Behavioural Biology, University of Groningen, Groningen, The Netherlands.
PLoS One. 2012;7(8):e43088. doi: 10.1371/journal.pone.0043088. Epub 2012 Aug 14.
Mechanisms maintaining honesty of sexual signals are far from resolved, limiting our understanding of sexual selection and potential important parts of physiology. Carotenoid pigmented visual signals are among the most extensively studied sexual displays, but evidence regarding hypotheses on how carotenoids ensure signal honesty is mixed. Using a phylogenetically controlled meta-analysis of 357 effect sizes across 88 different species of birds, we tested two prominent hypotheses in the field: that carotenoid-dependent coloration signals i) immunocompetence and/or ii) oxidative stress state. Separate meta-analyses were performed for the relationships of trait coloration and circulating carotenoid level with different measures of immunocompetence and oxidative stress state. For immunocompetence we find that carotenoid levels (r = 0.20) and trait color intensity (r = 0.17) are significantly positively related to PHA response. Additionally we find that carotenoids are significantly positively related to antioxidant capacity (r = 0.10), but not significantly related to oxidative damage (r = -0.02). Thus our analyses provide support for both hypotheses, in that at least for some aspects of immunity and oxidative stress state the predicted correlations were found. Furthermore, we tested for differences in effect size between experimental and observational studies; a larger effect in observational studies would indicate that co-variation might not be causal. However, we detected no significant difference, suggesting that the relationships we found are causal. The overall effect sizes we report are modest and we discuss potential factors contributing to this, including differences between species. We suggest complementary mechanisms maintaining honesty rather than the involvement of carotenoids in immune function and oxidative stress and suggest experiments on how to test these.
维持性信号诚实的机制还远未解决，这限制了我们对性选择和潜在重要生理部分的理解。类胡萝卜素着色的视觉信号是研究最广泛的性展示之一，但关于类胡萝卜素如何确保信号诚实的假设的证据是混杂的。我们使用了 88 个不同鸟类物种的 357 个效应大小的系统发育控制荟萃分析，检验了该领域的两个突出假设：类胡萝卜素依赖性颜色信号 i）免疫能力和/或 ii）氧化应激状态。分别对性状颜色和循环类胡萝卜素水平与不同免疫能力和氧化应激状态测量值的关系进行了荟萃分析。对于免疫能力，我们发现类胡萝卜素水平（r = 0.20）和性状颜色强度（r = 0.17）与 PHA 反应呈显著正相关。此外，我们发现类胡萝卜素与抗氧化能力呈显著正相关（r = 0.10），但与氧化损伤无显著相关性（r = -0.02）。因此，我们的分析支持这两个假设，因为至少对于免疫和氧化应激状态的某些方面，预测的相关性得到了证实。此外，我们还测试了实验和观察研究之间的效应大小差异；观察研究中的较大效应表明协变量可能不是因果关系。然而，我们没有发现显著差异，表明我们发现的关系是因果关系。我们报告的总体效应大小适中，我们讨论了可能导致这种情况的潜在因素，包括物种之间的差异。我们建议补充机制来维持诚实，而不是类胡萝卜素参与免疫功能和氧化应激，并建议进行实验以测试这些机制。
