By Stephen Ornes

Male cichlids are mainly freshwater fish that may go after other fish who dare cross their path. A male cichlid will even lunge if that “other fish” happens to be himself: When some types of these cichlids see their own reflections in a mirror, they respond as aggressively as when they encounter a real fish opponent.

A new study suggests that even though these two situations may look the same, a fish’s brain actually reacts differently in each case. Researchers from Stanford University recently studied male cichlids that fight their own reflections. The team observed that the part of the brain associated with fear and other negative emotions becomes active when the fish fight their mirror images.

Julie K. Desjardins, one of the scientists who worked on the study, says it’s not clear whether the research is finding “fear”— that is, the scientists are not sure that the fish are afraid of themselves. Even if it’s not fear, the fish is having a negative response, something besides the aggression it usually shows toward another fish, she told Science News.

The study by Desjardins and Russell Fernald, her colleague at Stanford, is the first to show that a fish’s brain reacts differently when the fish sees its own reflection. That doesn’t mean, however, that the fish recognizes itself.

Scientists use mirrors to try to study the consciousness of animals. Previous studies have shown that great apes, elephants, dolphins and magpies (a type of bird) look into a mirror and know they see themselves, Diana Reiss told Science News. Reiss is a scientist at Hunter College in New York City who tries to understand animal cognition, or how animals think. She says not every animal knows its mirror self — monkeys and fish, for example, don’t seem to recognize themselves in the mirror.

In the new study, Desjardins and Fernald did not observe a difference in the behaviors of fish going after other fish compared with fish going after their own reflections. And when the scientists looked at hormones in the fish, they didn’t see a difference. But when they looked in the fish’s brain, using a technique called immediate early gene (IEG) expression, they found a difference.

With this technique, the scientists watched particular genes in the fish that were associated with particular regions of the brain. Measuring IEG helped the scientists to determine which areas of the brain were more active than others. “It’s a kind of fishy MRI,” Desjardins told Science News. MRI stands for magnetic resonance imaging, a tool that gives scientists an idea of what’s going on inside the brain.

When a fish went after its own reflection, the scientists found that the fish brain was especially active in a region similar to the amygdala. In human beings and other animals, the amygdala is associated with fear and other negative emotions.

When the male cichlids went after other fish, they didn’t have the same activity in their amygdala regions — showing that their brains reacted differently when they looked themselves in the fishy face.

Using IEG expression to study fishy fear is new and unusual, but some older studies have shown similar results in other animals. Monkeys, for example, have not been shown to recognize themselves — but they do act differently around their own reflections than they do around other monkeys.

This experiment shows how mirrors can be used to study brain activity, even for animals that don’t recognize themselves.