Data from: Large-brained birds suffer less oxidative damage

Large brains (relative to body size) might confer fitness benefits to animals. Although the putative costs of well-developed brains can constrain the majority of species to modest brain sizes, these costs are still poorly understood. Given that the neural tissue is energetically expensive and demands antioxidants, one potential cost of developing and maintaining large brains is increased oxidative stress (‘oxidation exposure’ hypothesis). Alternatively, because large-brained species exhibit slow-paced life histories, they are expected to invest more into self-maintenance such as an efficacious antioxidative defence machinery (‘oxidation avoidance’ hypothesis). We predict decreased antioxidant levels and/or increased oxidative damage in large-brained species in case of oxidation exposure, and the contrary in case of oxidation avoidance. We address these contrasting hypotheses for the first time by means of a phylogenetic comparative approach based on an unprecedented dataset of 4 redox state markers from 85 European bird species. Large-brained birds suffered less oxidative damage to lipids (measured as malondialdehyde levels) and exhibited higher total non-enzymatic antioxidant capacity than small-brained birds, while uric acid and glutathione levels were independent of brain size. These results were not altered by potentially confounding variables and did not depend on how relative brain size was quantified. Our findings partially support the ‘oxidation avoidance’ hypothesis and provide a physiological explanation for the linkage of large brains with slow-paced life histories: reduced oxidative stress of large-brained birds can secure brain functionality and healthy lifespan, which are integral to their lifetime fitness and slow-paced life history.

相较于躯体尺寸的大容量脑，可能会为动物带来生存适合度收益。尽管发达大脑的推测性演化成本，会将多数物种的脑容量限制在中等水平，但这类成本的具体机制仍未得到充分阐明。鉴于神经组织的能量消耗极高且依赖抗氧化剂，发育并维持大容量大脑的潜在成本之一，是氧化应激水平升高（氧化暴露假说，oxidation exposure hypothesis）。与之相对，由于脑容量较大的物种具有较慢的生活史节奏，它们被预期会在自我维持上投入更多资源，例如构建高效的抗氧化防御系统（氧化规避假说，oxidation avoidance hypothesis）。若符合氧化暴露假说，我们预测大容量脑物种的抗氧化剂水平会降低，或氧化损伤程度会升高；若符合氧化规避假说，则结果恰好相反。

我们首次依托涵盖85种欧洲鸟类、共4种氧化还原状态标志物（redox state markers）的全新数据集，运用系统发育比较方法（phylogenetic comparative approach）对这两个对立假说进行检验。研究结果显示，相较于脑容量较小的鸟类，大容量脑鸟类的脂质氧化损伤（以丙二醛水平，malondialdehyde levels衡量）程度更低，总非酶抗氧化能力更高；而尿酸与谷胱甘肽（glutathione）水平则与脑容量无显著关联。上述结果未受潜在混淆变量的影响，且不受相对脑容量量化方式的影响。

我们的研究结果部分支持氧化规避假说，并为大容量脑与较慢生活史节奏之间的关联提供了生理学解释：大容量脑鸟类的氧化应激水平更低，可维持大脑功能与健康寿命，而这二者正是其终身适合度与慢生活史节奏的核心组成部分。