Short experimental heatwaves have sublethal impacts on male reproduction in a model insect.

Heatwaves are becoming more common and severe. Previous work has highlighted male insects as being particularly vulnerable to multi-day continuous heatwaves, yet our understanding of short duration heatwave impacts on insects is limited. Here, we assessed the impacts of short, simulated heatwave exposures (2, 5 and 10 h) using ecologically relevant temperatures (42, 44, 46, 48 and 50°C) on survival, reproductive output, testes volume and sperm length in Tribolium castaneum. We show that reproductive output is compromised at lower temperatures than survival, especially during the shortest heatwaves, supporting the notion that thermal fertility limits are lower than thermal viability limits. Furthermore, testes volume was reduced by 40% after a 10 h exposure at 42°C and sperm length decreased by 2.7% after an exposure of 42°C for just 2 h. This highlights that even short heat exposure can impact male fertility and reproductive trait morphology at temperatures below viability limits.