Aggression and social plasticity in odorous house ants (Tapinoma sessile)

Odorous house ants (Tapinoma sessile) are a cosmopolitan North American species and common urban pest with a plastic social system. In their native forest habitats, they live in small, subdominant colonies containing a single queen. In the urban environment, T. sessile forms large polygynous colonies with hundreds of queens, and is often the second most abundant ant after the pavement ant (Tetramorium caespitum). Other behavioral effects of this change in lifestyle are poorly understood, including possible altered interactions between colonies. Due to polygynous colonies necessarily accepting greater diversity within the colony, we expected urban colonies to be less aggressive towards outsiders compared to natural colonies. Colonies of T. sessile from natural and urban habitats around Tippecanoe County, Indiana were collected and paired in a series of behavioral assays. Three different aggression assays were used: (1) worker dyad interactions, (2) small group interactions with five workers from each colony, and (3) colony fragment interactions which included limited water and nesting space. Contrary to our prediction, results demonstrate higher levels of aggression in pairings containing urban colonies. It is possible that conflict avoidance, rather than acceptance, is a mechanism driving this result. While it is unclear whether aggressive ants are more likely to colonize urban environments or if urban environments drive ants within them to become more aggressive, these results may help explain the observed dominance and competitive ability of odorous house ants in urban environments despite their relatively meek natural behaviors.