Spatial and temporal variation in pheromone composition of ant foraging trails

Many social insects use pheromones to communicate and coordinate their activities. Investigation of intraspecific differences in pheromone use is a new area of social insect research. For example, interindividual variation in alarm pheromone content has been found in physical castes of polymorphic ants. Many ant species use multiple trail pheromones. Here we present novel research into trail pheromone variations between behavioral subcastes of pharaoh ants, Monomorium pharaonis. Monomorium pharaonis is attracted to trail pheromones found in its poison glands (monomorines) and Dufour’s glands (faranal). We show that the most abundant monomorines, I (M1) and III (M3), can be readily detected in pheromone trails. A behaviorally distinct subcaste known as ‘‘pathfinder’’ foragers can relocate long-lived pheromone trails. Chemical analysis showed that pathfinder foragers had low M3:M1 ratios (mean 3.09 6 1.53, range 1.03–7.10). Nonpathfinder foragers had significantly greater M3:M1 ratios (38.3 6 60.0, range 3.54–289). We found that M3:M1 ratio did not differ between foragers of different age but was correlated with behavioral subcaste at all ages. The relative abundance of M3:M1 on foraging trails ranged from 3.03–41.3 over time during pheromone trail build-up. M3:M1 ratio also varied spatially throughout trail networks, being lowest on trail sections closest to a food source (M3:M1 1⁄4 1.9–3.61) and highest near the nest (M3:M1 1⁄4 67–267). Our results indicate a functional role for differences in pheromone trail composition, whereby pathfinder foragers might preferentially mark sections of pheromone trail networks for future exploration.