Aggregation as a Source of Mates

In aggregation assays, B. germanica males displayed a stronger response to paper conditioned by virgin females than to paper conditioned by any other category, whereas the female response did not differ when presented with the residues of males, females, and juveniles (Wileyto et al., 1984). These authors postulated that males unassoci-ated with an aggregation may be using the sexual information present in the residues to determine the composition of a group, and therefore to locate potential mates. They concluded that their results were consistent with the hypothesis that cockroaches aggregate for the purposes of mating.

Functional separation of aggregation pheromone and sexual pheromone is not always possible; sex ratios and reproductive status have a complex relationship with aggregation behavior in B. germanica (Sommer, 1974; Bret et al., 1983). Because females of this species produce a nonvolatile as well as a volatile sex pheromone (Nishida et al., 1974; Tokro et al., 1993), it is not surprising that males respond to their residues. Encounters between potential mates are increased by gregarious behavior; newly emerged virgin females occur in close proximity to males, and sexual communication over long distances is not required for mate finding (Metzger, 1995). A virgin, then, would not remain one for long in a group that already included adult males. The hypothesis of Wileyto et al. (1984) would be stronger if wandering males were attracted to groups that contained female nymphs in their penultimate instar, so that they were already present to compete for newly emerged virgins. The argument, however, has other flaws. Virgins leave residues regardless of whether they are isolated or in a group, and residues in a harborage are a mélange of all stages present. It is also unclear whether mating takes place within the aggregation in free populations. Rivault's (1989) work suggested that prior to the imaginal molt, B. germanica gather in high-density areas in the middle of the aggregate, looking for sexual partners. However, in a number ofspecies, including B. germanica (Nojima et al., 2005), females produce volatile sex pheromones, and may move out of the group to release them. Females of Blab. giganteus, for example, have been observed calling on the outside of a tree that contained a large aggregation of conspecifics (C. Schal, pers. comm. to WJB). The age, sex, and kinship structure of a group will determine the optimal mating strategies open to an individual (Dunbar, 1979), and the disadvantages of mating in a group should not be ignored. Cockroaches typically require 30 min or longer to transfer a spermatophore (Roth and Willis, 1954b) and may be subject to harassment during that period of time (Chapter 6). The suggestion that cockroaches aggregate for the purposes of mating, then, may be true in some species or in some circumstances, but cannot be applied universally to gregarious species.

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