A farewell to EQ: A new brain size measure for comparative primate cognition

Abstract

Both absolute and relative brain size vary greatly among and within the major vertebrate lineages. Scientists have long debated how larger brains in primates and hominins translate into greater cognitive performance, and in particular how to control for the relationship between the non-cognitive functions of the brain and body size. One solution to this problem is to establish the slope of cognitive equivalence, that is the line connecting organisms with an identical bauplan but different body sizes. Here, we suggest that intraspecific slopes provide the best available estimate of this measure. This approach was abandoned because slopes were too low by an unknown margin due to estimation error. We control for the error problem by focusing on highly dimorphic primate species with large sample sizes and fitting a line through the mean values for adult females and males. We obtain the best estimate for the slope of ca 0.27, a value much lower than those constructed using all mammal species, and close to the value expected based on the genetic correlation between brain size and body size. We also find that the estimate of cognitive brain size based on cognitive equivalence fits empirical cognitive studies better than the encephalization quotient (EQ), which should therefore be avoided in future studies on primates, and presumably mammals and birds in general. The use of residuals from the line of cognitive equivalence may change conclusions concerning the cognitive abilities of extant and extinct primate species, including hominins.

Why big brains? A comparison of models for both primate and carnivore brain size evolution

“…For example, a recent analysis of mammalian brain size found the brain-to-body relationship to uncover more than just selection on brain size, indicating relative brain size measures, both residuals and EQ scores, are not accurately capturing brain size variation, and are not suitable for comparisons across species with different evolutionary histories [ 64 ]. Thus, van Schaik et al, [ 65 ] suggest the use of encephalisation quotients should be avoided in future studies, as EQs repeatedly fail to accurately predict brain size, and thus, varying levels of cognitive ability. For example, Deaner et al, [ 57 ] found absolute brain size measures, over statistically produced methods i.e., residuals, to be the best predictors of primate cognitive abilities.…”

No evidence for general intelligence in a fish

“…highly preferred food) in the feeding against preference task size and brain size are systematically larger in ectotherms than in endotherms (van Schaik et al, 2021;Tsuboi et al, 2018). Data on sexually dimorph primate species and on cleaner fish suggest that despite the different intraspecific slopes, cognitive performance is uncorrelated with body size (van Schaik et al, 2021;. Thus, the slopes seem to indicate equivalence in cognitive performance.…”

“…a: number of trials to complete reversal learning task; b: mean number of head bumps into Plexiglas separation in the detour task; c: % correct choices in the numerical competence task; d: mean number of flakes eaten before a prawn item (i.e. highly preferred food) in the feeding against preference task size and brain size are systematically larger in ectotherms than in endotherms (van Schaik et al, 2021;Tsuboi et al, 2018). Data on sexually dimorph primate species and on cleaner fish suggest that despite the different intraspecific slopes, cognitive performance is uncorrelated with body size (van Schaik et al, 2021;.…”

“…However, recent analyses show that endotherm vertebrate brain development and organisation are fundamentally different from ectotherm vertebrates. For example, the early exponential growth phase of brains is much prolonged in endotherms (Tsuboi et al, 2018), and the intraspecific slopes between body size and brain size are systematically larger in ectotherms than in endotherms (van Schaik et al, 2021; Triki et al, 2021; Tsuboi et al, 2018). Data on sexually dimorph primate species and on cleaner fish suggest that despite the different intraspecific slopes, cognitive performance is uncorrelated with body size (van Schaik et al, 2021; Triki et al, 2021).…”

The evolution of quantitative sensitivity

“…Criticisms of using brain size measurements in comparative biology include that these measures do not actually capture variation in brain size across taxa well [69,70]. Some researchers contend that the number of cortical neurons, rather than brain size, is a better proxy for cognitive ability [71].…”