Cost-effective eavesdropping between competing bee species: Empirical and theoretical support

Background/Question/Methods

Interactions between competitors can be indirect, such as foraging decisions based on "social information" gained from monitoring others' interactions with the environment. Eavesdropping, or exploitation of evolved signals aimed at others, in particular, has the potential to strongly affect dynamics within a foraging guild. Despite its ecological implications, little is known about eavesdropping within a trophic level. To investigate rules underlying such behavior, we tested heterospecific eavesdropping on food recruitment pheromone by stingless bees (Hymenoptera: Apidae, Meliponini). Anecdotes and previous eavesdropping studies suggested that the dominant Trigona hyalinata would be attracted to the pheromone of T. spinipes. Dominant species should eavesdrop on subordinate species to facilitate discovery of high-quality resources that they can take over. We used preference tests with free-foraging bees to test this hypothesis. We then developed a decision-analysis model to test the fitness effects of the eavesdropping strategy documented in our experiments.

Results/Conclusions

Trigona hyalinata foragers showed a concentration-dependent response to T. hyalinata food recruitment pheromone. They were attracted to low concentrations, but avoided the higher concentrations maintained by an actively-foraging T. spinipes colony. Empirical results suggested that dominant eavesdroppers will avoid heterospecific food location signals when the conflict costs indicated by those signals are too high. These costs are incurred not only during physical interactions, but also during "access time" when the dominant species is displacing subordinates. Modeling results confirm that matching eavesdropping behavior to non-physical costs maximizes net benefit to the focal colony, and should thus increase the colony's fitness. Our work significantly improves understanding of eavesdropping within a trophic level. It indicates that eavesdroppers respond to perceived costs, and highlights the importance of indirect conflict costs such as access time.