Small‑number discrimination in a solitary ground beetle (Anthia thoracica)

<title>Abstract</title> <p> Numerical abilities are essential for survival, as animals must often discriminate quantities to optimize foraging and predation. In vertebrates, such abilities are typically linked to an evolutionarily ancient “Number Sense” supported by two systems: the Object File System (OFS), enabling precise tracking of small sets, and the Analogue Magnitude System (AMS), supporting approximate estimation of larger numerosities. In invertebrates, most evidence comes from social Hymenoptera, where numerical cognition is closely tied to central‑place foraging and colony organization, making it difficult to disentangle social influences from more general cognitive mechanisms. Here, we examined spontaneous and learned numerical discrimination in <italic>Anthia thoracica</italic> , a solitary ground beetle not known to be a central‑place forager. In spontaneous choice tests, beetles preferred the larger set of food items only in the 1 vs. 3 condition, failing in 1 vs. 4 contrast, and showed no bias in equal‑numerosity controls. This pattern suggests a small‑number limit consistent with OFS‑like processing rather than AMS‑based ratio sensitivity. In a second task, beetles were trained to associate a visual stimulus depicting three elements with a reward. They successfully learned this association and generalized it to novel spatial configurations, indicating that their proto‑numerical abilities extend beyond biologically relevant stimuli. Overall, these findings demonstrate that a non‑social beetle can discriminate small numerosities in both ecological and abstract contexts, with a limit of about three items. This supports the idea that basic numerical abilities in insects rely on ancient and widespread cognitive mechanisms, and can arise independently of eusociality or central‑place foraging. </p>