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。然而，脑组织具有极高的代谢速率。因此，演化出更大尺寸的大脑，要么需要降低其他组织的能量需求，要么需要提升整体能量消耗。过往研究针对这两种假说得到了相互矛盾的证据，使得脑化（encephalization）演化过程中的代谢成本与限制机制仍不明确。象鼻鱼科电鱼（Mormyrid electric fishes）具有与灵长类相当的极端脑化特征。本研究发现，象鼻鱼科物种间的大脑尺寸差异极大，且几乎没有证据表明大脑尺寸与其他器官尺寸存在权衡关系，反而与静息氧耗速率呈显著相关。此外，我们还发现大脑尺寸越大，物种的低氧耐受能力越弱。因此，我们的数据为极端脑化（encephalization）提供了非哺乳类的例证，这类脑化演化可通过提升整体能量消耗来实现。过往研究在那些未经历过类似灵长类与象鼻鱼科电鱼那般极端脑化演化的类群中，发现了大脑尺寸变化与能量权衡的相关证据。据此我们认为，能量权衡仅能解释大脑尺寸的中等程度增幅演化，而极端脑化的能量需求，可能需要提升整体的能量投入。