In the Japanese horned beetle, Trypoxylus dichotomus (L.), males have a long forked head horn that they use in fights with other males over access to sap sites that attract females. Because of the high risk of injury from these contests, males should assess the fighting potential of their rivals before escalating to direct combat. Indeed, male rhinoceros beetles only escalate to intense fighting when matched with equal-sized rivals. Males often tap their opponents with their head horn before and during fights, so it is likely that beetles assess the size of their competitors via sensory input from their horns. Here, we used scanning electron microscopy to examine the density and distribution of sensory hairs along the length of the males’ horn. To assess the potential functional significance of variation in hair density, we combined our microscopy observations with a behavioral analysis of how males use their horns during fights. We found a strong correlation between the density of sensory hairs and the regions of the horns that were used most during combat. The distal tips of the horns had the highest hair density, and were also the region of the horn most frequently in contact with an opponent. Given the shaft and socket morphology of these hairs, which is the characteristic morphology of mechanoreceptors, we expect that they provide mechanosensory input. Thus, although beetle horns are often described as dedicated weapons, our results suggest that the head horns of T. dichotomus also play an important sensory role.
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