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Investigations of aggregation behaviour in five lizard species (three diurnal rock-dwelling species, one nocturnal rock-dwelling species, and one diurnal terrestrial species) occurring in the arid western regions of South Africa revealed that there is no single set of causal factors explaining this behaviour. Aggregating behaviour in Cordylus macropholis is simply the result of environmental constraints, namely a shortage of preferred shelter sites. Individuals of Ouroborus cataphractus aggregate to benefit from the early predator detection effect. In Platysaurus broadleyi and Chondrodactylus bibronii aggregation behaviour appears to be the result of environmental constraints as well as possible benefits from the dilution effect. At present there is no conclusive information on the reasons for aggregation in Namazonurus peersi. In the rock-dwelling species investigated, the extent of adaptation to counter the effects of intra-group competition appears to be correlated with the proportion of activity time spent at the communal shelter. The proportion of time spent at the shelter, in turn, appears to be determined by the morphology of the species and whether it is diurnal or nocturnal.