Age-dependent decline of fighting performance in the Japanese rhinoceros beetle Trypoxylus dichotomus (Coleoptera: Scarabaeidae)

Abstract

Age can affect the outcomes of male–male contests, but its effects vary across species. In some species, winning probability increases with age due to increased investment in reproduction as a strategy of terminal investment, while in others, it decreases due to senescence. In the Japanese rhinoceros beetle, Trypoxylus dichotomus, males use their horns to compete for resources and mates. Previous research has shown that the horn and body size influence contest outcomes. We investigated the effect of age on winning probability in male–male contests. We also measured the daily changes in lifting and grasping forces as indicators of fighting ability. Both body size and age differences significantly affected contest outcomes, with larger or younger males being more likely to win. Fighting ability declined with age; specifically, males over 20 days of age exhibited decreased grasping and lifting forces. Given the short reproductive season and low survival rate of natural populations of this species, selection probably favors individuals that fight with full effort regardless of age. Consequently, contest outcomes are primarily determined by differences in physical abilities, and older males with reduced fighting performance are more likely to lose to younger males. This study emphasized the importance of morphological and physiological factors in determining competitive outcomes and highlighted the impact of life history on sexual selection dynamics.