Scientists Train a Tropical Fish to Tell Human Faces Apart

Despite having a primitive brain, the tropical archerfish is able to distinguish between dozens of faces.

One of the world's most recognizable fish returns to the big screen next weekend in the long-awaited sequel to the 2003 animated film classic Finding Nemo. The eponymous star of Finding Dory is an affable blue tang fish who suffers from lifelong short-term memory loss—a movie jab at the myth that some fish have just a three-second memory. New research published in Scientific Reports proves that at least one species of fish has a good enough memory to learn and remember human faces. The study marks the first time fish—vertebrates that lack the sophisticated visual cortex of primates—have demonstrated this ability.

For their study, scientists from the University Oxford and University of Queensland first positioned a computer monitor that showed images of human faces above aquariums containing four archerfish—black-and-white fish that are a couple of inches long with a protruding jaw and the standout feature of a specialized mouth that allows them to deliver powerful blows of water to aerial prey, like insects. Once the tropical fish were trained to recognize a particular face by spitting jets of water at it, they were shown the same face mixed in with a series of 44 new ones. They performed admirably, distinguishing the friendly face with 81 percent accuracy.

Archerfish spits to recognize human face
Despite having a primitive brain, the tropical archerfish is able to distinguish between dozens of faces.

The second experiment tested whether four other archerfish (not involved in the first experiment) could be trained to identify human faces even when cues such as brightness, head shape and color were made the same. This time, the fish were tested on whether they could select the previously learned face among 18 other novel faces. In this case, the fish were even better at remembering the face, firing water at it with 86 percent accuracy.

Scientists expect such sophisticated visual recognition abilities from humans and other primates that possess a neocortex, the brain's outer layer that is involved in higher-order mental functioning. Brains of fish are considered simpler because they lack this brain structure, says study lead author Cait Newport of Oxford's department of zoology. Nonetheless, archerfish at least were found to have impressive visual memory, similar to damselfish, which were found capable of distinguishing individual faces of their own species and a similar species in a 2015 study.

The research team felt that because the human brain has a specialized region for recognizing human faces, there's something special about faces themselves. "To test this idea, we wanted to determine if another animal with a smaller and simpler brain, and with no evolutionary need to recognize human faces, was still able to do so," says Newport.

The results were surprising: In all cases, the fish spit at the face they had been trained to recognize, proving that they were capable of telling the learned face apart from many others. "Even when we did this with faces that were potentially more difficult because they were in black and white and the head shape was standardized, the fish were still capable of finding the face they were trained to recognize," says Newport.

Newport can't speculate as to what makes the archerfish brain capable of this sort of higher-order visual recognition. Though human facial recognition has never previously been demonstrated in fish, it has been shown in birds. However, unlike fish, birds possess neocortex-like structures, and most studies involved domesticated pigeons, which, unlike archerfish, may have had reason to adapt to recognize human faces. The archerfish study brings up the question of whether recognizing faces requires a large or complex brain after all.

'The fact that archerfish can learn this task suggests that complicated brains are not necessarily needed to recognize human faces," Newport says. So why, then, do humans have special brain structures for facial recognition? Newport suggests it might be "so that they can process a large number of faces very quickly or under a wide range of viewing conditions."