Brown rats and house mice eavesdrop on each other's volatile sex pheromone components.

Mammalian pheromones often linger in the environment and thus are particularly susceptible to interceptive eavesdropping, commonly understood as a one-way dyadic interaction, where prey sense and respond to the scent of a predator. Here, we tested the "counterespionage" hypothesis that predator and prey co-opt each other's pheromone as a cue to locate prey or evade predation. We worked with wild brown rats (predator of mice) and wild house mice (prey of brown rats) as model species, testing their responses to pheromone-baited traps at infested field sites. The treatment trap in each of two trap pairs per replicate received sex attractant pheromone components (including testosterone) of male mice or male rats, whereas corresponding control traps received only testosterone, a pheromone component shared between mouse and rat males. Trap pairs disseminating male rat pheromone components captured 3.05 times fewer mice than trap pairs disseminating male mouse pheromone components, and no female mice were captured in rat pheromone-baited traps, indicating predator aversion. Indiscriminate captures of rats in trap pairs disseminating male rat or male mouse pheromone components, and fewer captures of rats in male mouse pheromone traps than in (testosterone-only) control traps indicate that rats do eavesdrop on the male mouse sex pheromone but do not exploit the information for mouse prey location. The counterespionage hypothesis is supported by trap catch data of both mice and rats but only the mice data are in keeping with our predictions for motive of the counterespionage.