Radiology

Neuroscience

Evolutionary biology

Biology

Epistemology

Magnetic resonance imaging

Philosophy

Cognition

Psychology

Absolute (philosophy)

Encephalization

Neuron

Brain size

Medicine

Nature Ecology & Evolution

A longstanding 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 estimations of neuron numbers for 111 bird species, we show here that the number of neurons in the pallial telencephalon is positively associated with a major expression of intelligence: innovation propensity. 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. 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 significance 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. Using estimation data on neuron numbers in 111 bird species across 24 families, the authors show that number of neurons is positively associated with innovation propensity and encephalization.

Neuron numbers link innovativeness with both absolute and relative brain size in birds