Does a cichlid fish process face holistically? Evidence of the face inversion effect

Faces are the most important body part for differentiating among human individuals by humans. Humans read the face as a whole, rather than looking at its parts, which makes it more difficult to recognise inverted faces than upright. Some other mammals also identify each other based on the upright face and take longer to recognise inverted faces. This effect is called the face inversion effect and is considered as evidence for face-specific perception. This ability has rarely been observed in animals other than mammals, but it was recently reported that some fish species could distinguish among individuals based on the face. For example, the cichlid fish Neolamprologus pulcher rapidly recognises familiar conspecifics by faces rather than other body parts. Here, we examined the face inversion effect in N. pulcher, by showing photographs of conspecific fish faces and objects in both upright and inverted orientations. Subjects gazed at novel faces longer than familiar faces in upright presentation, whereas they did not show such a tendency for inverted faces. Although the object discrimination was difficult, we did not observe the difference between upright and inverted object photographs. Our results indicate that fish exhibits the inversion effect for faces. These findings suggest that N. pulcher may process their conspecifics' face holistically, like humans.