Intraspecific competition modulates heatwave effects on male reproduction in Drosophila prolongata

Heatwaves are increasing in intensity, frequency, and duration and may drive the responses of organisms to climate change more strongly than gradual increases in mean temperature. The effects of heatwave events on wildlife remain poorly understood because the magnitude of these effects often depends on environmental the context: additional environmental stressors can exacerbate the impact of extreme temperatures. Here, we test whether the impact of extreme heat on adult reproduction and survival depends on the intensity of intraspecific competition, a key environmental variable determining access to resources, in male Drosophila prolongata, a species with extended male-male contests. Contrary to expectations, more intense intraspecific competition partially mitigated the negative effects of the heatwave on male reproductive output. This pattern may reflect a compensatory response whereby males increase reproductive investment under stronger competition, but only when this competition occurs in combination with the additional stress imposed by a heatwave. Overall, our findings reveal that interactions between heat stress and intraspecific competition can shape the impact of heatwaves on reproduction, and highlight the need to consider such interactive effects when evaluating the risk that extreme climatic events pose to wildlife.