Fighting Through the Heat: How Male Aggression Influences Demography Under Recurrent Heatwaves.

Sexual selection is a potent evolutionary force that can enhance adaptation and reduce mutational load, while simultaneously reducing survival, or causing sexual conflict that reduces fitness of one or both sexes. Many populations face extreme, short-term stress events like heatwaves. The combined effects of sexual and environmental selection on population demography during and after such events remain poorly understood, even though such combined effects could be crucial for the persistence of small, endangered populations under climate change. In this study, we investigated how male aggression affects survival in a population during environmental stress. This was done by manipulating the expression of an aggressive male fighter morph in small populations of the male-dimorphic mite Sancassania berlesei, using pheromonal cues from high-density populations. We then exposed some of these populations to recurrent periods of extreme heat and monitored survival over eight generations. We found that heat exposure reduced survival, more severely in females than in males, and survival was lower in populations with higher fighter prevalence, but there was no interaction between temperature and fighter prevalence. Furthermore, survival declined across generations, and the decline was steeper in populations with lower prevalence of fighters, leading to the loss of their initial survival advantage by the last generation. Three populations exposed to heat went extinct from the reduced fighter expression regime. Our findings imply that despite its cost to individual survival, male aggression does not exacerbate population sensitivity to heatwaves over generations. Furthermore, we demonstrate that the mortality costs of male aggression are gradually compensated over successive generations, which could be a result of a more effective purging of inbreeding depression. Thus, while the additive effect of aggression and heatwaves on survival may increase demographic risks for bottlenecked populations in the short term, sexual selection may increase the resilience of populations to prolonged bottlenecks.