Species Of Fish Passes The Mirror Test For Self-Recognition

If the past 100 years or so of biology have taught us anything, it is that human beings are not as unique a species as we thought we were. More and more we are discovering that non-human animals possess mental capacities once thought to be the sole province of humankind. One of these mental capacities is self-awareness, seen as a sort of hallmark of consciousness.

The number of species that are able to recognize themselves in a mirror is small but surprisingly diverse; greater apes, a single species of elephant, dolphins, orcas, magpies, and even ants. Now, a scrappy species of fish has joined those prestigious ranks!


In a paper published on bioRxiv, a team of biologists at Osaka City University reports that the cleaner wrasse, a small species of fish, has passed the mirror self-recognition (MSR) test. The MSR test is meant to test an organism’s capacity for visual self-awareness. The classic MSR test involves placing an identifiable mark upon an organism and seeing if they react to the mark in a mirror.

According to the published study, the cleaner wrasse specimens passed all three criteria of their MSR test:  (i) social reactions towards the reflection, (ii) repeated 30 idiosyncratic behaviors towards the mirror (contingency testing), and (iii) frequent observation of their reflection. The implications for the study of animal cognition is profound, as the cleaner wrasse marks the first non-mammal, non-avian species to definitively pass the MSR test and the designed test represents an improvement on pre-existing MSR tests, which had mostly been developed for vertebrates capable of hand gestures and changing facial expressions.

MSR Test And Self-Recognition

The mirror self-recognition test was developed in the 1970s by psychologist Gordan Gallup. Gallup originally devised the test to see if animals are capable of self-recognition. In the classic version of the test, the animal is anesthetized and marked (by a sticker, paint, etc.) in a place the animal cannot normally see, such as on its forehead. The animal is then given access to a mirror. If the animal touches or investigates the mark, it is taken as evidence that the animal recognizes the reflection in the mirror as itself and not as some other animal.

Gallup performed his original tests on chimpanzees. His experiments indicated that, although initially hostile and aggressive to the reflected image, chimpanzees would eventually learn to use the mirror for self-directed actions such as grooming. Subsequent experiments showed that the chimps would react to changes in their appearance, such as the presence of a red dot on their forehead.

A rhesus monkey being administered the classic MSR mark test. Source: Chang, L.; et. al. (2018) “Mirror-Induced Self-Directed Behaviors in Rhesus Monkeys after Visual-Somatosensory Training” Current Biology. Vol. 25. no. 2. pp. 212-217. <>

Although the MSR test is a popularly used metric for gauging an organism’s self-awareness, it is not without criticisms. Most prevalently, the MSR test seems to be crafted specifically for species that rely on vision. Classic versions of the MSR test may not be suitable for species that rely on senses other than vision, such as taste or smell. Secondly, MSR tests could be prone to false negatives. Some animals may not regard the placed mark as abnormal or may be otherwise unmotivated to investigate it while nonetheless still being self-aware. Also, differences in the way the mirror is oriented and present the reflection could have an effect on behavior that is not due to self-recognition. Lastly, there is a considerable amount of debate whether recognition in a mirror necessarily implies self-awareness. Recognition in a mirror could arise from cognitive processes entirely different from those that give rise to a distinct subjective sense of self-hood.

Cleaner Wrasse And Self-Recognition

Even if the mirror test does not directly imply the presence of an abstract sense of self-hood, it certainly tests for the capacity for self-recognition. Such is the underlying premise of the current study, in which 10 cleaner wrasse specimens were given various iterations of MSR tests. Since cleaner wrasses do not have hand or limbs to examine mark with, the researchers had to look for other behaviors to determine self-recognition. Cleaner wrasses exhibit a characteristic cleaning behavior called scraping, which can be seen as self-directed, like the grooming behavior of chimps.

First, the team allowed to 10 fish to swim around in the tank with the mirror covered. As soon as the mirror was uncovered, 7 of the 10 fish started attacked their own reflection aggressively, suggesting they perceived their reflections as conspecific rivals. Eventually, the fish moved from aggressive displays of behavior to strange swimming patterns in front of their reflections. The fish would approach their reflection upside down, or dash back and forth while keeping their eyes on the reflection. None of these atypical behaviors have been recorded as social or aggressive actions of cleaner wrasses, and they were highly repeated by each fish, some more than 400 times a day. Most importantly, these behaviors only persisted in the presence of the mirror, indicating that the fish were, in fact, reacting to the mirror’s presence.

For the last phase of the test, the researchers anesthetized the fish and marked them with subcutaneous injections of colored gel. Since there is little tech for tracking fish eye movements, the scientists made the reasonable assumption that if the fish recognized the mark they would turn their body so the mark side was closer to the mirror. They found that the marked fish spent significantly more time with the marked portions of their body turned towards the mirror, indicating that they had recognized and were inspecting the mark. They also found that the more the fish spent with the marked sides of their bodies turned towards the mirror, the more they would attempt to remove the mark by scraping their throat against the surface. The self-directed behavior towards the marks is taken as evidence of self-recognition.

The researchers are clear to state the limitations on their study. MSR tests are somewhat controversial among animal cognitive scientists, and fish morphology may not lend itself to MSR-like tests. But the cleaning wrasses did show behavior types consistent with other aquatic species, such as dolphins or orca, that are considered to be capable of self-recognition. So the researchers claim that on pain of being consistent, their results should be taken as showing cleaning wrasses are capable of recognition in the mirror.


So it seems that the list of animals capable of recognition in the mirror has just gained a new member. Fish are normally considered a “lower” taxon less sophisticated than their mammalian brethren, so the discovery that some fish have cognitive capacities shared by mammals is an interesting one.



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