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Japanese rhinoceros beetle (Trypoxylus dichotomus) males have exaggerated horns used to compete for feeding territories. Larger males with larger horns generally win competitions, providing them the potential to mate with female beetles. However, agonistic interactions between males appear to begin with an initial assessment ritual, which often results in one beetle retreating without escalating to physical combat. It is unknown what information competing beetles may be able to communicate to each other during the assessment ritual. In many insect species, chemical signals can carry a range of information, including social position, nutritional state, morphology, and sex. Specifically, cuticular hydrocarbons (CHCs), which are waxes excreted on the surface of insect exoskeletons, are responsible for diverse forms of chemical communication in insects. Here, we asked whether CHCs in rhinoceros beetles carry information about body size and sex that males could use during assessment behavior. The CHCs of male and female Japanese rhinoceros beetles were extracted by washing the elytra of deceased beetles in hexanes. Samples were then analyzed through gas chromatography-mass spectroscopy (GCMS). Multivariate analysis of the composition of hydrocarbons observed in GCMS spectra revealed patterns associated with sex and multiple body size components in males (horn length, pronotum width, elytra length). We suggest that male rhinoceros beetles could communicate body size information through CHCs, explaining the decision-making behind escalating to combat and retreating behaviors after the initial assessment. We also suggest that male rhinoceros beetles could identify a conspecific's sex through analysis of CHCs. 

Japanese rhinoceros beetle (Trypoxylus dichotomus) males have exaggerated horns that are used to compete for territories. Larger males with larger horns tend to win these competitions, giving them access to females. Agonistic interactions include what appears to be assessment and often end without escalating to physical combat. However, it is unknown what information competitors use to assess each other. In many insect species chemical signals can carry a range of information, including social position, nutritional state, morphology, and sex. Specifically, cuticular hydrocarbons (CHCs), which are waxes excreted on the surface of insect exoskeletons, can communicate a variety of information. Here, we asked whether CHCs in rhinoceros beetles carry information about sex, body size, and condition that could be used by males during assessment behavior. Multivariate analysis of hydrocarbon composition revealed patterns associated with both sex and body size. We suggest that Rhinoceros beetles could be communicating information through CHCs that would explain behavioral decisions.