Group size, patterns of interactions, and chemical recognition cues inform the collective organization of pavement ant (Tetramorium caespitum) wars

The pavement ant (Tetramorium caespitum) is a tramp species that is found in disturbed habitats and closely associates with human habitation. It is a structural pest, building its nests under building foundations and sidewalks, a common household pest in urban and suburban areas, and an agricultural pest. The species is well known for its "ant wars" in which hundreds to thousands of workers from different colonies fight in a large group over food or territorial resources. These ant wars self-organize when ants from different colonies interact; additional workers are recruited to the fight. I show here that T. caespitum workers discriminate nestmates and non-nestmates by detecting cues coded in the mixture of hydrocarbons and that the cue workers use to recognize non-nestmate ants is coded in the relative abundance of methyl-alkane hydrocarbons in the hydrocarbon profile. The number of ants from two colonies fighting in a group affects the proportion of ants fighting. A lower proportion of ants fight in small groups and a larger proportion of workers fight in larger groups.