A new study led by the Osaka Metropolitan University has provided evidence for the first time that fish have a sense of self. Through a series of experiments, the researchers discovered that cleaner fish (Labroides dimidiatus) can recognize themselves in a mirror, and that, in this process of self-recognition, the determining factor was not seeing their own bodies, but rather their faces.
The experts presented each cleaner fish with four pictures: a photo of themselves, one of an unfamiliar conspecific, one of their own face on an unfamiliar cleaner’s body, and one of an unfamiliar cleaner’s face on their own body. The fish did not attack photos with their own faces, but did attack those with the faces of unfamiliar fish (an aggressive behavior known to exist in the wild). These findings suggest that the cleaner fish were capable to determine who was in the picture based on the face on the photo, but not the body.
“Here, we show that cleaner fish, Labroides dimidiatus, likely recognize their own mirror image using a mental image of the self-face comparable to humans. Mirror-naïve fish frequently attacked photographs of both themselves and strangers. In contrast, after passing the mirror test, aggression against their own photograph and composite photographs of own face/stranger body declined, but aggression remained toward unfamiliar and composite photographs of stranger face/own body. Our results suggest that cleaner fish […] can recognize their own mirror image based on a mental image of their own face, rather than by comparing body movements in the mirror,” the authors explained.
To assess whether the fish considered photos of themselves as very close companions, the experts conducted a photograph mark-test, by presenting fish with a picture where a parasite-like mark was placed on their throat. While six of eight fish that saw the photo of themselves with a parasite mark were observed to rub their throats to clean it off, when showing those same fish pictures of themselves without parasite marks or of a familiar cleaner fish with parasite marks, they did not rub their throats.
“This study is the first to demonstrate that fish have an internal sense of self. Since the target animal is a fish, this finding suggests that nearly all social vertebrates also have this higher sense of self,” concluded study lead author Masanori Kohda, an expert in Animal Behavior at Osaka.
The study is published in the journal Proceedings of the National Academy of Sciences.
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