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.

大脑体积较大可带来多方面认知优势。然而，脑组织的代谢速率极高。因此，演化出更大体积的大脑，要么需要降低其他生理过程的能量需求，要么需要提升整体能量消耗。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）拥有可与灵长类媲美的极端脑化程度。本研究发现，该科不同物种的大脑体积差异显著，且几乎无证据表明大脑体积与其他器官大小存在能量权衡关系，反而观察到大脑体积与静息耗氧速率存在相关性。此外，我们还发现大脑体积越大，机体的低氧耐受能力越差。综上，本研究数据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大脑体积。过往研究在未经历灵长类与象鼻鱼科鱼类这般极端脑化演化的类群中，发现了大脑体积变化与能量权衡的相关证据。因此我们认为，能量权衡仅能解释大脑体积的中等程度演化提升，而极端脑化的能量需求可能需要提升整体的能量投入。