Is presocial behaviour evolutionarily reversible in cockroaches?

The evolution of pre-social behaviour in two clades of cockroaches was studied in reference to phylogeny. In each clade, both solitary and pre-social behaviours exist. These behaviours are parsimoniously mapped on phylogenies obtained from previous morpho-anatomical studies. Solitariness appears to have evolved several times from pre-social ancestors. These reversals in pre-social behaviour stress once more the usefulness of phylogeny for revealing unexpected patterns, similar to the results obtained in studies of behavioural evolution in Hymenoptera. A simple model involving lack of adaptive radiation in solitaryderived clades and small size of selected ingroups accounts for the observed pattern of common evolutionary reversals.