Data from: To quiver or to shiver: increased melanisation benefits thermoregulation, but reduces warning signal efficacy in the wood tiger moth

Melanin production is often considered costly, yet beneficial for thermoregulation. Studies of variation in melanization and the opposing selective forces that underlie its variability contribute greatly to understanding natural selection. We investigated whether melanization benefits are traded off with predation risk to promote observed local and geographical variation in the warning signal of adult male wood tiger moths (Parasemia plantaginis). Warning signal variation is predicted to reduce survival in aposematic species. However, in P. plantaginis, male hindwings are either yellow or white in Europe, and show continuous variation in melanized markings that cover 20 to 90 per cent of the hindwing. We found that the amount of melanization increased from 40 to 59 per cent between Estonia (58° N) and north Finland (67° N), suggesting melanization carries thermoregulatory benefits. Our thermal measurements showed that more melanic individuals warmed up more quickly on average than less melanic individuals, which probably benefits flight in cold temperatures. With extensive field experiments in central Finland and the Alpine region, we found that more melanic individuals suffered increased predation. Together, our data suggest that warning signal efficiency is constrained by thermoregulatory benefits. Differences in relative costs and benefits of melanin probably help to maintain the geographical warning signal differences.

黑色素生成通常被认为存在代谢成本，却能为体温调节带来益处。对黑化（melanization）变异及其背后的对立选择压力的研究，极大地推动了我们对自然选择的认知。本研究旨在探究黑化的益处是否与捕食风险存在权衡，进而促成了成年雄性木虎蛾（Parasemia plantaginis）警戒信号的局地与地理变异。理论预测，警戒信号的变异会降低具有警戒色（aposematic）物种的存活率，但在欧洲分布的P. plantaginis中，雄性后翅呈黄色或白色，且其黑化斑纹覆盖后翅的比例存在连续变异，范围为20%至90%。我们发现，从爱沙尼亚（北纬58°）到芬兰北部（北纬67°），黑化程度从40%提升至59%，这表明黑化具备体温调节方面的益处。体温测量结果显示，黑化程度更高的个体平均升温速度快于黑化程度较低的个体，这在低温环境下可能有助于飞行活动。通过在芬兰中部与阿尔卑斯山区开展的大规模野外实验，我们发现黑化程度更高的个体面临的捕食压力更大。综合来看，我们的研究数据表明，警戒信号的效能受到体温调节益处的制约，黑色素生成的相对成本与收益差异，可能是维持该物种地理尺度上警戒信号变异的关键因素。