Numbers of neurons as biological correlates of cognitive capability

“2016), and it has been suggested that total number of pallial neurons is a better predictor of cognitive ability—including cylinder task performance—than absolute brain size (Herculano-Houzel 2017).”

“A comparison of the cognitive abilities of 75 species of mammals and birds demonstrated that the number of neurons in the cortex or avian pallium (see Glossary) correlates with measurements of cognition [4].”

“We propose that cognitive control over the Lombard effect in distinct model species depends on the relative size of the prefrontal cortex [104] and the strength of its interconnections with other cortical and subcortical brain structures.”

“The discrepancy between the large variation in body mass and number of brain neurons is consistent with the trend that we have revealed of much faster increases in body mass than in numbers of brain neurons (Herculano-Houzel, 2017).”

“In a recent opinion piece, Herculano-Houzel (2017) also argued that (cortical) neuron number outperforms total brain size as a predictor of behavioral performance in self-control tests reported by MacLean and colleagues (2014).”

“…cognitive skills available to the individuals of a certain species, which in turn determine the “typical” behavioural patterns expressed by individuals and populations of that species (Tinberg, 1951; Shettleworth, 2010; Miklosi, 2014; Herculano-Houzel, 2017; Santolin & Saffran, 2017; Moss, 2018).”

“Specifically, in most mammals, as brain volume increases, average neuron size tends to increase, and neurons become less densely populated in the brain (Herculano-Houzel 2014, 2017; Herculano-Houzel et al.”

“However, uncertainty remains about the other major component of the central nervous system (CNS), the spinal cord, in terms of numerical composition, for both the human and the nonhuman primate spinal cord (Herculano-Houzel, 2017).”

“As recently discussed by Herculano-Houzel [50], brain mass is only a proxy for the neuronal capability devoted to complex information processing.”

“It might be objected that comparing distantly related taxa on their cognitive performance based on brain measures only is not advised given the known differences in neuron densities in the brains of different clades [Herculano-Houzel, 2017] and the effect on brain size of highly divergent bauplans.”

“Indeed, a growing body of comparative evidence suggests that the absolute number of neurons in the telencephalon is a particularly good correlate of cognitive abilities (Dicke and Roth 2016; Olkowicz et al. 2016; Herculano-Houzel 2017).”

“In order to examine whether total numbers of pallial neurons (Herculano-Houzel 2017) and high performance in previous cognitive studies (Güntürkün and Bugnyar 2016) indeed link to such basic functions as motor self-regulation, we tested four species of large parrots in the cylinder task.”

“In line with these computational advantages is the behavioral finding that the number of neurons in the cortex of different species correlates with their cognitive performance (Herculano-Houzel 2017).”

“It should be noted that these findings disagree with the concept of the encephalization quotient (Jerison, 1973), as recently discussed (Herculano-Houzel, 2017).”

“(brain stem, diencephalon, striatum) (29, 30).”

“While it therefore seems quite plausible that observed effects of body size could be attributable to the correlation with brain weight (Bronson 1979; Carreira 2016; Horschler and MacLean 2019), perhaps due to an increased number of neurons in larger brains within a given lineage (Herculano-Houzel 2017; Jardim-Messeder…”

“1589 (Dicke & Roth, 2016; Herculano-Houzel, 2012a; Herculano-Houzel, 2017) and large pallial neuron numbers could also underlie corvid cognition (Herculano-Houzel, 2017).”

“Finally, the number of neurons in the mammalian cerebral cortex, or in the bird pallium, appears to be good predictors of inhibitory control (Herculano-Houzel 2017).”

“Although details about relative or absolute brain size and similar variables are debated [5], the idea that larger brains can in principle provide more neurons and thus more computing power is generally accepted [5,7].”

“52cm(3)) perform at 100%. With the inclusion of three corvid species, Kabadayi et al. (2016) showed that relative (or residual) brain volume also was a reliable predictor of performance in the cylinder task in birds.”

“The number of neurons in this part of the brain might indeed reflect levels of cognitive performance better than absolute or relative brain size (Herculano-Houzel, 2017; Jacobs et al., 2019).”

“…in these than in other mammals of similar body mass or even brain mass, given the clade-specific scaling of numbers of brain and body mass with the number of cortical neurons across endotherm species (Herculano-Houzel, 2017; Herculano-Houzel, Catania, Manger, & Kaas, 2015; Olkowicz et al., 2016).”

“The number of cortical neurons is related to information processing capacity, and can predict general cognitive abilities across species, such as the ability to control prepotent responses or make precise quantitative judgements 180,184,279,280 .”

“For instance, positive associations have been found between improved cognitive/behavioural abilities and overall brain size [1 – 10], brain region sizes [11 – 16], neuron cell numbers [17], and potentially neural connectivity [18].”

“…that the rodent brain can be considered as a distributed network with many discrete regional networks interacting to produce an immense spectrum of behavior (Koob and Volkow, 2016; Herculano-Houzel, 2017) and at each level this network is a quantitative network at a cellular and synaptic level.”

“This structure has been implicated with the evolution of complex cognitive capabilities in primates (Doty 2005; Passingham and Wise 2012; Herculano-Houzel 2017; Smaers et al. 2017).”

“For instance, it could be that changes in neural connectivity, neuronal activity or gene expression, while not essentially leading to volume changes, were responsible for the observed group performance differences (Weitekamp and Hofmann 2014; Herculano-Houzel 2017; Wallace and Hofmann 2021).”

“However, crossspecies comparisons in primates and birds suggest that neuron number has more behavioural explanatory power than cranial capacity, basedon the correlation between cortical orpallial neuron number and performance on a self-control task [71].”

“This subset of neurons, found in the cerebral cortex for mammals and in the pallium for birds, has been shown to correlate with cognition measurements (Herculano-Houzel 2017).”

“This conclusion holds even if controlled for taxonomic variation in neuron densities [57,58].”

“These are the signal processing units that make behavior flexible and complex and should thus constitute a primary determinant of signal processing capacity (Williams and Herrup, 1980; Herculano-Houzel, 2017; Ströckens et al., 2022).”

“One of the central questions, however, is whether the neuronal density was similar between paravians and palaeognaths, because the number of neurons is currently one of the best neurobiological correlates to cognitive performance (58, 59).”

“As Herculano-Houzel [61] emphasised, body mass is very variable and not tightly correlated with the number of neurones in the brain.”

“Neuronal and non-neuronal cells, Glia‚ are two highly specialized cells in the human brain that typically are responsible for direct and indirect supervision of whole body [27].”

“Where possible she correlated neuron numbers in the cortex of primates and pallium of birds with their cognitive performance [Herculano-Houzel, 2017].”

“Indeed, Herculano-Houzel (2011, 2017, 2023) has argued emphatically that neuron counts represent reliable estimates for cognitive abilities in extant vertebrates, markedly outperforming other measures such as relative or absolute brain size.”

“One study on brain size and neuron numbers was provided by the author FS based on prior knowledge 35 .”

“As the packing density of pallial neurons, regardless of pallial, brain, or body size, might be the best predictor of cognitive performance [64,83], these groups of birds cannot be overlooked in research on executive functions, and particularly in large-scale cross-species comparisons (e.”

“Combined with the data on the guppy and limited information on individual variation in mammals and birds [1,6,35] and our own unpublished data, it does not seem that there is generally a higher variability in brain traits in species with prolonged neurogenesis, although more species need to be investigated.”

“Primates have relatively high packing densities of neurons, such that they have more neurons than other mammals of identical brain sizes, and humans simply have the largest brains of primates, totaling 86 billion neurons (Herculano-Houzel, 2012, 2017).”

“It has been traditionally believed that species with larger brain size display greater cognitive abilities (reviewed in Jerison 2012), although current knowledge does not support the generality of this relationship (Healy & Rowe, 2013; Herculano-Houzel 2017; Perry et al., 2017).”

“Thus, number of pallial neurons seems to be a better correlate of cognitive abilities than number of all brain neurons or neurons in other brain parts [18]**,[31], even though abundant evidence also demonstrates the role of the cerebellum in various cognitive functions (for review, see [32,33]) and increase of its siz…”

“…the weight of its associated function on behavior ( Barks et al., 2015 ; Barton and Harvey, 2000 ; DeCasien and Higham, 2019 ; Ferrucci et al., 2022 ; Herculano-Houzel, 2017 ; Herculano-Houzel et al., 2016 ; Louail et al., 2019 ; Maguire et al., 2000 ; Sallet et al., 2011 ; Testard et al., 2022 ).”

“Insects, having smaller brains, might thus also be expected to perform poorly at tasks requiring MTT to the future (54, 55).”

“While the size and scale of nervous systems vary widely across species, it is striking that they nevertheless perform the key function of enabling the organism to respond appropriately to an ever changing environment [2].”

“Furthermore, while the scaling of neuron number with brain mass or region volume has been shown to differ between clades (Herculano-Houzel, 2011 , 2017 ), it is not known to vary within species.”

“…in the cerebral cortex (also commonly called neocortex) in humans (16.3 billion) has been reported to greatly exceed that in any other vertebrate group, including gorillas (9 billion) and chimpanzees (6–7 million) (Azevado et al., 2009; Herculano-Houzel, 2009, 2017; Olkowicz et al., 2016).”

“Furthermore, the total number of neurons seems to correlate with behavioural complexity and higher cognitive capabilities (Herculano-Houzel, 2017; Logan et al., 2018).”

“Although the correlation between brain size and cognitive ability has long been debated, the absolute number of neurons in the telencephalon has been shown to be a good proxy for cognitive abilities (Herculano-Houzel, 2017), this number being correlated with brain mass (Marhounová et al., 2019).”

“It might be objected that comparing distantly related taxa on their cognitive performance based on brain measures only is not advised given the known differences in neuron densities in the brains of different clades [Herculano-Houzel 2017] and the effect on brain size of highly divergent bauplans.”

“(Ngwenya et al. 2016 ; Dicke and Roth 2016; Olkowicz et al. 2016; Marhounová et al. 2019; Herculano-Houzel 2017) (Billings et al. 2020); Lazcano et al. 2021).”

“We reasoned that if we do find a clear brain size effect on test performance (cf. [26]), this argues for STM being an expression of domain-general cognitive abilities, rather than a domain-specific cognitive adaptation to a specific ecological (e.g. remembering the presence of temporarily invisible…”

“For example, despite field reports showing a relationship between cognition and innovation in birds and primates (Sayol et al. 2016; Herculano-Houzel 2017; Sol et al. 2022), experimental assays have revealed a broad range of alternative mechanisms of innovation (see Logan et al. 2018 for a review),…”

“, 2016; Nìmec and Osten, 2020), it has been suggested that neuron numbers or neuronal density might be a better indicator of the brain processing capacity rather than just looking at size (Herculano-Houzel, 2017; Kverková et al., 2022).”

“…size (supporting the theory that neuronal counts are a predictor of cognitive ability), factors such as neuronal size and density must also be considered (Herculano-Houzel, 2017; Herculano-Houzel, Messeder, Fonseca-Azevedo, & Pantoja, 2015) both of which are currently unknown for American crows.”

“The simplicity of the task constrains the rich and high-dimensional repertoire of neural activity that is observed during natural behavior [7, 24], and despite its apparent simplicity, the animals often need many trials to learn the task.”

“…that larger, gyrencephalic brains are characterized by low cortical neuron density with respect to small brains, a fact that has been related with higher dendritic complexity belonging to larger, more complex neurons (Herculano-Houzel 2017; Heyer et al. 2022; Galakhova et al. 2022; Brecht 2025).”

“Since then, alternative measures, such as cortical neuron number [13] or absolute brain size [50], have been discussed as brain measures that correlate with cognitive performance.”