Data from: Life in the unthinking depths: energetic constraints on encephalization in marine fishes

Several hypotheses have been proposed to explain the limitation of brain size in vertebrates. Here we test three hypotheses of brain size evolution using marine teleost fishes: the direct metabolic constraints hypothesis, the expensive tissue hypothesis, and the temperature-dependent hypothesis. Our analyses indicate that there is a robust positive correlation between encephalization and basal metabolic rate that spans the full range of depths occupied by teleosts from the epipelagic (< 200m), mesopelagic (200-1000m), and bathypelagic (> 4000m). Our results disentangle the effects of temperature and metabolic rate on teleost brain size evolution, supporting the direct metabolic constraints hypothesis. Our results agree with previous findings that teleost brain size decreases with depth, however, we also recover a negative correlation between trophic level and encephalization within the mesopelagic zone, a result that runs counter to the expectations of the expensive tissue hypothesis. We hypothesize that mesopelagic fishes at lower trophic levels may be investing more in neural tissue related to the detection of small prey items in a low-light environment. We recommend that comparative encephalization studies control for basal metabolic rate in addition to controlling for body size and phylogeny.

学界已提出多项假说，用以阐释脊椎动物脑容量的限制机制。本研究以海洋硬骨鱼类（marine teleost fishes）为研究对象，检验三项脑容量演化相关假说：直接代谢限制假说（direct metabolic constraints hypothesis）、昂贵组织假说（expensive tissue hypothesis）以及温度依赖假说（temperature-dependent hypothesis）。分析结果显示，脑化程度（encephalization）与基础代谢率（basal metabolic rate）之间存在稳健的正相关关系，该相关性覆盖了硬骨鱼类栖息的全部水深范围，涵盖大洋上层带（epipelagic，<200m）、中层带（mesopelagic，200-1000m）与超深渊带（bathypelagic，>4000m）。本研究结果厘清了温度与代谢率对硬骨鱼类脑容量演化的影响，支持直接代谢限制假说。本研究结果与既往研究结论一致，即硬骨鱼类的脑容量随水深增加而减小；但同时我们在中层带中检出了营养级与脑化程度之间的负相关关系，这一结果与昂贵组织假说的预期相悖。我们据此提出假说：营养级较低的中层带鱼类，可能会更多地投入资源发育与弱光环境下小型猎物探测相关的神经组织。我们建议，在脑化程度比较研究中，除了对体型与系统发育（phylogeny）进行校正外，还应纳入基础代谢率的校正因素。