Towards a unified biology of vertebrate number sense

Number sense, the ability to estimate and compare numerical quantities, emerges early in development and is widespread across species. Despite decades of work on behavioural and neural correlates of numerosity, the genetic and mechanistic foundations remain poorly integrated. We propose a review and perspective that brings together evidence from vertebrates. Our unique focus relies on how genetic variation, brain lateralisation, and neural circuit organisation across development shape number cognition. We highlight zebrafish (Danio rerio) and chicks (Gallus gallus) as complementary comparative models that can uncover conserved principles and novel mechanisms, opening new avenues for studying the biological bases of numerical cognition and its disorders from the very early developmental stages. This review integrates behavioural, genetic and neural evidence for number sense across vertebrates, with a focus on zebrafish and chicks. Current evidence indicates that numerical abilities rely on evolutionarily conserved mechanisms that emerge early in development and can be linked from genes to neural circuits.