Animal coloration is a multifaceted trait with many ecological roles and related to a variety of developmental and physiological processes. Consequently, coloration is often subject to a variety of selective pressures, leading to the evolutionary maintenance of variation. In this study, we investigated hypotheses related to the maintenance of dorsal color variation in wood frogs (Rana sylvatica). First, we tested for multimodality, and whether color correlates with body size or condition or varies by sex or age class. We combined behavioral trials with visual modeling to test for sex recognition. We also considered visual models for predators and tested for an interaction between discriminability indexes (JND) of color channel (chromatic vs. achromatic) and predator type (birds vs. snakes), as well as for a within individual trade‐off between the JND of chromatic and achromatic coloration. Finally, we tested for disruptive viability selection on color using predation trials, and for antagonistic directional selection between viability selection and reproductive investment of females. We found that wood frogs present continuous color variation that does not correlate with body size or condition, but that changes with age. Wood frogs present subtle sexual dichromatism, but we found no evidence for a role of color in sex recognition. Instead, we discuss the possibility that sex differences might, at least in part, have a demographic explanation. Predator visual models indicated that wood frogs cannot solely rely on dorsal coloration for camouflage. Moreover, different predators might present selective pressures in different color channels, while individuals’ achromatic and chromatic coloration trade‐off in JND. Therefore, different selective pressures caused by different predators might interact with ontogenetic changes and developmental/physiological trade‐offs to maintain color variation. We found no relationship between color and survival or reproductive investment, suggesting further work is required to fully understand selection on color. Our results highlight the importance of understanding evolutionary trade‐offs and developmental/physiological constraints in combination with one another, and suggest the potential for an interaction between these proximate and ultimate mechanisms in the evolutionary maintenance of variation. These results likely extend beyond color expression in amphibians, and exemplify a more general process for such evolutionary outcomes.

中文翻译：

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进化的取舍可能与生理限制相互作用，以维持颜色变化

动物着色是具有多种生态作用的多方面特征，与多种发育和生理过程有关。因此，着色通常要承受各种选择压力，从而导致变化的进化维持。在这项研究中，我们调查了与维持木蛙（Rana sylvatica）。首先，我们测试了多模态，以及颜色是否与身体大小或状况相关，还是随性别或年龄段而变化。我们将行为试验与视觉建模相结合，以测试性别识别能力。我们还考虑了捕食者的视觉模型，并测试了彩色通道（彩色与消色差）的判别指数（JND）与捕食者类型（鸟与蛇）之间的交互作用，以及在JND之间的权衡取舍有色和无色着色。最后，我们使用捕食试验测试了颜色的破坏性生存力选择，以及生存力选择和雌性生殖投资之间的拮抗方向选择。我们发现木蛙呈现出连续的颜色变化，该变化与体型或状况无关，但随年龄而变化。木蛙表现出微妙的性二色性，但我们没有发现颜色在性别识别中起作用的证据。相反，我们讨论性别差异可能至少部分具有人口统计学解释的可能性。捕食者的视觉模型表明，木蛙不能仅靠背侧伪装。此外，在JND中，不同的掠食者可能会在不同的颜色通道中呈现选择性压力，而个体的消色差和色差会有所折衷。因此，由捕食者引起的不同选择压力可能与个体发育变化和发育/生理折衷相互作用，以维持颜色变化。我们发现颜色与生存或生殖投资之间没有关系，这表明需要进一步的工作才能充分了解颜色的选择。我们的结果强调了相互理解进化折衷和发展/生理限制的重要性，并暗示了在变异的进化维持中这些最接近和最终机制之间相互作用的潜力。这些结果可能超出了两栖动物的颜色表达范围，并且例证了这种进化结果的更一般过程。