Division of labour underlies efficient colony defence in a clonal ant

The ecological success of social insects is often credited to the division of labour (DOL), yet empirical evidence directly linking DOL to colony efficiency is rare, in part because variation in DOL between colonies is often confounded by variation in colony traits (e.g., group size, genetic diversity) that can independently affect group efficiency. Here, we measure the link between DOL and efficiency in a crucial task, colony defence, in a social insect that affords precise experimental control over relevant individual and colony traits, the clonal raider ant (Ooceraea biroi). We find that DOL in defence behaviour emerges within colonies of near-identical workers, and is consistently associated with enhanced defence efficiency. This positive relationship is robust to variation in the social environment (group size and the presence and type of brood), indicating that the extent of behavioural variation between members of a social group can serve as a key indicator of group efficiency.