Oxytocin plays a role in the most basic form of empathy in zebrafish

Oxytocin plays an essential role in the spread of fear in zebrafish, according to a new study, the results of which suggest a deeply conserved role for the hormone in emotional contagion – the basal form of empathy – among vertebrates like fish. “The apparent concordance between mammals and fish of how oxytocin regulates empathetic behavior raises the intriguing possibility that the mechanisms underlying empathy and some forms of emotional contagion may have been conserved since fish and mammals last shared a common ancestor, ~450 million years ago,” write Ross DeAngelis and Hans Hofmann in a related Perspective. Empathy, the capacity to recognize and respond to the emotional state of another individual, is a hallmark of highly social mammals. However, a growing body of research suggests that the capacity for empathy may be more widespread across vertebrates. Emotional contagion, where individuals mirror the emotions and behaviors observed in others, like fear and distress, is considered the most ancestral form of empathy and forms the basis for more complex empathic behaviors. Although social fear contagion has been observed in zebrafish, it remains unknown whether the behavior shares the same underlying biological mechanisms that have been described to drive similar empathic behaviors in mammals. Across a series of experiments, Ibukun Akinrinade and colleagues evaluated the role of the oxytocin signaling system, known to regulate empathic behaviors in mammals, in the social fear contagion behaviors of zebrafish. Akinrinade et al. found that wild-type zebrafish observing distressed conspecifics in a separate tank exhibited a freezing behavior, a common proxy for fear across vertebrates. However, mutant zebrafish lineages that lacked genes encoding oxytocin and oxytocin receptors did not exhibit the fear response, suggesting that oxytocin is necessary and sufficient for social fear contagion in zebrafish. The authors also present evidence demonstrating the spread of fear contagion is an empathetic response and not simply behavioral copying by showing that wild-type fish preferred to approach and be near fish they previously observed to be in a distressed state – a behavioral response mutant fish also lacked. What’s more, by measuring the neural activity in these fish, the authors discovered that the brain regions involved in zebrafish fear contagion are generally the same as those implicated in emotional contagion in mammals.