In news that will surprise absolutely no one who has ever met a psychedelic enthusiast, psilocybin - the active compound in more than 200 species of mushrooms, mostly of the genus Psilocybe - appears to take the fight right out of aggressive fish. Researchers in Canada tested the substance on the amphibious mangrove rivulus fish (Kryptolebias marmoratus), and the findings, published in Frontiers in Behavioral Neuroscience, suggest that even a brief, low-dose exposure significantly reduces aggressive behavior without shutting down normal social interactions.
"We show that an acute, low dose of psilocybin significantly reduces activity and aggressive attack behavior during social interactions in adult mangrove rivulus fish, a species that is naturally highly aggressive," said first author Dayna Forsyth, a research associate and former MSc student at Acadia University in Nova Scotia. Senior author Dr. Suzie Currie, a biologist at The University of British Columbia, added that the findings provide "the first evidence that psilocybin can selectively reduce escalated aggression in a vertebrate model without suppressing social interaction." In mammals, psilocybin interacts with serotonin receptors in the brain, influencing mood, appetite, and aggression - but how it affects social interactions among animals has remained largely a mystery. Enter the fish.
Why mangrove rivulus? For one, they are naturally combative, particularly when they encounter another member of their species, and their aggressive responses are easy to recognize and measure - handy when you're trying to detect subtle behavioral shifts. They also have an unusual reproductive trick: they can fertilize themselves, producing offspring that are genetically identical. That lets scientists minimize genetic variation and more confidently attribute behavioral differences to psilocybin rather than inherited traits. The researchers worked with three genetically distinct lines of lab-bred fish: one group received psilocybin, another provided the fish used to provoke social interactions, and the third helped measure how much psilocybin entered and accumulated in the animals' bodies.
The experiment began with a baseline. Each fish was placed in a tank already occupied by another fish, separated by a fiberglass mesh barrier and an opaque cover. The mesh prevented physical contact while allowing the fish to see and smell each other once the cover was removed. After five minutes of adjustment, the cover came off and interactions were recorded. The next day - 24 hours after the initial observation - the same fish was placed in water containing dissolved psilocybin. After 20 minutes of exposure, it was returned to the tank with the same fish it had encountered previously, the cover was removed again, and researchers watched for changes.
The results were striking. Fish exposed to psilocybin moved around less and displayed fewer bursts of aggressive swimming than untreated fish. The scientists measured overall activity (time spent moving) and several signs of aggression, including swimming bursts - rapid, high-energy attack behaviors that indicate an encounter is escalating toward an attack, even without physical contact. "Swimming bursts are high-energy attack behaviors that represent an escalation of aggression towards the stimulus fish without making physical contact," Currie explained. "Other types of aggressive behaviors, like head-on displays, are more about communication and social assessment and require very little energy."
And that's the nuance: psilocybin didn't suppress every form of aggressive or social behavior. While the fish became less likely to engage in intense, energy-demanding attacks, their less physically demanding social displays remained largely unchanged. "Psilocybin's calming effect appears to selectively reduce energetically costly, escalated behaviors while lower-energy social display behaviors remained largely unchanged," Forsyth said. "This suggests that this compound can selectively dampen escalated social conflict rather than shutting down behavior altogether." The compound also reduced general activity - treated fish spent less time swimming than untreated controls when placed alongside another member of their species. Together, the findings suggest psilocybin may influence specific aspects of aggressive behavior rather than simply making animals less responsive to their surroundings.
The researchers believe animal studies like this could eventually help scientists better understand how psychedelic compounds influence behavior and identify possible directions for therapeutic research. By examining which social responses are most sensitive to psilocybin, they may uncover biological mechanisms that are difficult to investigate directly in people. But the team emphasized that the findings should not be interpreted as evidence of a treatment for aggression in humans. The experiment involved fish, did not evaluate any clinical intervention, and cannot establish whether similar effects would occur in people.
There are also open questions about how long the calming effect lasts. The researchers examined only single doses and short exposure periods - not repeated treatments, long-term behavioral changes, or whether the fish might adapt to the substance over time. Additional experiments will be needed to determine whether psilocybin's ability to reduce aggressive attacks can persist beyond the brief period examined. "Future studies can build on this work to explore how psilocybin alters neural signaling, which serotonin pathways are involved, and why some aspects of social behavior are affected while others are not," Currie concluded. "These are questions that are difficult or impossible to answer directly in humans."