Life history consequences of the prevalence of aggressive males carrying costly weapons

Sexually selected weapons used in intra-sexual competition for mates are among the most striking animal features, but how their evolution affects life history traits closely correlated with fitness, and hence species' evolutionary trajectories, is not well understood. Here, we selected for or against male morphs carrying a lethal weapon in a male-dimorphic mite Sancassania berlesei, and investigated how life histories evolve in populations with high vs. low proportion of weaponized, aggressive males called fighters and non-weaponized, non-aggressive males called scramblers. After 25 generations of experimental evolution, females from fighter-selected lines showed higher early-life fecundity compared to females from lines selected for non-aggressive scrambler males. Furthermore, both sexes matured earlier in fighter-selected lines compared to scrambler-selected ones. Larvae-to-adult survivorship was not affected by such selection treatment. Finally, we investigated whether adult survivorship under temperature stress was influenced by such selection treatment, and we found no difference between fighter-selected and scrambler-selected lines. Our results demonstrate that composite selection pressures resulting from the prevalence of costly intra-sexual aggression lead to an increase in key components of fitness, with likely consequences for population dynamics. However, we found no evidence that the response to such selection affects how individuals cope with environmental challenge.