Relative brain size and cognitive equivalence in fishes

Abstract

There are two well-established facts about vertebrate brains: brains are physiologically costly organs, and both absolute and relative brain size varies greatly between and within the major vertebrate clades. While the costs are relatively clear, scientists struggle to establish how larger brains translate into higher cognitive performance. Part of the challenge is that intuitively larger brains are needed to control larger bodies without any changes in cognitive performance. Therefore, body size needs to be controlled to establish the slope of cognitive equivalence between animals of different sizes. Potentially, intraspecific slopes provide the best available estimate of how an increase in body size translates into an increase in brain size without changes in cognitive performance. Here, we provide the first evaluation of this hypothesis for fishes. First, a dataset of 51 species that included only samples of [≥] ten wild-caught individuals yielded a mean brain-body slope of 0.46 (albeit with a large range of 0.26 to 0.79). This mean slope is similar to the encephalisation quotients for ectotherm higher taxa, i.e. teleost fishes, amphibians and reptiles ([~] 0.5). However, the slope is much higher than what has been found in endotherm vertebrate species ([~] 0.3). Second, we provide slope estimates for brain-body sizes and for cognition-body sizes in wild-caught cleaner fish Labroides dimidiatus as a case study. Brain-body slopes from two datasets gave the values of 0.58 (MRI scans data) and 0.47 (dissection data). Furthermore, we have cognitive performance data from 69 individuals tested in four different cognitive tasks that estimated learning, numerical, and inhibitory control abilities. In all four tasks, the cognitive performance did not correlate significantly with body size. These results suggest that the brain-body slopes represent estimates of intraspecific cognitive equivalence for this species. While subject to further studies on various species, our results suggest that endo- and ectotherm brain organisations and resulting cognitive performances are fundamentally different.