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，为动物带来诸多适合度益处。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，这暗示该色素存在进化权衡；不过目前鲜有研究量化类胡萝卜素色素沉着所带来的适合度成本。鲑鱼以其醒目的红色体色广为人知，但在奇努克鲑鱼（Chinook salmon，Oncorhynchus tshawytscha）中存在天然的遗传颜色多态性（红色与白色两种形态），会导致卵与其他组织出现基于类胡萝卜素的颜色差异。已有研究证实，卵内类胡萝卜素含量对孵化存活率具有适合度益处，但类胡萝卜素色素沉着也会使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存在存活能力方面的代价，但捕食压力降低所带来的潜在益处，或可解释该颜色多态性的维持机制。本研究借助野生奇努克鲑鱼的红色与白色卵开展实验，结果证实鲑鱼卵的类胡萝卜素含量越高，其被捕食的风险也越高。我们发现，两群野生型虹鳟（rainbow trout，一种生态相关的捕食者）在选择实验中显著偏好红色卵而非白色卵：红色卵被率先取食的频率是白色卵的两倍，且被取食的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进化权衡，为自然界中该物种独特的颜色多态性维持提供了一种进化机制，同时也首次证实了鲑鱼体内类胡萝卜素所带来的直接适合度成本。