Phylogénie Et Evolution Du Comportement Social Chez Les Blattes ...

An n e x e sinteractions and to induce spacing or dispersal. Previous studies of cockroaches already failedto derive any general trend in this respect (e.g., Bell et al., 1979; Breed et al., 1975; Gortonand Gerhardt, 1979).However, some behavioral acts showed weak but interesting differences between T.a.,P.b. and the three gregarious species. Interactions were less often initiated with the behavioralact “approach” in T.a. and resulted more from fortuitous encounters since the frequency ofinteraction was not smaller in this species. Agonistic acts were present even if they werenot very common, and some dominance-like acts (like “stilt-posture and antennation”) wereat their maximum. Finally, the avoiding act “full crouch” was much more frequent. On thewhole, fewer promoting acts and more dominance-like acts seem to characterize the solitaryway of life of T.a., whereas the converse characteristics typify more reciprocal interactionsin gregarious species. These results are in agreement with the definition of social behavior byMichener (1969) and Wilson (1971).According to these observations, solitary T.a. cockroaches may be characterized asnot refusing social interaction but not searching for it. In the laboratory experiments, whenthey are forced to remain close together, they are able to interact without breaking contactimmediately. They do not space strongly and only avoid too narrow contacts. From thispoint of view, the occurrence of solitariness in the field seems to be readily explained onlyby the lack of intraspecific attraction: individuals do not search for conspecifics, exceptfor reproduction, spreading slowly and continuously in the understory after their birth(Grandcolas, 1993a). No encounter between individuals of T.a. has been recorded during twomonths of continuous observations in the field together with a marking-recapture protocol(Grandcolas, 1993a), which suggests that interactions events are rare in the wild and thatphysical interactions are not central in the life of this species.Similarly, the evolution toward subsociality for P.b. was not supported by deepbehavioral changes. Again, the behavioral repertoire is comparable to the one of gregariousspecies and sequences of interactions are neither shorter nor longer than for other species. Asfor the solitary species, P.b. displayed more dominance-like acts (“climb”) and less promotingacts (“approach”). However, the correlation test between acts displayed by initiators and325