern shape and coverage—reflects this consistent difference between host and parasite eggs. These methods could be applied to quantify other phenotypes including stripes and waved patterns. Furthermore, by painting scribbles onto cuckoo finch eggs and testing their rate of rejection compared to control eggs (i.e. a response-based approach to quantify mimetic fidelity), we show that prinias do not discriminate between eggs based on the absence of scribbles. Overall, our results support relaxed selection on cuckoo finches to mimic scribbles, since prinias do not respond differently to eggs with and without scribbles, despite the existence of this consistent trait difference.

自然界中缺陷拟态（imperfect mimicry）的持续存在，始终是拟态理论面临的挑战。针对缺陷拟态存在的诸多假说，其预测结果会因拟态保真度（mimetic fidelity）的测量方式不同而存在显著差异。本研究以巢寄生-宿主系统（brood parasite–host system）为研究对象，同时采用基于性状（trait-based）与基于响应（response-based）的两类拟态保真度测量方法进行量化分析。杜鹃凤鹀（Anomalospiza imberbis）所产卵类属于不完善拟态卵，其缺乏常见宿主褐胁鹪莺（Prinia subflava）鸟卵所特有的精细纹路特征。基于闵可夫斯基泛函（Minkowski functionals）的性状判别分析——该方法运用与卵斑形状及覆盖范围相关的几何与拓扑形态计量学手段——清晰反映出宿主卵与寄生卵之间存在这一稳定差异。此类方法可被用于量化包括条纹与波纹状图案在内的其他表型特征。此外，本研究通过在杜鹃凤鹀卵表面人工绘制纹路，并将其与对照组卵的拒卵率进行对比（即基于响应的拟态保真度量化方法），证实褐胁鹪莺并不会通过是否存在纹路来区分鸟卵。总体而言，本研究结果支持如下结论：尽管宿主卵与寄生卵间存在稳定的性状差异，但由于褐胁鹪莺对带有纹路与不带纹路的鸟卵并无差异化响应，因此杜鹃凤鹀在模拟纹路方面所承受的选择压力已趋于松弛。