So far, the list of animals known to be able to recognize their own reflection is slim, but now it seems the unsuspecting cleaner fish (Labroides dimidiatus) could be the latest addition.
A study into mirror self-recognition (MSR) has investigated the reactions of cleaner fish to images of themselves and of other members of their species. Looking at aggression behaviours, the team found that the fish appeared to be able to recognize their own individual faces.
Testing the fish’s ability to recognize its own reflection, the team first conducted mirror mark tests. In these tests, a mark resembling an ectoparasite was placed on the throats of 10 fish before being shown their own reflection in a mirror. All the sample fish passed this test by exhibiting throat scraping behavior along the bottom of the tank in an attempt to remove the suspected parasite.
Video credit: Kohda et al, PNAS 2023 (CC BY 4.0)
As cleaner fish are known to act aggressively towards unfamiliar members of their species, further testing involved presenting them with still images of cleaner fish. The researchers found that those who had not participated in the mirror mark tests acted aggressively to all images of cleaner fish regardless of if it was their own image being displayed. This suggests the mirror tests acted as a way for the fish to learn what their own reflection looked like.
To test self-face recognition abilities, the team presented the fish with either single images of themselves or of an unfamiliar fish. In addition, they also showed images composed of the fish’s own face on an unfamiliar fish’s body, or an unfamiliar fish’s face on their own body.
Interestingly, the results showed the fish acted aggressively to any images in which showed an unfamiliar fish’s face, suggesting that cleaner fish exhibit self-face recognition much the same as humans.
In additional testing to establish whether or not the fish were viewing the self-images as themselves, the two tests were combined by putting a mark on the throat of the fish in the different images. Results from this round of testing found that 75 percent of the fish tested exhibited throat scraping behaviors on the images of themselves, but not on the images of other fish, suggesting the cleaner fish were recognizing the images as being of themselves.
While the mechanisms underlying the ability remain unclear, and MSR’s relation to self-awareness a controversial assumption, it appears self-recognition may be more widespread across vastly differing species than previously thought.
The paper is published in Proceedings of the National Academy of Sciences.