Data from: The costs of a big brain: extreme encephalization results in higher energetic demand and reduced hypoxia tolerance in weakly electric African fishes

A large brain can offer several cognitive advantages. However, brain tissue has an especially high metabolic rate. Thus, evolving an enlarged brain requires either a decrease in other energetic requirements, or an increase in overall energy consumption. Previous studies have found conflicting evidence for these hypotheses, leaving the metabolic costs and constraints in the evolution of increased encephalization unclear. Mormyrid electric fishes have extreme encephalization comparable to that of primates. Here, we show that brain size varies widely among mormyrid species, and that there is little evidence for a trade-off with organ size, but instead a correlation between brain size and resting oxygen consumption rate. Additionally, we show that increased brain size correlates with decreased hypoxia tolerance. Our data thus provide a non-mammalian example of extreme encephalization that is accommodated by an increase in overall energy consumption. Previous studies have found energetic trade-offs with variation in brain size in taxa that have not experienced extreme encephalization comparable with that of primates and mormyrids. Therefore, we suggest that energetic trade-offs can only explain the evolution of moderate increases in brain size, and that the energetic requirements of extreme encephalization may necessitate increased overall energy investment.拥有较大的大脑可带来多项认知优势。然而，脑组织的代谢速率极高。因此，演化出更大的大脑，要么需要降低其他生理过程的能量需求，要么需要提升整体的能量消耗水平。过往相关研究针对这两类假说得到了相互矛盾的证据，使得脑化（encephalization）程度提升的演化过程中，其代谢成本与限制机制仍不明确。象鼻鱼科电鱼（Mormyrid electric fishes）拥有可与灵长类媲美的极端脑化程度。本研究发现，该科各物种种间大脑尺寸差异显著，且几乎没有证据表明大脑尺寸与其他器官尺寸存在权衡（trade-off）关系；相反，大脑尺寸与静息氧耗速率呈显著相关。此外，本研究还发现，大脑尺寸越大，物种的低氧耐受能力（hypoxia tolerance）越低。因此，本研究的数据提供了一个非哺乳类的极端脑化实例，该类群通过提升整体能量消耗来适配更大的大脑尺寸。过往研究在未经历如灵长类与象鼻鱼科电鱼这般极端脑化的生物类群（taxa）中，发现了大脑尺寸变异与能量权衡的关联。据此，我们提出：能量权衡仅能解释大脑尺寸适度增大的演化过程，而极端脑化的能量需求，可能需要提升整体的能量投入水平。