A cleaner wrasse looking at itself in the mirror.
Photo: Alex Jordan

New research seems to demonstrate that a fish called the cleaner wrasse has passed the famous mirror test for self-recognition—and the results have ignited discussion about animal intelligence and the meaning of the test itself.

When first confronted with a mirror, animals initially act as if the individual they see in the reflection is another animal of the same species. But individuals from some species seem to figure out that they’re looking at themselves, including some chimpanzees, elephants, dolphins, magpies, and, of course, humans, leading us to believe these animals possess a concept of “self.” Scientists are now wondering: Has the most recent research sufficiently demonstrated that a fish species passes the test? And would that mean that cleaner wrasse are self-aware, or does the test tell us less than we think? And what’s so strange about a smart fish, anyway?

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“A lot of people think fish are vacant animals with three-second memories,” study author Alex Jordan, a principal investigator at the Max Planck Institute Department of Collective Behaviour in Germany, told Gizmodo. “But if you educate yourself on what these animals can do, it shouldn’t be surprising that they can do something more complex.”

Psychologist Gordon Gallup first published the results of a mirror test in 1970 in the journal Science. Gallup introduced a mirror into the cages of four wild-born chimps. At first, they behaved as if they’d seen another individual by vocalizing and threatening. But after getting used to the mirror, they began using it to groom parts of their body they couldn’t otherwise see and to pick food from their teeth. To add evidence that the chimps knew they were looking at themselves, the scientists removed the mirrors, then put red dye on the chimps’ faces under anesthesia. They observed the chimp not touch their faces until they put the mirror back next to the cage, after which the chimps tended to the marks. Several monkeys were tested as well, and none demonstrated the self-directed behavior shown by the chimps. The paper concluded that the recognition would require an “advanced form of intellect” and would “seem to qualify as the first demonstration of a self-concept in a subhuman form.” The test has since been re-performed on the animals we now consider “smart,” like dolphins, elephants, and magpies.

Fish don’t have limbs, so the experiment was a little more challenging to adapt to the cleaner wrasse. The team, composed of researchers from Osaka City University in Japan at the time of the experiment, introduced the fish to the mirrors and observed as some of the subjects behaved as if another fish were there. But the fish then begin to act strangely, by swimming upside-down or spreading their fins and quivering. When the scientists injected a colorless mark into the fish under anesthesia, the fish behavior didn’t seem to change much. Nor was there a behavior change when the scientists injected a colored mark into their throats. But after seeing the mirror again, three out of four tested fish appeared to posture in front of the mirror to better see the mark, then scrape their face on the ground, according to the paper published in PLOS Biology.

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Some researchers Gizmodo spoke with wondered if enough evidence had been presented in the form of video or images to prove definitively that the fish had actually passed the test.

“It’s certainly interesting,” researcher Diana Reiss, professor of psychology at Hunter College of the at the City University of New York, told Gizmodo, “but I think it really provokes more investigation.” She noted that the fish performed some of the “strange” behaviors without a mirror present. She also pointed out that one must carefully observe the behavior of the individual animals to determine if the behavior is truly strange for them.

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Another researcher, Frans de Waal from Emory University, agreed that the study was interesting and provocative, but told Gizmodo that “the behavior they show in the mirror test is open for interpretation, which is why some scientists, including myself, remain skeptical.” He, too, noted that fish do not have hands or arms, making the results more difficult to interpret.

Jordan noted that the lab is working on replicating the study, and that he is devising further quantitative tests that he hopes will provide the overwhelming evidence that scientists are hoping for. He said that human prejudices about fish intelligence are causing the scientific community to hold his research to higher standards—it’s taken several years for the paper to pass through peer review, including journal rejections. He also pointed out that previous mirror tests have been performed on only a handful of representatives from the species, and often only two or three individuals actually pass the mark test.

But should further experiments strengthen the results, you might choose to write off the mirror test as a whole—and that’s something that the paper addresses. Perhaps the real issue is Gordon’s 1970 assertion that passing the mirror test implies some surprising, monolithic intellectual depth. “The minimum required explanation is not that these animals are looking in the mirror, then having a moment like ‘whoa, I exist’ while having an acid trip about their place in the universe,” said Jordan. “They’re probably just using the mirror as a mechanism to see their body in the same way they can turn their head to see their body.”

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Some of the issues could involve the language we use to talk about animal intelligence, and how we project some of our own experiences onto the animals. “Some of the confusion arises because the terms we use to describe an animal’s behavior are also terms we use for ourselves,” Victoria Braithwaite, professor in fisheries and biology at Penn State University not involved in the study, told Gizmodo. “When we apply these to animals, we sometimes mistakenly consider that the full degree of sophistication we have is transferred to the animal—but this is not the case.”

Perhaps the test just shows which species have faced selection pressures that require some individuals to recognize others among their ranks. “Many and a variety of kinds of approaches and tests should be used” to better understand animal intelligence, study author Masanori Kohda from Osaka City University in Japan, told Gizmodo.

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And perhaps the real question is, why do we write off fish? There are over 33,000 known species, more than mammals and birds combined. Some fish have been observed using tools—as far back as 1958, scientists have known that some fish lay their eggs and glue them onto surfaces like leaves so they can carry all the eggs simultaneously. Trevor Hamilton, associate professor in psychology from MacEwan University in Canada, agreed that he’d like to see a paper replicating this one—but he wasn’t quite surprised by the results.

“They chose the right fish species to run this task,” he said. “The cleaner wrasse has an incredibly complex social dynamic, so I’m not surprised that if any fish were to show positive results on this test, it would be them. The cleaner wrasse has a great memory capacity, great cognitive capacity, and their social dynamics are pretty incredible.”

Ultimately, intelligence is multifaceted, and there’s much we don’t understand about animal cognition. Plenty of animals can perform incredible feats of “intelligence,” like chickadees that can remember the locations of food caches, honeybees that can do math, or octopuses with their surprising problem-solving abilities. Slime molds can solve mazes. 

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Jordan and the rest of the researchers on the team are continuing to work on this research, and the conversation will continue over what the mirror test means. It’s just science in action, refining methods and continuing to research in order to get closer to the truth.

Said Jordan: “I’m just pleased that people are engaging and discussing what the mirror test means, and offering alternate hypotheses for us to test in the lab.”