| Summary: | Many territorial animals reduce aggression toward their neighbors once territorial boundaries are established. This relationship is called the dear enemy phenomenon, hypothetically based on a conditional strategy like tit for tat (TFT). However, studies on territorial animals such as male songbirds do not fully support this hypothesis. We tested the TFT hypothesis in females of the territorial cichlid Neolamprologus pulcher, which exhibits dear enemy relationships, under laboratory conditions. Focal fish attacked invading neighbors as frequently as invading strangers, but they immediately stopped attacking the dear neighbor after the neighbor returned to its own territory, whereas they kept attacking strangers even after they stopped invading. These responses of territory owners are consistent with the predictions of TFT theory. Interestingly, fish were likely to remain vigilant toward the neighbor after its invasion and subsequent return to its own territory. This vigilance, while not a required condition for TFT, is consistent with TFT if a betrayer tends to repeat its intrusion.
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