Abstract

True individual recognition (TIR), the ability to distinguish multiple familiar members individually, is more elaborate than class-level recognition, and evidence for the ability to perform TIR is reported from primates, some other social mammals, birds and lizard in vertebrates. These animals exhibit a highly social structure, wherein TIR is essential for their social interactions. Such high sociality has been documented in fish, but clear evidence of TIR has been limited. The cichlid, Neolamprologus pulcher, a cooperative breeder that guards a territory, exhibits the dear enemy relationship. Here, we show that this fish distinguishes two familiar neighbours individually, i.e., TIR ability, using one-way mirrors in experimental tanks. Focal fish established the dear enemy relationship with two neighbours, N A and N B , and displayed limited aggression towards these familiar neighbours. However, their aggressiveness towards neighbour N B increased when they were shifted from the original side of the tank after N A was removed, suggesting that they distinguished N B from N A or regarded N B as a stranger. Interestingly, this aggression level against the shifted neighbour N B largely decreased within 1 min. This decrease contrasted with the longer and more frequent aggressiveness towards unfamiliar strangers. These results suggest that focal fish recognised neighbour N B as a familiar stranger but probably punished N B that moved beyond its territory, that is, betrayed the dear enemy relationship. We prevented the effects of the behavioural reactions of exposed individuals using a one-way mirror. Thus, we conclude that this fish species displays TIR and discuss that TIR is prevalent in territorial animals in which the dear enemy effect is common.