Data from: Redder isn’t always better: cost of carotenoids in Chinook salmon eggs

Carotenoids provide animals with many fitness benefits through increased mating success, immune function, gamete quality and antioxidant capacity. Despite these benefits, carotenoids are not utilized equally by all animals, implying trade-offs associated with the pigments; although, few studies have quantified fitness costs of carotenoid pigmentation. Salmon are known for their conspicuous red coloration; however, amongst Chinook salmon (Oncorhynchus tshawytscha), a natural genetic color polymorphism exists (red and white morphs) which results in carotenoid-based color differences in eggs and other tissues. While the fitness benefit of egg carotenoid content on egg incubation survival has been demonstrated, carotenoid pigmentation also results in highly visible eggs vulnerable to predation. Therefore, although white Chinook salmon eggs experience costs in terms of viability, a potential benefit in terms of reduced predation could help explain the maintenance of the polymorphism. Here, using red and white eggs from wild Chinook salmon, we show that increased carotenoid content of salmon eggs leads to greater predation risk. We found that two populations of wild-type rainbow trout (an ecologically relevant predator) showed a significant bias for red eggs over white eggs under choice experiments, where red eggs were consumed first twice as often and significantly faster than white eggs. Our study suggests that trade-offs between red and white Chinook salmon during the egg stage provide an evolutionary mechanism promoting the maintenance of the unique Chinook salmon color polymorphism in nature, while also, for the first time, demonstrating a direct fitness cost of carotenoids in salmon.类胡萝卜素（Carotenoids）可通过提升交配成功率、免疫功能、配子质量与抗氧化能力，为动物带来多重适合度收益。尽管具备此类益处，但并非所有动物都能同等利用类胡萝卜素，这暗示这类色素存在相关权衡代价；不过目前鲜有研究量化类胡萝卜素色素沉着的适合度成本。奇努克鲑鱼（Oncorhynchus tshawytscha）以其醒目的红色体色广为人知，但其天然存在遗传颜色多态性，包含红色与白色两种形态，这会使卵与其他组织出现基于类胡萝卜素的颜色差异。虽然卵类胡萝卜素含量对卵孵化存活率的适合度收益已得到证实，但类胡萝卜素色素沉着也会让卵变得极易被捕食者发现，从而面临被捕食的风险。因此，尽管白色形态奇努克鲑鱼的卵会在生存能力上付出代价，但凭借降低被捕食风险所带来的潜在收益，或可解释该颜色多态性的维持机制。本研究利用野生奇努克鲑鱼的红色与白色卵开展实验，结果表明，鲑鱼卵的类胡萝卜素含量越高，其被捕食风险也越高。我们发现，两个种群的野生型虹鳟（rainbow trout）作为生态相关的捕食者，在选择实验中均表现出对红色卵的显著偏好：红色卵的首次被取食频率是白色卵的两倍，且被取食速度显著快于白色卵。本研究表明，奇努克鲑鱼卵阶段红色与白色形态间的权衡，为自然界中独特的奇努克鲑鱼颜色多态性的维持提供了一种进化机制，同时也是首次在鲑鱼中证实类胡萝卜素存在直接的适合度成本。