The Upside of Recognition Error? Artificially Aggregated Colonies of the Stingless Bee Tetragonula Carbonaria Tolerate High Rates of Worker Drift

Defence behaviours allow animals to evade predation or other attack, but in some cases carry significant costs to deploy. Costly defences are more likely to be favoured where the risks of mistaking harmless interactions for attacks ('false alarms') are low. Here we investigate if and how the stingless bee Tetragonula carbonaria (Apidae: Meliponini) avoids deploying costly 'defence swarms' (used to repel conspecific attack) in response to the accidental drift of foreign workers into the nest (false alarms). We found that colonies in artificial aggregations regularly received drifted workers (0-42% of returning foragers) without triggering their swarm response. This tolerance depended on the proximity of the drifted worker's nest: workers from nearby colonies were more accepted than workers from distant colonies. We suggest this tolerance is unlikely to be the result of natural selection, because we found naturally occurring colonies in native forest were rarely aggregated and did not experience worker drift. Rather, the acceptance of foreign intruders observed among artificially aggregated colonies likely arises because such conditions reduce the cues available for successful discrimination between nestmates and non-nestmates. Such recognition error benefits T. carbonaria under artificial conditions, as it prevents colonies from launching costly defensive swarms in response to harmless drifters.