Reaction-Time Signatures Reveal Divergent Cognitive Strategies Underlying Numerical Decisions in Monkeys and Crows

Non-symbolic numerical competence, supported by the approximate number system (ANS), is widespread across animals, yet it remains unclear whether similar numerical performance reflects shared cognitive mechanisms. We compared two rhesus monkeys and two carrion crows on an extended delayed match-to-numerosity task. Animals viewed a sample numerosity (1–4), maintained it across a delay, and searched sequentially through up to three test displays for a match. Behavioral accuracy was comparable across species, with both showing classical ANS signatures such as numerical distance and size effects. However, reaction-time analyses revealed striking species differences: monkeys’ responses slowed with increasing numerosity and later test phases, whereas crows responded faster over later test phases and showed numerosity-invariant reaction times. Classifier decoding further confirmed that numerosity and test phase were strongly reflected in monkeys’ reaction times but only weakly in crows’. These findings demonstrate that equivalent numerical performance can arise from fundamentally different cognitive strategies, as revealed by distinct reaction-time dynamics, highlighting how evolutionary constraints shape species-specific solutions to the same computational problem.