We recognize how others are doing and react to it: The ability to empathize shapes our social behavior and it is also evident in some “highly developed” animal species. Now a study shows that fish also show basic forms of empathy. The special thing about it is that the oxytocin hormone also plays a role in them. It causes zebrafish to be ‘contagious’ to fear signals from others and even ‘care’ for peers they have previously observed in a state of stress. The results suggest that the oxytocin-mediated empathy system may have arisen early in evolution, the scientists say.
In humans, the complex ability to put oneself in the shoes of others is of great importance: empathy forms the basis for cooperation and prosocial behavior, among other things. Complex forms of empathy can also be clearly identified, especially in social mammals such as primates, elephants or dolphins. But in the meantime, forms of empathy have also been shown in other living beings. The most basic version is the so-called emotional contagion: If an individual shows signs of fear, for example, other people will recognize this and behave in the same way.
On the trail of emotional contagion
Studies of the neural basis of empathy and emotional contagion in mammals have already provided evidence that the “prominent” hormone oxytocin plays a role in the system. Various effects of this messenger substance in the context of emotional behavior in animals and humans are already known. The researchers led by Ibukun Akinrinade from the Portuguese Gulbenkian Research Institute in Oeiras have now investigated whether oxytocin also plays a role in forms of emotional contagion in fish. They carried out their investigations on a typical research model animal: the zebrafish (Danio rerio). These are schooling fish with complex interaction behavior.
In their study, the researchers conducted comparative experiments with “normal” zebrafish (wild type) and with individuals who have mutations in their oxytocin genes, meaning that they lack the hormone. First of all, the researchers were able to show that the wild-type fish, which were observing frightened conspecifics in a separate tank, also became terrified. In contrast, the control animals of the mutant line, which lacked the oxytocin effect, did not show this induced fear response. However, if the hormone was artificially administered to them, the observations showed that they too were infected by the fear signals of the fish in the neighboring tank.
To investigate the neural basis of this oxytocin effect, the scientists examined the brains of the fish using neuroscientific methods that can show how intensively certain areas were activated. They report that they found evidence that the brain regions involved in emotional contagion in zebrafish correspond to those previously associated with this process in mammals.
Do fish “comfort” each other?
It could be argued that the reaction in fish known as emotional contagion is based on a rather simplistic copying of behavior observed in their own kind. However, further results of the researchers point to an actually more empathetic phenomenon: They presented their test animals with an obviously frightened fish and at the same time a relaxed specimen. After the observation phase, the animals were given the opportunity to make contact. It turned out that in contrast to the mutants, the wild-type fish were more likely to join the fish in which they had seen the signs of fear. So was it a kind of comforting behavior, as has already been observed in some mammals?
At least it seems clear that oxytocin apparently also plays an important role in emotional contagion – the most basic form of empathy – in vertebrates such as fish. “The similarity between mammals and fish in the way oxytocin regulates empathetic behavior makes it conceivable that the mechanisms underlying empathy and forms of emotional contagion may have predates even the most recent common ancestors of fish and mammals approximately 450 million years ago,” write Ross DeAngelis and Hans Hofmann of the University of Texas at Austin in an article accompanying the study. “To test this possibility, however, further studies should be carried out on different species from all major vertebrate lineages,” say DeAngelis and Hofmann.