A common antidepressant detected in rivers and streams worldwide is disrupting how fish learn, and the impact is strikingly one-sided.
New research led by Monash University shows the drug amitriptyline impairs spatial learning in wild fish, but only in males. Females remain unaffected.
The international study, led by Jack Manera, PhD candidate in the School of Biological Sciences at Monash University, and published in Environmental Science & Technology, adds to growing evidence that pharmaceutical pollution is reshaping animal behaviour in unexpected ways.
"Every day, traces of human medications wash into our rivers and streams through incorrect drug disposal and wastewater systems that simply aren't designed to completely remove them," said Mr Manera.
"Many of these compounds are specifically engineered to alter brain chemistry in humans. So, we wanted to know what this means for the animals living downstream."
To test this, researchers exposed wild-caught guppies to amitriptyline at concentrations already found in natural waterways. The fish were then repeatedly challenged to navigate a maze, allowing researchers to track how quickly and accurately they learned over time.
All fish improved with practice, becoming faster and making fewer wrong turns. But the pattern shifted sharply under pollution.
Drug-exposed males made up to 34 per cent more errors than unexposed males, losing the performance advantage they typically hold. Females showed no decline in learning.
"Under normal conditions, males were actually more accurate than females and also improved faster," said Mr Manera. "But exposure to the pollutant erased that advantage entirely. At high concentrations, males ended up performing much worse than females."
Beyond the sex-specific effect, the study also revealed stable individual differences in performance. Some fish were consistently more accurate, others more error-prone, but all improved at similar rates, suggesting learning capacity itself remains intact.
"Spatial learning is fundamental to survival, as fish rely on it to find food, locate mates, and evade predators," Mr Manera said. "If pollution selectively undermines those abilities in one sex, the consequences could ripple through entire populations and ecosystems."
The findings highlight a gap in how environmental risks are assessed, with researchers calling for greater focus on behavioural and cognitive effects, not just survival.
"This isn't just about whether animals survive exposure," said Mr Manera. "It's about whether they can still function effectively in their environment."
The study was an international collaboration between Monash University (Australia) and the Swedish University of Agricultural Sciences (Sweden).
