You may have heard that âfish donât feel pain.â Itâs a common, persistent myth that dates back to 17th-century French philosophy. Yet, regardless of RenĂ© Descartesâs 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 fishesâ 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.
Oxytocin, which is sometimes misleadingly referred to as the âlove hormone,â is not sufficient on its own to fully explain animal or human feelings and behaviors. Oxytocin doesnât âcause love,â nor dictate morality. Past research of the chemicalâs 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 agoâbefore 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.
âIt kind of raises questions about what it actually means to be human, and what it means to be an animal,â 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 âI personally donât have a problem saying these fish have sentience. They have, if you will, the ability to have an emotional life.â It may be very different from the emotional life of humans and other animals, Hofmann added as a caveat. Yet nonetheless, itâs 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 wildâwhen hurt or threatened by a predator, a fish will release an âalarm substanceâ or chemical signal that notifies others nearby of the threat.
Though the single fishes in the experiment were sequestered in separate tanks and werenât 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 fishesâ fear went way downâby about half. The researchers then dosed some of the mutant fish with oxytocin and found that their response changedâbecoming more similar to the standard fish.
In another set of trials comparing normal âwild typeâ 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, didnât show a significant preference for either side.
Finally, the researchers dissected lots of zebrafish brains following these fear trials to find out where the animalsâ 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 fishesâ fear and social contagion were âhomologousââ meaning both developmentally and functionally similarâto 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 Lisbon, 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 empathyârecognizing emotions in oneâs peersâ must be at least as old as the split between fish and mammals, which began between 350 and 400 million years ago.
Though for now, this remains unproven, Oliveira noted. âTo properly test this hypothesis in detail, one would need to run a comparative analysis of social contagion across [many different] species.â His co-author, Kyriacos Kareklas, another neuroscience researcher at the Institute of Applied Psychology, agreed. After all, emotional recognition through oxytocin couldâve evolved multiple, separate times.
Additionally, though the new research shows that zebrafish respond to fearful displays from their peers, Oliveira says that doesnât mean the fish necessarily see the world the way people do. âThis study does not show that fish have human-like empathy or feelings,â he wrote. âWe 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.â
Itâs 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 whatâs going on in a zebrafishâs brainâwe have to suspend projection and do more research.
Kyriacos, the co-lead researcher, said that next, heâd like to try to figure out how and why oxytocin motivates zebrafish behavior. Do fish transfer âcalmâ to one another, as well as fear? Is it adaptive for a zebrafish to approach another fish in distressâif 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 heâs 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 heâd like to see more evidence that zebrafish âcare aboutâ their peersâ emotional stateâand arenât 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.
Itâs not just mammals and zebrafish that produce oxytocin or similar hormones, Hofmann explained. Research has demonstrated that insects, crayfish, worms, and even âfreaking little leaches,â make oxytocin or related chemicals in their bodies, he said. Though, right now, thereâs 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 weâre swimming in a whole ocean of emotional animals.
