We analysed quantity discrimination data from 672 subjects across 33 bird and mammal species, using a novel Bayesian model that combined phylogenetic regression with a model of number psychophysics and random effect components. This allowed us to combine data from 49 studies and calculate the Weber fraction (a measure of quantity representation precision) for each species. We then examined which cognitive, socioecological and biological factors were related to variance in Weber fraction.

We found contributions of phylogeny to quantity discrimination performance across taxa. Of the neural, socioecological and general cognitive factors we tested, cortical neuron density and domain-general cognition were the strongest predictors of Weber fraction, controlling for phylogeny. Our study is a new demonstration of evolutionary constraints on cognition, as well as of a relation between species-specific neuron density and a particular cognitive ability.

…Quantitative sensitivity is an aspect of cognition that is ubiquitous among many species, and many researchers debate the nature of its evolutionary basis across taxa, including in humans and other primates.^1–6 Baboons use numerical estimation to guide troop movement,^7,8 desert ants and fiddler crabs navigate by keeping track of the number of steps they have taken,^9,10 and social species like hyenas and lions vocalize or approach other conspecific groups only when their group has a numerical advantage.^11–17 A diverse range of animals—from primates to reptiles, fish and insects—can discriminate numerical quantities in laboratory tasks, for example, comparing computerized arrays or sequences of pure tones to peck, touch or approach the numerically larger set.^2,4,18–22 Moreover, animals represent numerical values cross-modally^23–27 and under conditions where dimensions such as area, density and duration are equated, uncorrelated with numerical value or otherwise controlled.^{2,4,20,28–30}

…This is the first study to measure the origins of quantitative cognition with these methods. We found contributions of phylogeny to quantity discrimination performance across taxa, indicating evolutionary constraints on quantitative cognition. Additionally, a subset of neuronal and cognitive variables predicted species’ quantitative sensitivity—the strongest predictors were neuron density and general cognitive ability. The results indicate that when selecting an animal from the world at random, we can roughly predict its Weber fraction by knowing its species.

An individual’s Weber fraction was related to its species-typical cortical neuron density. Individuals from species with higher cortical neuron density had more precise Weber fractions. Thus, one constraint on an individual’s quantitative cognition is the biological capacity for information processing in their brain, as determined genetically and developmentally for each species.^74–81 Additionally, quantitative precision was related to neuron density in the cerebellum, a brain structure that has been overlooked in studies of vertebrate brain evolution and cognition.^89–91 We found that density of neurons was a more accurate proxy for quantitative ability than brain volume when comparing species across taxa. Caveats to the interpretation of the neuron density findings include (1) the number of species with cortical and cerebellar neuron density values is lower than for neuronal number, and (2) the relationship between neuron number and neuron density differs across animal groups. An increase in number of neurons in a primate brain structure does not mean larger neurons (and lower density), whereas in most non-primate mammals more neurons means larger neurons and lower density.^75,115 Our study is a rare demonstration of a relation between neuron number or neuron density and a particular cognitive ability. Previous within-species comparisons showed that neuron number in multiple brain regions did not predict performance on a battery of behavioral tasks in mice,¹³⁵ and though raccoons who performed best on a puzzle box task had more cells in their hippocampus than lower performing individuals, this difference may have been driven by glial cells.¹³⁶ However, cross-species comparisons in primates and birds suggest that neuron number has more behavioral explanatory power than cranial capacity, based on the correlation between cortical or pallial neuron number and performance on a self-control task.⁷¹ Our cross-species finding from birds and mammals implies that quantitative sensitivity is yoked to species-specific developmental programmes for neuronal density; therefore, some species are well-equipped to develop precise quantitative sensitivity whereas others may be unable to do so.

Our finding that primate species’ quantitative sensitivity improved with their domain-general cognition score indicates that general cognitive functions, perhaps in tandem with specialized quantitative functions, impacted the evolution of quantitative precision across species.

…Our novel analysis shows that biological features of a species’ evolutionary history likely modulate the development of individuals’ numerical cognition, a crucial finding that emphasizes the importance of phylogenetic constraints on cognition. Natural selection has biologically prepared some species to develop high neuronal densities and general cognitive capacities that yield precise quantitative representations. These data begin to reveal the evolutionary pressures that shaped numerical cognition across species and bring us closer to understanding the evolutionary precursors that sparked human mathematical cognition.

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