It’s almost automatic—if you notice a smudge when you look in the mirror, you wipe it off. Seems simple, but only a few particularly clever species such as orangutans and dolphins share this ability with humans.
But now, incredibly, new research suggests that the cleaner wrasse—a tiny, tropical reef fish—can recognize itself too, making it the first fish to do so.
Scientists have long used a mirror test to evaluate whether an animal is capable of visual self-recognition—and potentially self-awareness. Self-awareness involves having a working knowledge of your own mental states, like thoughts and emotions, along with an understanding of how you physically appear; self-recognition, in contrast, is limited to knowing the latter. It’s unclear how much self-recognition implies self-awareness.
By placing a dot or mark on a subject, and then placing the animal in front of a mirror, researchers can observe if the creature investigates the mark. Passing the test suggests an animal understands that the marred reflection is a representation of its own marked body, and not just another member of its species.
Only those regarded as the brainiest non-human species on Earth have passed the mirror test: great apes, dolphins, elephants, and magpies. But the new research, first released online in BioRxiv and now published in PLOS Biology, raises the possibility that an advanced sense of self and self-awareness is far more widespread in the animal kingdom than scientists had thought.
The research team, led by Masanori Kohda, a biologist at Osaka City University in Japan, had originally tried the mirror test on a different species of fish, a cichlid, one thought to be particularly smart.
“Some social cichlids are so intelligent that they discriminate familiar members individually like primates—true individual recognition,” says Kohda.
But the fish failed to recognize itself in the mirror.
The team then turned to another candidate: the bluestreak cleaner wrasse (Labroides dimidiatus). These small fish, only about the length of a human finger, are found throughout the warm, shallow reefs of the Indo-Pacific. They get their name from their unique role: nibbling dead skin, mucus, and parasites off other fish.
Cleaner wrasse already have a reputation as sophisticated thinkers among their finned brethren, manipulating their “clients” in various ways to maximize their nutrient intake, while also keeping other fish happy. The wrasses appear to keep track of hundreds of different animals and their relationships with each. These little creatures even indirectly boost the brainpower of other fish through their cleaning services by removing harmful, distracting parasites.
A first glimpse
How would these intelligent fish fare on the mirror test?
To find out, Kohda and his team put 10 wild-caught wrasses in individual tanks outfitted with a mirror. Many of the fish reacted aggressively towards their reflections at first (engaging in “mouth-fighting”), apparently seeing the reflection as another cleaner wrasse in their space.
But eventually, this behavior gave way to something far more interesting. The fish began behaving strangely, approaching their reflections upside-down, or dashing towards the mirror quickly, only to stop right before touching it. At this phase, the researchers say, the cleaner wrasse were “contingency-testing”—directly interacting with their reflections, and perhaps just starting to understand that they were looking at themselves and not another wrasse.
Once the fish were acquainted with the mirrors, the researchers injected a benign, brown gel under the skin of eight of them. Importantly, some of these injections were in places the wrasse couldn’t see without the aid of a mirror, such as on their throats. When the fish saw their reflections—and the spots on their skin—they appeared to attempt to scrape it off on surrounding surfaces, probably identifying the mark as a parasite.
The wrasses scraped their throats only in the presence of a mirror, and only when the mark they sported was colored. Fish injected with a clear mark didn’t scrape, and neither did those with a colored mark when no mirror was present. Only when the fish could see their mark in a mirror did they try to scrape it off, suggesting that they recognized their reflections as their own bodies.
When Kohda saw this behavior, he was floored. Literally.
“When I observed the scraping behavior of [the] marked throat in the video the first time, I was so surprised that I fell down from my chair,” he says.
The wrasses also displayed their throat markings to the mirror before and after scraping, as if they were angling their bodies for a closer look and checking to see if they managed to remove the “parasite.”
Redouan Bshary, a biologist at University of Neuchâtel in Switzerland who studies cleaner wrasse behavior and cognition, is impressed by the findings, and maintains that the movements the wrasses were making in front of the mirrors are wholly unique.
“I never saw a cleaner [wrasse] swimming on its back, and I never saw a cleaner [wrasse] scratching its throat,” says Bshary, who wasn’t involved in the study. “These are new behaviors that are apparently tightly linked to the mirror.”
Bshary also lauds the researchers for tracking the wrasses for such a long period of time, noting that he’s seen cleaner wrasses “mouth-fighting” with mirrors upon initial exposure, and that ending observations at that stage cuts the story short.
But Gordon Gallup—an evolutionary psychologist at the State University of New York at Albany, and inventor of the mirror test—isn’t convinced. Gallup argues that the wrasse’s inherent preoccupation with ectoparasites on the bodies of other fish—a consequence of its cleaning lifestyle—may explain some of the behavior.
“It should come as no surprise that they show longer viewing times to color marks that resemble ectoparasites on what appears to be another fish that can only be seen in a mirror,” says Gallup.
Gallup adds that the strange posturing in front of the mirror may be how they’ve learned to manipulate what they think is another fish in the mirror so they can see the mark better.
“Scraping the throat where the mark is may simply represent an attempt to call the attention of the other fish in the mirror to the presence of an apparent ectoparasite on its throat,” says Gallup.
Kohda counters this alternative explanation of the behavior by noting that it doesn’t explain why the wrasse would check itself in the mirror after scraping its throat.
“Only the hypothesis that cleaner fish recognize the mirror reflection as [their] own body will explain all of our results,” says Kohda.
Sense of self?
If cleaner wrasse have indeed passed the mirror test, does that make them self-aware? Well, possibly. However, it may also mean that the test itself doesn’t show us what we think it does.
This is something Kohda and his colleagues readily admit as a possibility, and call for reinterpreting the mark test in the newest version of their work, in PLOS Biology. Cleaner wrasses may have passed the mark test as it's been defined in years past, but is the test as informative about self-awareness as we think it is, across a wide range of species?
In the new publication, primate psychologist Francis de Waal similarly argues in a corresponding piece the the findings argue for a more nuanced view of mirror self-recognition where animals exist somewhere on a continuum of understanding their own reflections, rather than a binary, pass-fail system.
For Michael Platt—a cognitive neuroscientist at the University of Pennsylvania not involved with this study—the research is “fascinating and well-executed.”
Platt says that the study demonstrates that either many more animals have a sense of “self” than we’ve realized, or the mirror test has little to do with recognizing “self.” Instead, learning to use mirrors may just be a way of helping an animal define the boundaries of its own body.
“It's impossible to know which of these two conclusions is correct, since nonhuman animals cannot provide self-report or otherwise discuss their experiences with us,” says Platt.
And if the test really does reveal this abstract sense of self-awareness? It means that fish—and potentially many other animals with rarely considered internal lives—may have minds that are surprisingly similar to our own.
Editor's note: This story was originally published Sep. 11, 2018, just after the research was first featured in BioRxiv, a pre-print server that doesn't include peer-review. It was updated and re-published Feb. 7, 2019, when the study was published in PLOS Biology, a peer-reviewed journal.