The mandible opening response: quantifying aggression elicited by chemical cues in ants

Social insects have evolved efficient recognition systems guaranteeing social cohesion and protection from enemies. To defend their territories and threaten non-nestmate intruders, ants open their mandibles as a first aggressive display. Albeit chemical cues play a major role in discrimination between nestmates and non-nestmates, classical bioassays based on aggressive behaviour were not particularly effective in disentangling chemical perception and behavioural components of nestmate recognition by means of categorical variables. We therefore developed a novel bioassay that accurately isolates chemical perception from other cues. We studied four ant species: Camponotus herculeanus, C. vagus, Formica rufibarbis and F. cunicularia. Chemical analyses of cuticular extracts of workers of these four species showed that they varied in the number and identity of compounds and that species of the same genus have more similar profiles. The antennae of harnessed ants were touched with a glass rod coated with the cuticular extract of (a) nestmates, (b) non-nestmates of the same species, (c) another species of the same genus and (d) a species of a different genus. The mandible opening response (MOR) was recorded as the aggressive response. In all assayed species, MOR significantly differed among stimuli, being weakest towards nestmate odour and strongest towards odours originating from ants of a different genus. We thus introduce here a new procedure suitable for studying the chemical basis of aggression in ants.