Facial Recognition in a Discus Fish (Cichlidae): Experimental Approach Using Digital Models.

Shun Satoh,Hirokazu Tanaka,Masanori Kohda

doi:10.1371/journal.pone.0154543

Abstract

A number of mammals and birds are known to be capable of visually discriminating between familiar and unfamiliar individuals, depending on facial patterns in some species. Many fish also visually recognize other conspecifics individually, and previous studies report that facial color patterns can be an initial signal for individual recognition. For example, a cichlid fish and a damselfish will use individual-specific color patterns that develop only in the facial area. However, it remains to be determined whether the facial area is an especially favorable site for visual signals in fish, and if so why? The monogamous discus fish, Symphysopdon aequifasciatus (Cichlidae), is capable of visually distinguishing its pair-partner from other conspecifics. Discus fish have individual-specific coloration patterns on entire body including the facial area, frontal head, trunk and vertical fins. If the facial area is an inherently important site for the visual cues, this species will use facial patterns for individual recognition, but otherwise they will use patterns on other body parts as well. We used modified digital models to examine whether discus fish use only facial coloration for individual recognition. Digital models of four different combinations of familiar and unfamiliar fish faces and bodies were displayed in frontal and lateral views. Focal fish frequently performed partner-specific displays towards partner-face models, and did aggressive displays towards models of non-partner’s faces. We conclude that to identify individuals this fish does not depend on frontal color patterns but does on lateral facial color patterns, although they have unique color patterns on the other parts of body. We discuss the significance of facial coloration for individual recognition in fish compared with birds and mammals.

Highlights

Animals that repeatedly encounter each other in communities structured by dominance hierarchies or territoriality often develop social signals facilitating the recognition of individuals [1,2,3,4]
We proposed three hypotheses to explain why individual color signals of fish tend to develop in the facial area [15]: (1) ‘Encountering’ hypothesis: When fish encounter each other, they will often approach in a ‘head-to-head’ position, where signals on the frontal parts of the fish will be most effective. (2) ‘Main body’ hypothesis: The critical social signal should be on the main parts of the body, not on peripheral parts that could be damaged or lost in conflict, such as the fins. (3) ‘Face-specific’ hypothesis: The face will inevitably be the site of visual signals for individual recognition
Since fish consistently conducted partner-displays toward partner models (PfPb) and non-partner-displays toward non-partner models (NfNb) laterally presented, this experiment showed that this fish is capable of distinguishing between pair partner and non-partner

Summary

Introduction

Animals that repeatedly encounter each other in communities structured by dominance hierarchies or territoriality often develop social signals facilitating the recognition of individuals [1,2,3,4]. Such signals may be beneficial for both signal senders and receivers on average [2]; for example, they may help to avoid costly physical fighting associated with social conflict. Visual signals for individual identification are common among mammals and birds, and a number of species use facial patterns as visual recognition cues (e.g. primates [5,6,7], sheep [8,9] and birds [10,11,12]). Permanently territorial damselfish may recognize neighbor individuals based on their facial coloration, which varies among individuals [14,15,18]

Methods

Results

Conclusion