Cognitive advantages of large brains persist during an experimental heatwave

Heatwaves negatively impact behaviour with associated cognitive impairment in humans. A growing body of literature reports negative effects of heatwaves on cognition also in animals. A larger brain is known to generate enhanced cognitive abilities that may buffer against environmental changes and thereby potentially increase fitness in large-brained individuals. Yet, how a larger brain buffer against adverse effects on cognitive abilities induced by thermal stress, such as that experienced during heatwaves, remains unknown. We examined detour problem solving and working memory during an experimental heatwave in guppies artificially selected on brain size with matching differences in neuron number. Overall, detour problem solving was impaired among guppies during the heatwave, while working memory was unaffected. Large-brained guppies outperformed small-brained guppies in detour problem solving and working memory in both the heatwave and control temperature treatments. During the heatwave, large-brained guppies exhibited cognitive performance levels comparable to those of small-brained guppies under normal temperature conditions in the detour task. Our study thus suggests that small-brained individuals might have lower fitness also during heatwaves if increased temperature impair cognitive abilities required for survival and reproduction. Furthermore, our results open up the possibility that cognition-driven brain size evolution may have been influenced by abiotic factors.