Ant foraging path use responds to different types of risk and their encounter probabilities

Abstract

Ant colonies are likely able to access food locations by multiple paths that can vary predictably in length or mortality risk. To favor one path over another requires ants to perceive, communicate and act upon important differences in length and risk between paths. Here, we present replicate Linepithema humile colonies with four equal-length paths to a sugar source. The paths vary in probabilities of encountering a risk cue that range from 0 to 100%. The risk cues were either live workers of an aggressive competitor (Liometopum occidentale: LO) or only formic acid (FA), a defensive chemical common in many formicine species. Both the probabilities of encounter and type of cue affected path preferences. Although across both cues the zero-risk path was most used, the use was not exclusive and patterns of response to the cues differed significantly. More ants were on all the paths when the cue was LO rather than FA, leading to a higher success rate at finding food. Worker numbers on paths with LO did decline over time as consistent with recognizing neighbors to be ‘dear enemies’ that are a reduced threat to the colony. Similarly, changes in path usage suggested that the FA cue also became viewed as less threatening over time when competitors were never simultaneously present. The results are consistent with L. humile exhibiting the behavioral plasticity and communicative ability needed to categorize and predict risk to efficiently collect food while defending against other ant species.