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 for in order 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 slope estimates for brain-body sizes and for cognition-body in wild-caught cleaner fish Labroides dimidiatus. The cleaners' cognitive performance was estimated from four different cognitive tasks that tested for learning, numerical, and inhibitory control abilities. The cognitive performance was found to be rather independent of body size, while brain-body slopes from two datasets gave the values of 0.58 (MRI scans data) and 0.47 (dissection data). These values can hence represent estimates of intraspecific cognitive equivalence for this species. Furthermore, another dataset of brain-body slopes estimated from 14 different fish species, gave a mean slope of 0.5, and hence rather similar to that of cleaners. This slope is very similar to the encephalisation quotients for ectotherm higher taxa, i.e. teleost fishes, amphibians and reptiles (0.5). The slope is much higher than what has been found in endotherm vertebrate species (0.3). Together, it suggests that endo- and ectotherm brain organisations and resulting cognitive performances are fundamentally different.

Sex differences in the cognitive abilities of a sex-changing fish species Labroides dimidiatus

“…Nevertheless, the males did outperform the females in this task despite being larger in body size. Performance in the other tasks, in both males and females, were independent of body size; a finding that fits previous outcomes resulting from a larger dataset on female cleaner fish [41]. It is noteworthy to mention that the observed and tested female cleaners were sexually mature, which means they were at least 1 year old (i.e.…”

“…In other words, larger individuals (within or between sexes) are more likely to be older [ 40 ]. Previous research on cleaner fish has shown that performance levels from various cognitive tasks do not correlate with body size (a proxy for age) [ 41 ]. Therefore, it is unlikely that males will outperform females in any of our four cognitive tasks just because they are larger and older.…”

A Farewell to the Encephalization Quotient: A New Brain Size Measure for Comparative Primate Cognition

“…The latter vary between 0.4 and 0.5 within species [Tsuboi et al, 2018]. For fishes, Triki et al [2021] obtained a mean intraspecific slope of 0.49 that is independent of body size variation. Furthermore, studied one fish species, i.e., the cleaner fish Labroides dimidiatus in more detail.…”

Differences in brain morphology of brown trout across stream, lake, and hatchery environments

“…Therefore, our sample in this respect adequately represents the reality of the size distributions in the sampled populations. The lack of positive correlation between the brain size and FL shown here in hatchery individuals is unusual for wild fishes, where body size is a strong predictor of brain volume (e.g., Triki et al, 2021 ). Previous studies on the evolution of encephalization in mammals have suggested that relative brain size depends on selection pressure on the size of the brain as well as on the overall body size (Smaers et al, 2012 ).…”