Department of Evolutionary Ecology, Estación Biológica de Doñana/CSIC, Seville, Spain.
J Evol Biol. 2011 May;24(5):999-1006. doi: 10.1111/j.1420-9101.2011.02232.x. Epub 2011 Feb 21.
Eumelanin and pheomelanin are the most common vertebrate pigments. They generate different colours and are synthesized under different physiological conditions. While pheomelanogenesis requires high levels of a key intracellular antioxidant (glutathione, GSH), eumelanogenesis is inhibited by GSH. This implies that species that present the molecular basis to produce large amounts of pheomelanin might be more limited to perform other costly processes that generate oxidative stress than species that produce eumelanin. Brain development requires large amounts of energy and antioxidants during ontogeny, so that large-brained species may be constrained in their simultaneous synthesis of large amounts of pheomelanin, but not in their synthesis of eumelanin. Here, we tested this hypothesis in a large dataset of 323 bird species. After controlling for the effects of phylogeny, latitude and sexual dichromatism, the proportion of pheomelanic plumage colour was strongly negatively related to the relative brain mass of species, whereas no relationship was found for the proportion of eumelanic colour. This indicates that the production of pheomelanin is a costly process that cannot evolve together with complex neural structures and thus with large cognitive capacity. This is the first time that the expression of melanic traits is found to correlate with another phenotypic character across species.
真黑素和褐黑素是最常见的脊椎动物色素。它们产生不同的颜色，并且在不同的生理条件下合成。虽然褐黑素的生成需要高水平的关键细胞内抗氧化剂（谷胱甘肽，GSH），但真黑素的生成却受到 GSH 的抑制。这意味着，与产生真黑素的物种相比，那些具有产生大量褐黑素分子基础的物种可能更受限制，无法同时进行其他产生氧化应激的昂贵过程。大脑发育在个体发生过程中需要大量的能量和抗氧化剂，因此，大脑较大的物种可能会受到同时合成大量褐黑素的限制，但不会受到合成真黑素的限制。在这里，我们在一个包含 323 种鸟类物种的大型数据集上测试了这一假设。在控制了系统发育、纬度和性二色性的影响后，发现羽色中褐黑素的比例与物种的相对脑质量呈强烈负相关，而真黑素的比例则没有关系。这表明，褐黑素的产生是一个昂贵的过程，不能与复杂的神经结构以及因此与大的认知能力一起进化。这是第一次发现黑色素特征的表达与跨物种的另一个表型特征相关。
