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.较大的大脑可带来多项认知优势，但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，要么提升整体能量消耗。既往针对这些假说的研究得出了相互矛盾的结论，使得脑化（encephalization）扩增演化过程中的代谢成本与限制机制仍不明确。 象鼻鱼科（Mormyridae）电鱼具有与灵长类相当的极端脑化特征。本研究发现，该科物种间的脑容量差异极大，且几乎没有证据表明脑容量与其他器官大小存在能量权衡（trade-off）关系，反而脑容量与静息氧耗速率呈显著相关。此外，我们还发现脑容量越大，物种的低氧耐受能力越弱。本研究的数据因此提供了一个非哺乳类的极端脑化案例，该类群通过提升整体能量消耗来适配更大的脑容量。 既往研究在未经历灵长类与象鼻鱼科电鱼这般极端脑化的类群中，发现了脑容量变异与能量权衡的相关证据。据此我们提出，能量权衡仅能解释脑容量的中度扩增演化，而极端脑化的能量需求可能需要提升整体能量投入。