Detailed contest behaviour of the Japanese horned beetle, Trypoxylus dichotomus septentrionalis, was examined in the field. Male-male interactions have three sequences, and in these sequences four behavioural stages were recognized. After two males encountered (Stage 1), they always faced and shoved each other with their horns (Stage 2: 'Shoving'). Then, if the horn length or body size difference between the contestants was large, one male began to retreat and was chased by the other male, and the interactions terminated (Stage 4A: 'Chasing'). If the difference was small, the interactions proceeded to the escalated fighting stage (Stage 3: 'Pry'), in which two males put their horns under their opponents and push and try to flip them up each other. The interactions, which proceeded to Stage 3, have two ways of termination. If the body size difference was large, one male was flipped up by the other male, and the interactions was terminated quickly (Stage 4B). If the difference was small, the interactions was not terminated so quickly and continued until one male began to retreat, proceeding to Stage 4A. It is suggested that males with shorter horns relative to the opponents avoid the escalated fighting stage, 'Pry', after perceiving the horn length difference during 'Shoving', which would be an appraising behaviour. Thus, 'Shoving' is the most important stage among all the interaction processes in that the highest proportion of judgement is made here. The great allometric variation of horn length would presently function more greatly for enhancing the efficiency of mutual appraisal than that in actual fighting.

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Exaggerated weapons of sexual selection often diverge more rapidly and dramatically than other body parts, suggesting that relevant agents of selection may be discernible in contemporary populations. We examined the ecology, reproductive behavior, and strength of sexual selection on horn length in five recently diverged rhinoceros beetle (Trypoxylus dichotomus) populations that differ in relative horn size. Males with longer horns were better at winning fights in all locations, but the link between winning fights and mating success differed such that selection favored large males with long horns at the two long‐horned populations, but was relaxed or nonexistent at the populations with relatively shorter horns. Observations of local habitat conditions and breeding ecology point to shifts in the relative abundance of feeding territories as the most likely cause of population differences in selection on male weapon size in this species. Comparisons of ecological conditions and selection strength across populations offer critical first steps toward meaningfully linking mating system dynamics, selection patterns, and diversity in sexually selected traits.

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