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Japanese rhinoceros beetles Tropoxylus dicotomus are distinguished by large, pronged horns extending from the heads of the males. Male beetles use these horns to battle over females and sap feeding territories on trees. These exaggerated structures likely evolved as weapons or as a signal to other beetles of the resource holding potential of an individual. To understand how these structures are used in resolving competition, we staged a series of interactions between males. Half of the beetles were calorie restricted to manipulate condition and trials were conducted with beetles both size matched, and with pairings made by random. Winners and losers were tracked for each fight and behavioral sequences were documented and analyzed. Most interactions did not end with physical fights between the beetles, instead there were contacts and what appeared to be assessment, then one of the beetles retreated and the other claimed the territory. However, in some cases, the horns were used to throw the other beetle from the territory. Both horn size and body size, but not male condition were found to be significant factors predicting fight outcome.