Longevity and detection of persistent foraging trails in Pharaoh's ants, Monomorium pharaonis (L.)

Pheromone trails provide a positive feedback mechanism for many animal species, and facilitate the sharing of information about food, nest or mate location. How long pheromone trails persist determines how long these environmental memories are accessible to conspecifics. We determined the time frame over which Pharaoh's ant colonies can re-establish a long-lived trail and how the behaviour of individual workers contributes to trail re-establishment. We used artificially constrained pheromone trails on paper to investigate trail longevity and individual responses. Trails formed by traffic of 1000–2000 ant passages could be re-established after 24 h, and after 48 h for 4000–8000 ant passages. Only 27.5% of individual foragers were highly successful in a bioassay testing the ability to detect trails established 24 h earlier. Trail-finding ability was significantly correlated with a low antennal position. Long-lived trail detection scores increased significantly in 57% of foragers after 5 h of food deprivation and isolation, but declined again after feeding. In a control study, only 9% of foragers changed their scores, when isolated with food present. A high degree of conservatism was found such that, regardless of treatment, 21% always failed the bioassay and 17% always succeeded. Our demonstration of long-lived components in Pharaoh's ant trails and of a behavioural specialization in ‘pathfinding’ shows that pheromone trails are more complex at the individual level than is generally recognized.