Ants adjust their pheromone deposition to a changing environment and their probability of making errors

Abstract

Animals must contend with an ever-changing environment. Social animals, especially eusocial insects such as ants and bees, rely heavily on communication for their success. However, in a changing environment, communicated information can become rapidly outdated. This is a particular problem for pheromone trail using ants, as once deposited pheromones cannot be removed. Here, we study the response of ant foragers to an environmental change. Ants were trained to one feeder location, and the feeder was then moved to a different location. We found that ants responded to an environmental change by strongly upregulating pheromone deposition immediately after experiencing the change. This may help maintain the colony's foraging flexibility, and allow multiple food locations to be exploited simultaneously. Our treatment also caused uncertainty in the foragers, by making their memories less reliable. Ants which had made an error but eventually found the food source upregulated pheromone deposition when returning to the nest. Intriguingly, ants on their way towards the food source downregulated pheromone deposition if they were going to make an error. This may suggest that individual ants can measure the reliability of their own memories and respond appropriately.

Keywords: Lasius niger; environmental uncertainty; memory; metacognition; metamemory; pheromone trails.

Figure 1.

Experimental T-maze. Pheromone depositions were counted in the grey rectangular area. Ants were considered to have chosen one arm of the T-maze when their antennae had crossed a decision line. In every experiment, the feeder was first placed at the end of one arm of the T-maze, and after a set number of visits by the ants moved to the other arm.

Figure 2.

Main figure: mean pheromone depositions of ants returning from the food source to the nest, by the visit number relative to the food source location change. Note that pheromone deposition is upregulated immediately after experiencing the change event. For clarity, this figure shows data only from experiments in which ants made 15 training visits to the original feeder location before the food source location was changed. See the electronic supplementary material figure S1 for similar figures from all change treatments. Inset: mean pheromone depositions of returning ants by whether or not they had experienced an environmental change event, and whether or not they had made a branch choice error before finally locating the food. This figure shows data from both ants which did and did not deposit pheromone. See the electronic supplementary material, S1 for two similar figures (figures S2 and S3) for deposition probability of ants and deposition intensity of depositing ants. Electronic supplementary material, S1 also contains all statistical results for the post hoc pairwise analyses. (Online version in colour.)

Figure 3.

When travelling outwards towards a food source, ants deposited less pheromone if they were going to make an error at an upcoming bifurcation. When returning to the nest, ants which had made an error deposited more pheromone than ants which had made a correct decision. Different letters denote statistically significant values. These patterns are mainly driven by the proportion of ants choosing to deposit pheromone, and not by the modulation of the number of pheromone depositions by depositing ants. See the electronic supplementary material, S1 for similar figures (figures S5 and S6) in which pheromone laying probability and intensity are separated and for statistical details. Data from all visits and all treatments are pooled for clarity in this figure. (Online version in colour.)