Conspecific ant aggression is correlated with chemical distance, but not with genetic or spatial distance

Five possible mechanisms might underlie kin recognition in social groups: spatial location, familiarity through prior association, phenotype matching, recognition alleles, or rejecting unfamiliar cues. Kin recognition by phenotype matching relies on a strong correlation between genotype and phenotype. Aggression bioassays are the standard method for investigating recognition in animals, particularly social insect interactions among nestmates and non-nestmates. These bioassays typically pay little regard to how outcomes are determined by differences in chemical recognition cues of the test subjects, because the system of signal coding was unknown until recently. We exploited the known nestmate recognition system of the ant Formica exsecta to investigate aggression between 24 pairs of colonies across a range of chemical (Z9-alkene & n-alkanes), genetic, and spatial distances. The whole Z9-alkene chemical profile was the only significant (p < 0.001) predictor of aggression levels. Aggression was a nonlinear step function of Z9-alkene chemical distance, where a small change in chemical profile resulted in a rapid behavioural transition from non-aggression to overt aggression. These findings raise questions surrounding our current understanding of recognition systems, because they support phenotype matching to a colony chemical profile without a significant genetic or spatial component.