Supplementary Material for: Relative Brain Size and Its Relation with the Associative Pallium in Birds

Despite growing interest in the evolution of enlarged brains, the biological significance of brain size variation remains controversial. Much of the controversy is over the extent to which brain structures have evolved independently of each other (mosaic evolution) or in a coordinated way (concerted evolution). If larger brains have evolved by the increase of different brain regions in different species, it follows that comparisons of the whole brain might be biologically meaningless. Such an argument has been used to criticize comparative attempts to explain the existing variation in whole-brain size among species. Here, we show that pallium areas associated with domain-general cognition represent a large fraction of the entire brain, are disproportionally larger in large-brained birds and accurately predict variation in the whole brain when allometric effects are appropriately accounted for. While this does not question the importance of mosaic evolution, it suggests that examining specialized, small areas of the brain is not very helpful for understanding why some birds have evolved such large brains. Instead, the size of the whole brain reflects consistent variation in associative pallium areas and hence is functionally meaningful for comparative analyses.

尽管学界对大脑扩容演化的关注度与日俱增，但脑尺寸差异的生物学意义仍存在争议。这类争议的核心在于，大脑结构究竟是以彼此独立的方式演化（镶嵌进化（mosaic evolution）），还是以协同统一的模式进行（协同进化（concerted evolution））。倘若大型大脑的演化源于不同物种的不同脑区扩张，那么对全脑尺寸进行比较研究可能就失去了生物学意义。这一论点曾被用于批判那些旨在解释物种间全脑尺寸现存差异的比较研究。本研究发现，与领域通用认知（domain-general cognition）相关的大脑皮层（pallium）区域在全脑中占比极高，在脑容量较大的鸟类中这类区域的占比显著偏高；且在合理校正异速生长效应（allometric effects）的前提下，这类区域的尺寸可准确预测全脑尺寸的变化。该结论并未否定镶嵌进化的重要性，但却表明，针对大脑特化的微小区域开展研究，对于理解部分鸟类演化出大容量大脑的原因并无显著助益。与之相反，全脑尺寸反映了关联型大脑皮层区域的一致性差异，因此在比较分析中具备明确的功能意义。