Intraspecific behavioural and environmental contexts influence collective risk of microplastic ingestion in a social fish

Plastic pollution is a growing ecological threat, yet the behavioural and social factors influencing susceptibility to microplastic ingestion remain poorly understood, particularly under varying environmental conditions. We tested how social dominance, group dominance structure and water flow interact to shape microplastic ingestion in a group-living freshwater fish, the European minnow (Phoxinus phoxinus). Capture, ingestion, retention and swallowing of food pellets and weathered microplastics by individuals within social groups in static and flowing water were quantified in a controlled experiment. The results showed that dominant individuals consistently captured and ingested more microplastics than subordinates in both flow regimes, but swallowing rates increased with dominance rank only in flowing water, suggesting that environmental factors can modulate rank-related asymmetries in microplastic exposure. Groups with steeper dominance hierarchies did not ingest more microplastics overall, but instead rejected them more frequently, indicating enhanced collective discrimination in groups with more defined dominance structures, despite higher ingestion rates by dominants. Flowing water reduced total microplastic ingestion across individuals and groups, probably due to the transient availability of items, yet also intensified the disproportionate exposure of dominants. These findings show that abiotic and social contexts jointly influence microplastic exposure risk, and that in polluted environments, traits that normally confer a competitive advantage may instead create a socially mediated evolutionary trap.