Variation and repeatability of male agonistic hiss characteristics and their relationship to social rank in Gromphadorhina portentosa

Male Madagascar hissing cockroaches, Gromphadorhina portentosa, produce a variety of sounds or ‘hisses’ during social interactions. The agonistic signals produced during male-male competition were examined, specifically the intra- and inter-individual variation of agonistic hisses, to determine if these sounds could be reliable indicators of male rank or male size. The relationship between characteristics of the agonistic hiss and social rank were also examined. Among males, agonistic hisses ranged from sounds without clear frequency bands to those with frequency modulation. Agonistic hisses varied in dominant frequency, which was measured at the beginning, middle, and end of the hiss, duration, and hiss rate (number per min). With males, the characteristics of a hiss were repeatable within and between days. Dominant males hissed significantly more than middle-ranking or subordinate males. While dominant, middle-ranking and subordinate males differed in the rate of hissing, there were no significant differences in the duration or dominant frequency of their hisses. Finally, the relationship between male weight and features of agonistic hisses was examined. Both duration and frequency were significantly correlated with male weight. Larger males had longer, lower frequency hisses than smaller males. The possible function of agonistic hissing during male-male interactions and the role of hisses in discrimination between individuals is discussed.