Department of Biological Sciences, Auburn University, Auburn, AL, 36849, USA
School of Biological Science, Monash University, Clayton, VIC, 3168, Australia.
J Exp Biol. 2019 Mar 18;222(Pt 6):jeb188102. doi: 10.1242/jeb.188102.
Carotenoid-based coloration in birds is widely considered an honest signal of individual condition, but the mechanisms responsible for condition dependency in such ornaments remain debated. Currently, the most common explanation for how carotenoid coloration serves as a reliable signal of condition is the resource trade-off hypothesis, which proposes that use of carotenoids for ornaments reduces their availability for use by the immune system or for protection from oxidative damage. However, two main assumptions of the hypothesis remain in question: whether carotenoids boost the performance of internal processes such as immune and antioxidant defenses, and whether allocating carotenoids to ornaments imposes a trade-off with such benefits. In this study, we tested these two fundamental assumptions using types of domestic canary () that enable experiments in which carotenoid availability and allocation can be tightly controlled. Specifically, we assessed metrics of immune and antioxidant performance in three genetic variants of the color-bred canary that differ only in carotenoid phenotype: ornamented, carotenoid-rich yellow canaries; unornamented, carotenoid-rich 'white dominant' canaries; and unornamented, carotenoid-deficient 'white recessive' canaries. The resource trade-off hypothesis predicts that carotenoid-rich individuals should outperform carotenoid-deficient individuals and that birds that allocate carotenoids to feathers should pay a cost in the form of reduced immune function or greater oxidative stress compared with unornamented birds. We found no evidence to support either prediction; all three canary types performed equally across measures. We suggest that testing alternative mechanisms for the honesty of carotenoid-based coloration should be a key focus of future studies of carotenoid-based signaling in birds.
鸟类的类胡萝卜素着色被广泛认为是个体状况的诚实信号，但负责这种装饰物条件依赖性的机制仍存在争议。目前，类胡萝卜素着色如何作为状况可靠信号的最常见解释是资源权衡假说，该假说提出，将类胡萝卜素用于装饰物会减少其对免疫系统的可用性或对氧化损伤的保护。然而，该假说的两个主要假设仍然存在疑问：类胡萝卜素是否增强了内部过程（如免疫和抗氧化防御）的性能，以及将类胡萝卜素分配给装饰物是否与这些益处产生权衡。在这项研究中，我们使用不同类型的家养金丝雀（）来检验这两个基本假设，这些金丝雀可以进行实验，以便严格控制类胡萝卜素的可用性和分配。具体来说，我们评估了三种不同的颜色培育金丝雀的遗传变异体的免疫和抗氧化性能指标，这些金丝雀在类胡萝卜素表型上仅存在差异：有装饰物、富含类胡萝卜素的黄色金丝雀；无装饰物、富含类胡萝卜素的“白色显性”金丝雀；无装饰物、缺乏类胡萝卜素的“白色隐性”金丝雀。资源权衡假说预测，富含类胡萝卜素的个体应该比缺乏类胡萝卜素的个体表现更好，并且与无装饰物的鸟类相比，将类胡萝卜素分配给羽毛的鸟类应该会以降低免疫功能或更大的氧化应激为代价。我们没有发现任何证据支持这两种预测；所有三种金丝雀类型在所有测量指标上的表现都相同。我们认为，测试类胡萝卜素着色诚实性的替代机制应该是未来鸟类类胡萝卜素信号研究的一个关键重点。
