No evidence for general intelligence in a fish

Abstract

Differences in human general intelligence or reasoning ability can be quantified with the psychometric factor g, because individual performance across cognitive tasks is positively correlated. g also emerges in mammals and birds, is correlated with brain size and may similarly reflect general reasoning ability and behavioural flexibility in these species. To exclude the alternative that these positive cross-correlations may merely reflect the general biological quality of an organism or an inevitable by-product of having brains it is paramount to provide solid evidence for the absence of g in at least some species. Here, we show that wild-caught cleaner fish Labroides dimidiatus, a fish species otherwise known for its highly sophisticated social behaviour, completely lacks g when tested on ecologically non-relevant tasks. Moreover, performance in these experiments was not or negatively correlated with an ecologically relevant task, and in none of the tasks did fish caught from a high population density site outperform fish from a low-density site. g is thus unlikely a default result of how brains are designed, and not an automatic consequence of variation in social complexity. Rather, the results may reflect that g requires a minimal brain size, and thus explain the conundrum why the average mammal or bird has a roughly 10 times larger brain relative to body size than ectotherms. Ectotherm brains and cognition may therefore be organized in fundamentally different ways compared to endotherms.

Relative brain size and cognitive equivalence in fishes

“…For the cognitive performance and body size data, we compiled data from the study by Aellen et al [2021 Preprint]. The dataset comprises cognitive performance of 69 wild-caught female cleaner fish tested in four laboratory tasks (For further details, please refer to the original study by Aellen et al [2021 Preprint]). Here, we used these data to explore the relationship between body size and cognitive performance.…”

“…Aellen et al [2021 Preprint] tested fish learning and flexibility abilities in a reversal learning task. The experiment was based on a simultaneous two-choice task with colour and shape cued Plexiglas plates, i.e., yellow triangle vs green round plates.…”

“…To test for fish numerical abilities, Aellen et al [2021 Preprint] used Plexiglas plates with various numerical quantities in the form of black squares depicted on a white background. There were nine quantities ranging between one to nine black squares.…”

“…Such event can be reproduced in laboratory conditions with Plexiglas plates as surrogates for clients, and fish flakes and mashed prawn as surrogates for client ectoparasite and mucus, respectively [Bshary, and Grutter, 2006]. Aellen et al [2021 Preprint] used similar laboratory set-up to test whether cleaner fish can inhibit their preferences for prawn and feed on fish flakes. During test trials, a Plexiglas plate with two fish flakes items and two prawn items was presented to the fish.…”

“…Here, we used published data on brain size [Triki et al, 2019a; Triki et al, 2020a] and cognitive performance in cleaner fish [Aellen et al, 2021 Preprint] to explore the relationship between body size, brain size and cognitive performance. The resulting brain-body and cognition-body slopes provide a first test to what extent intraspecific brain-body slopes in fishes may reflect cognitive equivalence.…”