Face recognition in the Tanganyikan cichlid Julidochromis transcriptus

The face is an important cue for discriminating conspecifics in some primates (including humans), other mammals and birds. Although there is considerable evidence that fish can discriminate between conspecifics based on familiarity, the actual traits used to do so remain unclear. However, recent studies showed that two cichlid species used face colour patterns similarly to other vertebrates, and have suggested that social signals have evolved around the eyes, even in fish (face-specific hypothesis). In this study, we tested whether the striped, rock-dwelling Tanganyikan cichlid Julidochromis transcriptus can discriminate between familiar and unfamiliar conspecifics using facial patterns alone. We used a monitor to present fish with four digital models of combinations of facial and body patterns from familiar and unfamiliar fish. Focal fish spent more time near the monitor when presented with unfamiliar face models regardless of the body pattern. Therefore, we concluded that J. transcriptus recognizes familiar conspecifics using facial patterns alone, despite having distinct stripe patterns on the whole body. Our results are consistent with the face-specific hypothesis and indicate that social signals have evolved around the eyes of fish.