Some fish can recognize their own faces in photographs and mirrors. This ability is commonly attributed to humans and other animals considered particularly intelligent, such as chimpanzees, the scientists reported. The discovery of abilities in fish suggests that self-awareness may be much more common among animals than scientists previously thought.
“It is widely believed that animals with larger brains will be more intelligent than animals with small brains,” says zoosociologist Masanori Kohda of Osaka Metropolitan University in Japan. Perhaps it’s time to rethink that assumption, Kohda says.
Kohda’s previous research showed that wrasse can pass the mirror test, a controversial cognitive assessment that purports to reveal self-awareness, or the ability to be the object of one’s own thoughts. The test involves exposing the animal to a mirror and then secretly placing a tag on the animal’s face or body to see if it will notice it in its reflection and try to touch it on its body. Previously, only a few species with large brains, including chimpanzees and other great apes, dolphins, elephants and magpies, had been tested.
In a new study, cleaner fish that completed the mirror test were then able to distinguish their own faces from those of other cleaner fish in photographs. This suggests that fish identify themselves in the same way that humans think they do—by visualizing their faces, Kohda and colleagues report Feb. 6 in Proceedings of the National Academy of Sciences .
“I think it’s really remarkable that they can do this,” says primatologist Frans de Waal of Emory University in Atlanta, who was not involved in the study. “I think this is incredible research.”
De Waal is quick to point out that failing the mirror test should not be taken as evidence of a lack of self-awareness. Still, scientists are struggling to understand why some species known to have complex cognitive abilities, such as monkeys and crows, didn’t make the cut. Researchers also question whether the test is appropriate for species such as dogs, which rely more on their sense of smell, or pigs, which may not care enough about a mark on their body to try to touch it.
The mixed results in other animals make it even more surprising that a small fish can pass. In their first studies using mirrors, published in 2019 and 2022, Kohda’s team exposed wild-caught cleaner fish to mirrors in separate aquariums for a week. The researchers then injected a brown dye directly under the scales on the fish’s throat, making a mark similar to the parasites these fish eat on the skin of larger fish in the wild. When the tagged fish saw themselves in the mirror, they began hitting their throats against rocks or sand on the bottom of the tank, apparently trying to scrape off the tags.
In the new study, 10 fish that completed the mirror test were then shown a photo of their own face and a photo of the face of an unfamiliar cleaner fish. All fish were aggressive toward the unfamiliar photo as if it were someone else’s, but were not aggressive toward the photo of their own face.
When another eight fish that had spent a week near the mirror but had not been tagged before were shown a photo of their face with a brown tag on their throat, six of them began scratching their throats like the fish that had passed the mirror test. . But they didn’t scratch when shown a photo of another fish with a tag.
The researchers believe that animals that recognize their reflection in a mirror most likely first learn to identify themselves by seeing the animal’s movement in the mirror match their own movement. Because cleaner fish could also recognize their own faces in still images, they, and perhaps other animals in the mirror test, could identify themselves by creating an imaginary image of their face that they could compare with what they see in a mirror or on photos, say the authors.
“I think it’s a great next step,” says comparative cognitive psychologist Jennifer Vonk of Oakland University in Rochester, Michigan, who was not involved in the study. But she would like to see more research before jumping to conclusions about what is represented in the mind of a non-verbal creature like a fish. “As with most other studies, this still leaves some room for further observation.”
Kohda’s lab plans to conduct additional experiments to continue to explore what’s going on in the cleaner fish’s brain, and to test the new photo recognition method on another popular research fish, the three-footed stickleback ( Gasterosteus aculeatus ).
Animal behaviorist Jonathan Balcombe, author of ” What does a fish know? ,” is already convinced, describing the new study as “robust and pretty brilliant.” Humans shouldn’t be surprised that fish can be self-aware, given that they already have complex behaviors, including tool use, planning and cooperation, Balcombe says. “It’s time to stop thinking of fish as lesser members of the vertebrate pantheon.”
