Dung beetle distress signals may be correlated with sex and male morph: a case study on Copris lunaris (Coleoptera: Scarabaeidae, Coprini)

Insect sounds are predominantly produced by stridulations, where specialised body parts contact repeatedly to induce acoustic pulse trains. We studied the stridulatory organ and sound emissions in Copris lunaris, by focussing separately on females, and on major and minor males. Results highlighted an isometric growth of pars stridens in response to body size, and identified a wing-pygidium locking structure that assists sound emission. Sex-specific acoustic differences of some degree were detected between major males and females, as sounds emitted by majors showed higher frequencies and shorter pulses with elevated impulse rates. This cannot be immediately explained by size differences in the components of stridulatory apparatus. Rather, divergence might be an indicator of some underlying behavioural difference in response to distressing events. In minor males, acoustic properties overlapped with both females and major males, although incomplete stridulations had a lower ratio in minor than major males. This paper provides the very first perspective of the potential role of sex and male polymorphism on sound production. However, future categorisations of sounds coupled to behavioural observations of specific interactions are needed to reveal the function of sex and morph-specific differences.