LAUSR

Abstract According to the dear enemy phenomenon, territory owners decrease costs of ownership by decreasing aggression toward neighbors once territory boundaries have been established. To maintain this cooperation, when territorial neighbors are caught in the act of “cheating” by failing to respect territory boundaries, they should be “challenged” via increased aggression. Most studies examining this prediction tested neighbor recognition through playback of advertisement calls. We tested whether territorial terrestrial salamanders, Plethodon serratus , would identify cheating neighbors via chemical or visual cues. In separate experiments, focal residents were exposed to either territorial substrate markings from another salamander or to a mirror (simulating a territorial neighbor) on one side of their territory for several days. During subsequent testing, the chemical or visual (mirror) cues were presented at either the trained location (cooperating) or on the opposite (cheating) side. Either olfactory or visual cues alone were sufficient for discrimination of cheating/cooperating neighbors, with cheating neighbors receiving the most aggressive displays and chemosensory sampling behavior from focal residents. Aggressive displays were particularly high in trials with visual cues, which likely indicated a more immediate threat. Moreover, responses to the visual threat may have been more escalated because the “neighbor” did not back down following threat posturing from the focal resident. Qualitatively, focal residents attempted to physically interact with the mirror images even in cooperating neighbor treatments. Therefore, in accordance with the dear enemy hypothesis, cheating neighbors elicited a higher level of threat displays than cooperating neighbors, but even cooperating neighbors could sometimes be challenged at the territory border. Significance statement For territorial neighbors, cooperating by respecting territorial boundaries can reduce the cost of ownership by reducing aggressive interactions between neighbors. Cooperating neighbors (“dear enemies”) remain in their own territories, whereas cheating neighbors travel beyond their territory boundaries. Evolutionarily, cooperation can be maintained if neighbors that “cheat” must pay a cost. Thus, location of individuals (detected via advertisement signals) is predicted to influence aggressive behavior by adjacent neighbors. We demonstrated that territorial southern red-backed salamanders responded with a higher level of aggressive displays to cheating neighbors whose presence was detected via either chemical (substrate markings) or visual (mirror) displays. In mirror trials, focal residents paced back and forth with their mirror images, exhibiting short bursts of threat posturing and tapping the image with their snouts, which is consistent with behavior toward other salamanders. Therefore, cooperation between neighbors can be maintained via increased aggression toward cheaters.