Social interactions and aggression among male Madagascar hissing cockroaches (Gromphadorhina portentosa) in groups (Dictyoptera: Blaberidae)

Abstract

We studied agonistic interactions among male Madagascar hissing cockroaches,Gromphadorhina portentosa, in groups of five (low-density) or 10 (high-density) males. Consistent with previous studies of male pairs, we observed aggression (Abdomen Flick, Abdomen Push, Butt, Lunge), submission (Crouch, Retreat), and noncontact behavior (Abdominal Extension, Abdomen Thrash, Agonistic Hiss, Stilt). Males at both densities performed all acts. However, males in the high-density group performed Abdomen Push significantly more often than males at a low density. The rate of each remaining act was unaffected by density. Regardless of density, males within social groups varied in aggression. More aggressive males utilized frontal assaults (Butt and Lunge) during interactions, while males displaying lower levels of aggression preferentially used the abdomen during interactions. More aggressive males performed Abdomen Flick more frequently, while males displaying lower levels of aggression performed Abdomen Push. We also investigated the relationship between male aggression and the four noncontact behaviors. We found that Abdominal Extension, Abdomen Thrash, and Agonistic Hiss were positively correlated with our aggregate measure of male aggression suggesting these are aggressive displays. Stilt was positively correlated with measures of both aggression and submission, leaving its function unclear. None of the behavioral acts examined in this study were highly correlated with male weight. Our results are discussed in light of possible hypotheses addressing the function of specific behavior during male-male competition.