Costs of Aggregation

Two noteworthy potential costs of group living in gregarious cockroaches are the transmission of pathogens (Chapter 5) and the risk of cannibalism. Both the higher humidity and the intimate physical association typical of aggregations help promote infectious diseases. The cost may be direct, resulting in illness or death, or indirect, in the form of trade-offs ensuing from increased investment in the immune system. Cannibalism is usually a density-dependent behavior, in that high population levels may decrease the local food supply and lower attack thresholds. Injuries also may be more common in dense aggregations, resulting in scavenging of the crippled and dead. Vulnerable life stages such as oothecae and young or molting nymphs may be at risk regardless of group size (Dong and Polis, 1992; Elgar and Crespi, 1992). Young cockroaches typically suffer the highest mortality of any developmental stage (e.g., B. germanica—Sherron et al., 1982; P. americana—Wharton et al., 1967), in part because frequent ecdyses expose nymphs to injury and cannibalism. However, if the local food supply adequately meets the needs of the older group members, the advantages of living in a multigenerational group should outweigh the risks for young stages. Cannibalism is relatively unstudied in cockroaches (but see Gordon, 1959; Wharton et al., 1967), and the information we do have is sketchy. Young nymphs are described as the most cannibalistic in P. americana (Wharton et al., 1967, Roth, 1981a), but the behavior is rare in first to third instars of B. germanica (Pettit, 1940). While these findings may reflect species-specific differences, variation in cannibalistic behavior either within or among species may also be attributed to laboratory culture under different densities or feeding regimens.

There are additional costs to social behavior, particularly when groups become too large. These include increased competition for resources, decay in habitat quality, and increased attractiveness to predators (Parrish and Edelstein-Keshet, 1999). Overcrowded cockroaches may exhibit a breakdown in circadian rhythm, enhanced aggression, a prolonged nymphal period, supplementary juvenile stages, increased mortality, and decreased body size (Wharton et al., 1967). Optimal group size is no doubt variable and depends on both the taxon in question and available resources, but it has been calculated for one cockroach. Deleterious effects from crowding begin to occur in B. germanica when they exceed a level of 1.2 individuals/cm2 in a harborage (Komiyama and Ogata, 1977). That the net gain of living in a group diminishes after the aggregate reaches a certain size is also reflected in cockroach chemical communication. The composition of the aggregation pheromone in Eub. distanti is known to vary with cockroach population density (Brossut, 1983), and dispersal pheromones have been found in the saliva of the German cockroach (Suto and Kumada, 1981; Ross and Tignor, 1986a). This pheromone counteracts fecal attractants and is most concentrated in the saliva of crowded, gravid females. It is thought to function as a space regulator within aggregations, force dispersal from crowded or otherwise unfavorable conditions, and deter cannibalism of young nymphs. Adult males react most strongly to the pheromone and are thought to be the main target group (Ross and Tignor, 1985; Faulde et al., 1990).

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