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.

Centre for Biodiversity and Environment Research Department of Genetics Evolution and Environment University College London London UK

CSIC CREAF Cerdanyola del Vallès Catalonia Spain

Departament de Genètica Microbiologia i Estadística Facultat de Biologia and Institut de Recerca de la Biodiversitat Universitat de Barcelona Barcelona Catalonia Spain

Department of Biology McGill University Montréal Québec Canada

Department of General Zoology University of Duisburg Essen Essen Germany

Department of Zoology Faculty of Science Charles University Prague Czech Republic

Montana Cooperative Wildlife Research Unit University of Montana Missoula MT USA

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