The red and the black: habituation and the dear-enemy phenomenon in two desert Pheidole ants

Many species of territorial animals are more aggressive toward strangers th an neighbors, a pattern of aggression referred to as the 'dear-enemy phenom enon.' In many cases, the mechanism by which neighbors are discriminated fr om strangers and the function of neighbor-stranger discrimination remain co ntroversial. We investigated the spatial patterns of inter-colony aggressio n within and between two Pheidole species of seed-harvesting ants in the Mo jave Desert of California by quantifying aggression between colonies in sta ndardized staged encounters. We also tested whether the level of fighting b etween workers of two colonies is affected by previous exposure to each oth er. We show that neighbors (i.e., colonies less that 2.6 m away) of either species are treated less aggressively than more distant colonies and that h abituation may be a mechanism by which this discrimination is achieved. The variation in aggression among spatially distant colonies also suggests tha t additional genetic or environmental factors are involved in recognition. The function of the dear-enemy phenomenon in these ant species may be relat ed to the greater risk to the resources of a colony presented by strange wo rkers than workers from a neighboring colony.