Mate competition and relatedness among males mediate the evolution of lethal fights in bulb mites.

Abstract

Aggression can enhance direct fitness by aiding the aggressor's in securing reproductive resources, but it negatively impacts inclusive fitness when directed toward kin. We investigated the trade-off between the indirect fitness costs of aggression among kin and the direct benefits of increased success in mate competition using the bulb mite Rhizoglyphus echinopus, a male dimorphic species in which male fighters kill rivals with their modified third legs, while unmodified male scramblers seek unguarded females to mate. Our experiments showed that fighter aggression was never directed toward females, suggesting that fighter aggression evolved as a tactic to monopolize mates. Fighters grab other males before killing them, and grabbing behavior increased in the presence of a female, regardless of the presence of kin. Scrambler mortality increased with fighters' grabbing activity, but whereas the presence of mate competition increased lethal aggression, kinship decreased it, as higher mortality was observed among non-kin. These findings suggest that aggressive behavior intensifies under mate competition, but the decision to escalate aggression to lethal levels is influenced by kinship. In conclusion, this study provides insight into the trade-offs underlying kin-discriminatory aggression and direct benefits.