Data from: Mate competition and relatedness among males mediate the evolution of lethal fights in bulb mites

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.