Fish May Feel Each Other's Fear Through the Same Mechanism That Drives Human Empathy

Photo of zebrafish in an aquarium
Photo of zebrafish in an aquarium

Zebrafish are both a common aquarium fish and biology research model. Yet the animals, often considered simple, could have complex emotional lives.

You may have heard that ‘fish don’t feel pain.’ It’s a common, persistent myth that dates back to 17th-century French philosophy. Yet, regardless of René Descartes’s many other worthwhile ideas, the scientific consensus is that he was flat-out wrong on this one. Fish, and all other classes of vertebrate animals, seem to suffer from bumps, cuts, and punctures similarly to how humans and other mammals do. Fish do feel physical pain. In fact, fish may even experience emotional distress, according to multiple studies. And now, new research adds to our increasingly nuanced understanding of fishes’ complex inner lives.

Fish, or at least zebrafish, may have their own version of empathy, enabled by one of the same chemical pathways thought to drive human social behavior. Fear is contagious for zebrafish, and the hormone oxytocin is responsible for fright catching on, according to a study published Thursday in the journal Science.

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Oxytocin, which is sometimes misleadingly referred to as the “love hormone,” is not sufficient on its own to fully explain animal or human feelings and behaviors. Oxytocin doesn’t “cause love,” nor dictate morality. Past research of the chemical’s effects on people has been seriously skewed. And, in fact, the hormone is probably involved in just as many negative behaviors as it is positive, according to recent research in mice. Yet regardless, oxytocin is an important chemical messenger, shown to mediate all sorts of emotional responses in humans and other mammals. And it seems to play a big role for fish, too.

Much like humans might pick up on and be influenced by the emotional state of other people, zebrafish seem to recognize fear in their peers and respond to it as if afraid themselves. But if you remove oxytocin from the equation, that recognition and subsequent reaction mostly go away, the new study shows. Moreover, the researchers found that the brain region responsible for mediating fish fear contagion is comparable in some ways to the emotional center of the mammalian brain.

Taken altogether, the findings suggest that empathy may have a much longer evolutionary history than previously thought, stretching back more than 375 million years ago—before the existence of the last common ancestor between fish and humans. The research means zebrafish, the small striped swimmers already commonly used in biology experiments, could prove useful for studying behavior broadly across the animal kingdom, according to the study authors. It also means that people might have much more company in the realm of sentience than we often think we do.

“It kind of raises questions about what it actually means to be human, and what it means to be an animal,” said Hans Hofmann, a neurobiologist at the University of Texas at Austin, in a phone call with Gizmodo. Hofmann studies the evolution of social behavior and was not involved in the new research. But based on the zebrafish findings, he said “I personally don’t have a problem saying these fish have sentience. They have, if you will, the ability to have an emotional life.” It may be very different from the emotional life of humans and other animals, Hofmann added as a caveat. Yet nonetheless, it’s there.

To examine that possible, fishy emotional life, the study researchers conducted multiple, linked experiments. In one set of trials, they held a single zebrafish in a tank next to another tank containing multiple zebrafish. Then, they dosed the multiple fish with a substance known to trigger fear behavior (like freezing or erratic swimming) in the species. Zebrafish are social and travel in schools in the wild—when hurt or threatened by a predator, a fish will release an “alarm substance” or chemical signal that notifies others nearby of the threat.

Though the single fishes in the experiment were sequestered in separate tanks and weren’t exposed to that alarm substance, the researchers observed that a large proportion of the fish still froze as if afraid themselves when they saw their peers reacting through two layers of glass.

However, when the scientists tried the same experiment with genetically modified fish that lacked the ability to make or detect the hormone oxytocin, the percentage of sequestered fish that froze in response to the other fishes’ fear went way down—by about half. The researchers then dosed some of the mutant fish with oxytocin and found that their response changed—becoming more similar to the standard fish.

In another set of trials comparing normal “wild type” fish with the no-oxytocin mutants, zebrafish were simultaneously shown two videos displayed on either side of their tank of the same fish in two different emotional states: frozen, fearful fish vs. neutral and chill fish. Then, the videos were replaced by a second set of footage that was just the same fish displaying unafraid behavior in both instances. During the experiment, both mutant and wild type fish paid more attention to the video of the fearful fish than they did on the unperturbed fish. But when the videos swapped out, wild type fish moved to be closer to the formerly afraid video side of their tank. The study authors interpreted this to mean the zebrafish were emotionally associating (and possibly even trying to comfort) the fish that had displayed fear. The mutant fish, lacking oxytocin, didn’t show a significant preference for either side.

Finally, the researchers dissected lots of zebrafish brains following these fear trials to find out where the animals’ response originated from and where oxytocin seemed to be acting in the brain. By slicing the fish brains into very thin layers, dyeing them, and examining the bits under a microscope, the researchers were able to locate the primary regions of activity that differed between the wild type and mutant fish. They found that the two brain areas which seemed to be most involved in the fishes’ fear and social contagion were “homologous”— meaning both developmentally and functionally similar—to parts of the brain thought to be involved in emotional expression and empathy in mammals (humans included).

The tidiest explanation for how this is possible is that the process of emotional recognition in vertebrate animals has been conserved throughout evolutionary time, said Rui Oliveira, a neuroscientist at the Institute of Applied Psychology in Lison, Portugal and the senior researcher on the zebrafish study, in an email to Gizmodo. Assuming the least number of genetic flips and changes, this basic expression of empathy—recognizing emotions in one’s peers— must be at least as old as the split between fish and mammals, which began between 350 and 400 million years ago when the first fish began to move onto land.

Though for now, this remains unproven, Oliveira noted. “To properly test this hypothesis in detail, one would need to run a comparative analysis of social contagion across [many different] species.” His co-author, Kyriacos Kareklas, another neuroscience researcher at the Institute of Applied Psychology, agreed. After all, emotional recognition through oxytocin could’ve evolved multiple, separate times.

Additionally, though the new research shows that zebrafish respond to fearful displays from their peers, Oliveira says that doesn’t mean the fish necessarily see the world the way people do. “This study does not show that fish have human-like empathy or feelings,” he wrote. “We can say that fish are able to recognize the state of fear in others and respond...but we cannot say that they experience the feeling of fear by observing others.”

It’s a subtle, yet important distinction that Hofmann too, took pains to make. People are so adept at emotional recognition, we often project feelings onto animals, computers, and even basic shapes that are unwarranted. To truly understand what’s going on in a zebrafish’s brain—we have to suspend projection and do more research.

Kyriacos, the co-lead researcher, said that next, he’d like to try to figure out how and why oxytocin motivates zebrafish behavior. Do fish transfer “calm” to one another, as well as fear? Is it adaptive for a zebrafish to approach another fish in distress—if so, why?

Hofmann, too, would like to see corresponding experiments studying more positive emotions and behaviors in zebrafish, like mate choice, to see if emotional contagion operates similarly in those circumstances. Oliveira said he’s hoping to pursue such studies.

Lars Westberg, a neuroscientist at the University of Gothenburg in Sweden who has also studied oxytocin in zebrafish but was uninvolved in the new research, told Gizmodo via email that he’d like to see more evidence that zebrafish “care about” their peers’ emotional state—and aren’t just mimicking physical motions. Westberg would also be interested in more detailed work charting the neural pathways involved so that the research could be better applied to studying humans and other animals beyond zebrafish.

It’s not just mammals and zebrafish that produce oxytocin or similar hormones, Hofmann explained. Research has demonstrated that insects, crayfish, worms, and even “freaking little leaches,” make oxytocin or related chemicals in their bodies, he said. Though, right now, there’s not enough data out there to know what these hormones do across species. Yet, as the science progresses, and as more researchers offload their mammalian biases, we could discover we’re swimming in a whole ocean of emotional animals.

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