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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.
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Male competition signaling in Japanese rhinoceros beetles
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.
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- Award ID(s):
- 2016125
- PAR ID:
- 10510583
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Integrative and Comparative Biology
- ISSN:
- 1540-7063
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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