A key issue to address in the analysis of any social behavior is the degree of relatedness of group members; in cockroaches the variation is considerable. At one end of the spectrum, cockroach aggregations are not always species specific (Table 8.1). No overt agonistic encounters are observed in mixed-species groups, but, given the choice, individuals will usually associate with conspecifics (Brossut, 1975; Rust and Appel, 1985). Blatta orientalis and B. germanica mixed in the laboratory soon form segregated groups (Ledoux, 1945). Initially separated taxa, however, may eventually mingle if their habitat requirements coincide. Everaerts et al. (1997) placed two closely related Oxyhaloinae species, Nauphoeta cinerea and Rhy-parobia maderae, together in laboratory culture. At first they stayed in monospecific groups, but the degree of mixing increased with time, and the taxa were randomly distributed by the fifth day. While intraspecific grouping in cockroaches should be considered the general rule, conditions of high density or scarcity of resources, such as suitable harborage or pockets of high humidity, may result in mixed groups. Mixed-species social groups also are reported from birds, hoofed mammals, primates, and fish, and these typically display gregarious behavior similar to that seen in single-species groups (Morse, 1980).
Although there are no available data on the relatedness of individuals in natural aggregations, populations of B. germanica within a building are more closely related than populations between buildings (C. Rivault, pers. comm. to CAN). There are also indications that aggregations are cohesive relative to other groups of the same species. In B. germanica almost no mixing of aggregations occurs, even if several are in close proximity (Metzger, 1995); mark-recapture studies show that only 15% of the animals left their initial site of capture (Rivault, 1990). In the cave cockroach Eublaberus distanti, 90% of individuals remained in the same group during a 30-day period (R. Brossut in Schal et al., 1984). Site constancy is also known in P. americana (Deleporte, 1976; Coler et al., 1987). It is
Table 8.1. Examples of mixed-species aggregations in cockroaches. Additional examples are given in Roth and Willis (1960).
Periplaneta americana, Eurycotis floridana
P. americana, Blatta orientalis, Blattella germanica
Schultesia lampyridiformis, Chorisoneura sp., Dendroblatta onephia B. germanica, P. fuliginosa, P. americana Aglaopteryx diaphana, Nyctibora laevigata, Cariblatta insularis
In stumps, under Dozier (1920)
bark, in corded wood
In cupboard of home
In bird's nest
In cracked telephone pole
In bromeliads, Jamaica
Variety of combinations: In sewers Blatta orientalis, P. americana, P fuliginosa, Parcoblatta spp.
Appel and Tucker (1986)
Eads et al.(1954, pers.comm.to
unclear, however, whether the insects are faithful to the group, to the physical location, or both.
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