Fish May Sense Each Other’s Fear

Zebrafish respond when their peers act afraid, an ability regulated by the same hormone that drives human empathy, a new study shows

Small striped fish swimming in water
Zebrafish experience what's known as "emotional contagion" and react when their peers are afraid. Oregon State University via Flickr under CC BY-SA 2.0

Fish may have their own version of empathy. Like humans and other social mammals, they appear to be able to recognize and respond when their peers feel afraid. Now, in a study published last week in the journal Science, researchers say the same mechanism responsible for human empathy might also be at play among fish.

This suggests the human ability to understand and share another’s feelings could have evolved much earlier than previously thought—around 450 million years ago, when fish and mammals split on the evolutionary tree, according to a perspective accompanying the paper.

From past research, scientists know that humans, elephants, dolphins and many other types of vertebrates have the capacity to be empathetic. They also know that fear can spread throughout groups of fish in a phenomenon known as “emotional contagion.” This can be helpful if, for example, one fish spots a predator before others do—it can then alert its peers by acting distressed.

But until now, scientists weren’t sure which biological systems, if any, led fish to catch onto another’s emotional state. To find out, they focused on zebrafish—tiny, striped members of the minnow family that are often used in research. Scientists concluded the hormone oxytocin, which helps regulate social behavior and empathy in mammals, may also drive contagious fear in fish.

In one of their experiments, the team removed genes linked to oxytocin production and detection from some zebrafish, then let them observe other zebrafish that were acting afraid in another tank. Many of the genetically modified animals did not respond to their peers’ fear, while a control group of unedited fish did. Then, when the researchers injected the genetically modified fish with oxytocin, they behaved more like the standard, unmodified fish and mirrored the fright behaviors of their peers.

In another experiment, unaltered zebrafish appeared to pay more attention to videos of fish that had previously shown distress, as opposed to videos of fish that always appeared calm. This suggests the fish may have been emotionally connecting with their fearful peers—and possibly, researchers say, even trying to console them.

“They respond to other individuals being frightened,” says study co-author Ibukun Akinrinade, a neuroscientist at the University of Calgary, to the Associated Press’ Christina Larson. “In that regard, they behave just like us.”

Finally, the researchers dissected zebrafish brains and, after examining the tissue under a microscope, found that the two areas most involved in emotional contagion were similar to the mammalian brain regions associated with empathy.

As a result of this work, researchers now have a better sense of the biology behind emotional contagion in zebrafish. But the team still has questions they hope to resolve with further study: For one, does emotional contagion work in a similar way with positive emotions, or only negative ones like fear that may be beneficial for survival? And exactly how—and why—does oxytocin affect the fish’s behavior?

Notably, while the scientists showed zebrafish recognize and respond to fear in others with a change in behavior, they didn’t prove the observing fish actually feel afraid, as study co-author Rui Oliveira, a neuroscientist at the Institute of Applied Psychology, points out to Gizmodo’s Lauren Leffer.

Still, the findings suggest that fish could be sentient, like humans and other species, even if their sentience is different from that of other creatures.

“They have, if you will, the ability to have an emotional life,” Hans Hofmann, a neurobiologist at the University of Texas at Austin who was not involved in the research but co-authored the accompanying perspective, tells Gizmodo.

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