Mice pass a key test of consciousness

Before you set a mouse trap, you may want to think — because the mouse might be thinking too.

A new study has found that mice appear to recognize themselves in the mirror — placing them in a rarified category alongside humans, primates, dolphins and the smartest birds.

According to the research published on Tuesday in Neuron, black lab mice painted with white paint and placed in front of a mirror will work diligently to clean the paint off — an early indication that they recognize that the mouse in the mirror is them.

Researchers tend to be very cautious about any finding that seems to convey animal sentience, and in this case, investigators from the University of Texas Southwestern Medical School were hesitant to draw the conclusion that mice are self-aware.

It is possible, they noted, that mice can detect a change in their appearance without having an underlying experience of consciousness that connects them to humans, elephants or magpies.

As such, researchers were careful to use phrases like "self recognition-like behavior" rather than drawing broader conclusions about the internal experience of their tiny experimental subjects.

Nonetheless, the finding that mice interact with the creature in the mirror as themselves — rather than as a friend or rival — opens the door to the possibility that mice are more self-aware than had been generally thought.

The mirror test originated half a century ago with the inquiries into animal consciousness that begin with our nearest relatives, the chimpanzee. 

Zoologist George Gallup Jr. painted chimpanzees' faces with odorless red dye and placed them in front of a mirror to see if they would interact with the spot.

When they did, the findings became a powerful argument that animals — often seen in mid-century scientific dogma as biological machines lacking an internal life — had a consciousness analogous to our own.

In some cases, that capacity surpassed that of humans: Children can generally walk and begin to talk before they can pass the mirror test.

Over the past half century, a wide array of other animals have passed the test — great apes in the 1970s, followed by bottlenose dolphins in the 1990s, elephants and magpies in the late 2000s and zebrafish in 2019.

While passing the mirror test is linked to consciousness, failing doesn't necessarily indicate a lack of it. In large part, that is because the test itself is based on a vision of intelligence in a sensory world that privileges humans — one in which sight is the dominant sense, touch with our limbs is the primary way of interacting with the world and grooming oneself is important.

For example, cats have not passed the mirror test, but this could be due less to a failure of cognition than their notorious lack of cooperation in lab settings — or any other. (As Science reports, cats tend to fail many intelligence tests that dogs pass because of their habit of wandering off in the middle of experiments.)

Cats' failure to pass the test might also stem from the fact that, like dogs, they prioritize smell as a means of separating themselves from others.

Tuesday's findings also reinforce the role of social learning in making sure animals even know what a mirror is, or what their face is supposed to look like. The UT researchers found that black-furred mice passed the test — but only those that had been raised alongside other black-furred mice, not those that had been raised alone or among white mice.

That finding suggests "that mice need to have social experiences alongside other similar-looking mice in order to develop the neural circuits required for self-recognition," researchers wrote in a statement.

In another important caveat, mice only passed the test if they had spent time previously becoming accustomed to — and grooming themselves in front of — mirrors.

They also needed a bit of a boost, the study's first author Jun Yokose said.

“The mice required significant external sensory cues to pass the mirror test—we have to put a lot of ink on their heads,” he said, which raises the possibility that the feeling of ink on their fur, plus the visual stimulus of seeing it reflected in the mirror, were required for the mice to notice the stain.

“Chimps and humans don't need any of that extra sensory stimulus,” Yokose added.

These caveats align with those observed in other species, which only passed the mirror test once allowances were made for their unique size or sensory data. 

Elephants only passed with adequately sized mirrors, and magpies — crow relatives which use their beaks to interact with the world rather than their limbs — required specially designed stickers they could remove with their mouths.

The primatologist Frans de Waal — who conducted the mirror tests on elephants — has noted that these sorts of findings are likely only the very beginning of inquiries into animal consciousness.

Quoting the physicist Wevern Heisenberg, de Waal wrote that “what we observe is not nature in itself, but nature exposed to our method of questioning.” 

Heisenberg, he noted, “made this observation regarding quantum mechanics, but it holds equally true for explorations of the animal.”