In theory, a male should change the allocation of fighting and mating efforts in relation to his age. Thus, the consequences of sexual selection may be complicated by changes in the male resource allocation due to aging. However, previous studies have focused on changes in female mate choice and male-male competition with aging separately, and the impact of aging on the relationship between mate choice and male competition is unknown. Here, we examined how male competitiveness and attractiveness and their relationship changes over male lifespan in Gnatocerus cornutus. In this species, males perform courtship displays and fight rival males for mates. Older males are more competitive in male fighting and aggressive toward females than younger males. The aggression is also directed toward females. Almost all older males who attacked a female failed to copulate, while younger males never attacked the females. As a result, copulation success decreased with age. When an older male did not mistake a female for a male, he exhibited more frequent courtship. However, mating with older males imposed direct costs on females in terms of both fecundity and lifespan, with no offsetting indirect benefits for her offspring. The courtship behavior of older males does not supply females with cues for mate-choice benefits but are used to coerce females into mating. Our results suggest that male–male competition constrains female preference for older males. Thus, female choice and male–male competition may not be reinforcing in older G. cornutus males.

Raw data for Older males are more competitive in male fights and more aggressive toward females in the broad-horned flour beetle Gnatocerus cornutus