Psilocybin, the psychedelic substance found in more than 200 species of mushrooms, appears to have an unexpected effect on aggressive fish. Most mushrooms containing this compound belong to the gilled mushroom genus Psilocybe. In mammals, psilocybin interacts with serotonin receptors in the brain, influencing emotions and behaviors such as mood, appetite, and aggression. However, scientists still know relatively little about how the substance affects social interactions among animals.
To investigate this question, researchers in Canada examined the effects of psilocybin on the amphibious mangrove rivulus fish (Kryptolebias marmoratus). Their findings, published in Frontiers in Behavioral Neuroscience, suggest that even a brief exposure to a low dose can significantly reduce aggressive behavior without disrupting 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.
“These findings provide the first evidence that psilocybin can selectively reduce escalated aggression in a vertebrate model without suppressing social interaction,” added senior author Dr. Suzie Currie, a biologist at The University of British Columbia.
Why Scientists Studied Aggressive Mangrove Rivulus Fish
Mangrove rivulus fish are naturally combative, particularly when they encounter another member of their species. Their aggressive responses are easy to recognize and measure, allowing researchers to detect even small changes in behavior.
These fish also have an unusual reproductive ability. They can fertilize themselves, producing offspring that are genetically identical. This makes them especially useful for experiments because scientists can minimize genetic variation and more confidently attribute behavioral differences to psilocybin rather than inherited traits.
For the experiment, the researchers worked with three genetically distinct lines of fish bred in the laboratory. One group received psilocybin, another provided the fish used to provoke social interactions, and the third helped researchers measure how much psilocybin entered and accumulated in the animals’ bodies.
The researchers first established a baseline for each fish’s behavior. They placed a fish into a tank already occupied by another fish, with the two 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 allowing five minutes for the fish to adjust to their shared environment, the team removed the opaque cover and recorded their interactions.
The following day, 24 hours after the initial observation, the researchers placed the same fish in water containing dissolved psilocybin. Following 20 minutes of exposure, they returned it to the tank with the same fish it had encountered previously. The opaque cover was then removed again so the researchers could observe any changes in behavior.
Magic Mushroom Compound Reduces Aggressive Attacks
The results revealed a striking change. Fish exposed to psilocybin moved around less and displayed fewer bursts of aggressive swimming than fish that had not received the substance.
To assess these changes, the scientists measured overall activity (time spent moving) and several signs of aggression (including swimming bursts). These rapid bursts are particularly important because they indicate that an encounter is escalating toward an attack, even when the fish cannot make physical contact.
“Swimming bursts are high-energy attack behaviors that represent an escalation of aggression towards the stimulus fish without making physical contact,” explained Currie. “Other types of aggressive behaviors, like head-on displays, are more about communication and social assessment and require very little energy.”
Interestingly, psilocybin did not 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,” said Forsyth. “This suggests that this compound can selectively dampen escalated social conflict rather than shutting down behavior altogether.”
The compound also reduced general activity. When placed alongside another member of their species, fish treated with psilocybin spent less time swimming than untreated controls.
Together, these findings suggest that psilocybin may influence specific aspects of aggressive behavior rather than simply making animals less responsive to their surroundings.
What the Findings Could Mean for Psychedelic Research
The researchers believe animal studies like this one 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, researchers may be able to uncover biological mechanisms that are difficult to investigate directly in people. Such insights could ultimately inform studies of potential clinical applications.
However, 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 important questions about how long the calming effect lasts. The researchers examined only single doses and short exposure periods. They did not investigate repeated treatments, long-term behavioral changes, or whether the fish might adapt to the substance over time.
Additional experiments will be necessary to determine whether psilocybin’s ability to reduce aggressive attacks can persist beyond the brief period examined in this study.
“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,” concluded Currie. “These are questions that are difficult or impossible to answer directly in humans.”
