Leg-biting fights reduce the number of sperm transferred by the loser and in draws in Zophobas atratus

Intra-sexual selection has been observed across a wide range of species. Male-male combat can not only determine the winner and loser but also affect subsequent reproductive success. The effects of combat outcomes on reproduction are thought to depend on the reproductive ecology of the target species. However, to our knowledge, studies examining the impact of combat outcomes on sperm competition and fitness remain limited. In the giant mealworm (Zophobas atratus), male’s combat involves biting each other's hind legs. Females mated to the losers of leg-biting contests had significantly fewer eggs and fewer offspring than females mated to males that were not in a contest. Possible explanations for this fitness reduction include the inability of males to transfer sperm effectively due to the combat outcome or the inability of their sperm to fertilize eggs due to female cryptic sperm choice, and the mechanisms underlying this reduction remain unclear. Previous studies have observed distorted mating postures in losing males, leading us to hypothesize that leg-biting during combat might affect sperm transfer. To test this, we allowed uncontested males, winners, losers, and males with a draw outcome to mate with females and compared the number of sperm within the female’s spermatheca. Additionally, we examined the correlation between combat duration and sperm count. Results showed that losers and males with draw transferred fewer sperm than non-combat males. Moreover, the longer the combat duration, the fewer sperm males were able to transfer. These findings suggest that the reduction in sperm transferred was affected by both losing in combat and prolonged combat duration in leg-biting encounters.