“Neuron Numbers Link Innovativeness With Both Absolute and Relative Brain Size in Birds”, Daniel Sol, Seweryn Olkowicz, Ferran Sayol, Martin Kocourek, Yicheng Zhang, Lucie Marhounová, Christin Osadnik, Eva Corssmit, Joan Garcia-Porta, Thomas E. Martin, Louis Lefebvre, Pavel Němec2022-07-11 (, )⁠:

A long-standing issue in biology is whether the intelligence of animals can be predicted by absolute or relative brain size. However, progress has been hampered by an insufficient understanding of how neuron numbers shape internal brain organization and cognitive performance.

On the basis of [substantially updated] estimations of neuron numbers for 111 bird species [in 24 avian families], we show here that:

the number of neurons in the pallial telencephalon is positively associated with a major expression of intelligence: innovation propensity [>4,400 published reports of bird species using novel foods or new feeding techniques in the wild]. The number of pallial neurons, in turn, is greater in brains that are larger in both absolute and relative terms and positively covaries with longer post-hatching development periods.

Figure 2: Neurons and innovation propensity. Relationship between neuron numbers and innovation propensity for the entire brain and the pallium, cerebellum and brainstem, as predicted by models. a, Absolute neuron numbers. b, Neuron numbers adjusted by body size by including body mass (previously subtracting brain mass) as covariate in the model. c, Density of neurons (cells per mg). All models account for the effect of phylogeny, biogeographic realm and confounding variables (Supplementary Tables 1 & 2). Lines show the values predicted by Bayesian phylogenetic mixed models and the lower and upper bounds are the credibility intervals representing the uncertainty interval of the prediction. Sample size is 99 species, as nocturnal specialists (owls) are excluded from the innovation database.

Thus, our analyses show that neuron numbers link cognitive performance to both absolute and relative brain size through developmental adjustments. These findings help unify neuro-anatomical measures at multiple levels, reconciling contradictory views over the biological importance of brain expansion. The results also highlight the value of a life history perspective to advance our understanding of the evolutionary bases of the connections between brain and cognition.

…the relationship between neuron numbers and brain size is complex. The relationship tends to be roughly linear for relative brain size, especially when we exclude owls, but only for the entire brain and the pallium (Figure 4b). In contrast, neuron numbers tend to asymptote at larger absolute brain sizes in all cases (Figure 4c). This last finding agrees with the notion that animals that have large brains merely because they have very big bodies are not necessarily the most intelligent, as it is the case for Ratites and large Galliformes.

Figure 4: Neuron numbers as a function of absolute and relative brain size. a, Bivariate dependence plots representing neuron numbers in the entire brain and main brain regions as a function of absolute and relative brain size, based on the predictions from random forests. Colors describe neuron numbers, with low numbers represented by dark-blue colors and higher numbers by yellow-green colors. Relative brain size was estimated by means of the normalized scaled brain index, with the allometric exponent estimated excluding clades that have been found to exhibit substantial grade shifts in brain:body allometries (NSBIgrades; Methods). b, Univariate representations (partial dependence plots) for relative brain size to further interpret the bivariate dependence plots. The plots show the dependence between neuron numbers and relative brain size, marginalizing over the values of absolute brain size. c, Univariate representation of the bivariate dependence plot for absolute brain size. In b and c, lines show the values predicted by random forests and the lower and upper bounds are the credibility intervals representing the uncertainty of the prediction. In all analyses, owls have been excluded. For analyses with the entire sample of species, see Supplementary Figures 6 & 7. (M = million).
Figure 4: Neuron numbers as a function of absolute and relative brain size. a, Bivariate dependence plots representing neuron numbers in the entire brain and main brain regions as a function of absolute and relative brain size, based on the predictions from random forests. Colors describe neuron numbers, with low numbers represented by dark-blue colors and higher numbers by yellow-green colors. Relative brain size was estimated by means of the normalized scaled brain index, with the allometric exponent estimated excluding clades that have been found to exhibit substantial grade shifts in brain:body allometries (NSBIgrades; Method). b, Univariate representations (partial dependence plots) for relative brain size to further interpret the bivariate dependence plots. The plots show the dependence between neuron numbers and relative brain size, marginalizing over the values of absolute brain size. c, Univariate representation of the bivariate dependence plot for absolute brain size. In b and c, lines show the values predicted by random forests and the lower and upper bounds are the credibility intervals representing the uncertainty of the prediction. In all analyses, owls have been excluded. For analyses with the entire sample of species, see Supplementary Figures 6 & 7. (M = million).

…The reason why the dual role of absolute and relative brain size in cognition has been underappreciated in the past probably reflects the common practice of removing the allometric effects of body size in comparative analyses of brain size. As suggested by2, this is probably legitimate when comparing brains of species with striking differences in body size, like an ostrich and a hummingbird. Yet by treating body size as a statistical nuisance, we appear to be missing important information.

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Neuron Numbers Link Innovativeness With Both Absolute and Relative Brain Size in Birds