Too hot to reason? Experimental heatwaves affect cognitive traits in male guppies

Heatwaves, increasingly common and intense due to climate change, are increasing mortality rates and disrupting vital functions. Recent research has begun exploring their impact on cognition. Since cognition underlies key fitness-related behaviors, such as foraging, predator avoidance, and mate choice, understanding the cognitive costs of heatwaves is crucial. Here, we investigate whether heatwaves impact cognition using male guppies (<i>Poecilia reticulata</i>) as a vertebrate model. We focused on males due to their behavioral consistency in cognitive tests and because they were previously observed to alter sexual behavior after a heatwave. Males were exposed to a 5-d experimental heatwave (32 °C) or control treatment (26 °C). The chosen temperatures are ecologically relevant for the species, fall within their natural habitat's thermal range, and reflect extreme climatic events that are projected to become even more frequent and severe under future climate scenarios. Following treatment, all fish were tested at 26 °C for spatial memory and learning, mate choice, inhibitory control, and anti-predator responses. We also conducted histopathological evaluations of brain tissue to investigate potential central nervous system lesions. The results show that heatwave exposure declined maze solving efficiency, affected mate choice-related cognitive capacities, and led to suboptimal anti-predatory responses. No effects were observed on inhibitory control or habituation. Importantly, heatwave exposure induced morphological alterations in the central nervous system, potentially explaining the observed changes in cognitive performance. Our study provides a comprehensive evaluation of heatwave impacts on cognitive function, highlighting the need of investigating their subtle yet significant effects to fully understand how heatwaves influence fitness beyond survival.