Why do geckos group? An analysis of "social" aggregations in two species of Australian lizards

Although reptiles of many species are often found in aggregations within retreat sites, there is little quantitative information on the size and composition of such aggregations. Such data may clarify the processes that stimulate aggregation, and that determine patterns of co-occurrence of individuals with respect to sex and body size. We gathered data on diurnal aggregations (under rocks) of two species of nocturnally-active gekkonid lizards (the gekkonine Christinus marmoratus and the diplodactyline Nephrurus milii) from two widely-separated localities in southern Australia (central Victoria and an island off the southern coast of Western Australia). Both species occurred at these localities, but geographic variation was evident in mean adult body sizes and in sexual size dimorphism. Both species actively aggregated, and the composition of groups differed significantly from that expected under a model of random assortment. For example, adult females of N. milii were rarely found with juvenile conspecifies, whereas pairs of juveniles were common. Most groups of C. marmoratus contained only a single adult male. In N. milii, members of an aggregation tended to resemble each other in body size. These significantly non-random patterns within aggregations of gekkonid lizards are suggestive of the kinds of processes that determine group size and composition, but experimental studies are needed to verify the causal factors involved.