New Evidence Suggests Cleaner Fishes Recognize Themselves In Mirrors

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An animal who tries to remove a mark from her body that is only visible when looking into a mirror displays mirror self-recognition (MSR), a capability often regarded as evidence for self-awareness. The MSR test has been applied to many species of vertebrates, with negative results in some apes, monkeys, pigs, dogs, cats, and parrots. Some elephants, dolphins, horses, magpies, and crows show evidence of MSR, although such interpretations have been widely criticized. In fact, chimpanzees and orangutans are the only species who have undoubtedly passed the MSR test and, by extension, demonstrate self-awareness.

A previous Faunalytics summary reported on an MSR test for cleaner wrasse, a type of tropical fish from Asia. The authors argued that this type of test might not be an appropriate method to assess self-awareness due to biological differences between animal groups and species (for example, not all species rely on visual cues). However, the results were promising in that most of the fishes seemed to recognize themselves in the mirror. However, to be confident that the fishes actually passed the test, critics wanted to see repeated studies showing positive results, that a reasonable number of individuals can pass the test, and additional experiments to eliminate other explanations for mirror-related actions. The research group that conducted the original study set out to do just that. 

Besides testing more fishes, the group looked into ecological context. In the last study, they marked cleaner wrasse with a brown color, as this is believed to mimic the hue of parasites they eat. In this study, they also marked fishes with blue and green to understand how this would change mirror-directed behavior. To rule out alternative explanations for the MSR test, the researchers took the following steps:

• They moved the mirror to test whether fishes were habituated to the mirror itself, as opposed to seeing themselves reflected in it.
• They tested individuals who were previously exposed to mirrors as well as those who weren’t, hypothesizing that only individuals exposed to mirrors should be able to pass the test.
• They placed cleaner fish pairs across from each other to see if they could distinguish other individuals of the same species with marks on their bodies.

Remarkably, all 14 fishes subjected to the standard mirror passed in the study. Together with the group’s previous attempt, this brings the number of cleaner wrasse tested to 18 — the largest sample size for any nonhuman species tested except for chimpanzees. With the exception of dolphins, these fishes also show the highest rate of passing at 94%. In contrast, only a small proportion of individuals were shown to pass the test in other successful animal species: 40% in chimpanzees, 50% in orangutans, 30% in gorillas, 30% in Asian elephants, and 40% in magpies.

In terms of potential alternative explanations, the researchers ruled out skin irritation as the fishes tried to remove the mark only upon seeing it in the mirror. When implanted deeper, the mark elicited scratching behavior without mirrors, further supporting that the original marks were noticed by sight alone. Fishes unfamiliar with mirrors took longer to notice the spots on their throats, and seeing another fish with a mark did not result in self-scratching behavior, confirming that the fishes can distinguish themselves from others. Finally, moving the mirror did not renew aggressive behaviors, which suggests the fishes recognized themselves rather than becoming habituated to a specific location of the mirror. This differs from rhesus macaques, who appeared to pass the MSR test but became aggressive once the mirror was moved. 

The authors highlight the importance of choosing environmentally relevant markers when testing different animals for mirror self-awareness. In this case, the cleaner wrasses reacted to brown marks (similar to their diet source in color and size) and ignored blue and green marks. Cleaner fishes also show a variety of other advanced cognitive abilities beyond self-recognition. These include using social tools, configurational learning, and making use of their conspecifics in decision-making. It’s important to note that we cannot say with complete certainty that cleaner fishes possess self-awareness. However, the evidence from both this study and the previous one are difficult to dismiss. In the end, the researchers suggest that either criteria for establishing self-awareness in animals or the validity of the mirror test needs to be revised to improve future investigations.

Although the degree of self-awareness may differ between species in ways that are independent of how a species performs in the mirror test, it is no less impressive what cleaner fishes are capable of. With fish farming on the rise, lumpfish and wrasse are widely used in the aquaculture industry for their remarkable ability to pick off parasites from other fish. Unfortunately, their welfare and needs might be largely overlooked, and animal advocates can use the results of this study to call this into question.