Squamate reptiles generally have been ignored in the search for a unified theory for the evolution of sociality due to the perception that they exhibit little social behavior beyond territoriality and dominance hierarchies and display polygynous mating systems. However a growing body of research has revealed unsuspected levels of social complexity and diversity in mating systems within the squamate lineage, particularly among the members of the Australian Scincid genus Egernia. Several species of Egernia are amongst the most highly social of all squamate reptiles, exhibiting stable social aggregations and high levels of long-term social and genetic monogamy. Social complexity is widespread within the Egernia genus, with reports of social aggregations in 23 of the 30 described species. The purpose of this review was to examine the potential for the Egernia genus as a model system for study of the evolution of sociality and monogamy within squamate reptiles.
Current evidence indicates there is substantial variability in social complexity both within and between species, with social organization covering the spectrum from solitary to highly social. Four highly social Egernia species are known to live in stable social aggregations consisting of closely related individuals (adults, subadults, juveniles; i.e., ‘family’ groups) that appear to utilize chemical cues to recognize group members (kin recognition). Enhanced vigilance against predators is one presumed benefit of group membership. Additionally, juveniles within social groupings appear to receive low levels of indirect parental care. Several Egernia species create scat piles that mark group territories. Three Egernia species exhibit long-term social and genetic monogamy and several inbreeding avoidance strategies have been documented. However, it is currently unknown whether monogamy is widespread within Egernia.
Egernia species occupy a broad range of habitats, although most are terrestrial, saxicolous or semi-arboreal. Several species display an attachment to a permanent home site, generally a rock crevice, burrow or tree hollow. Egernia species take 2–5 years to mature, live for 5–25 years, and are viviparous with litter size positively correlated with body size. Several Egernia species are herbivorous, with the degree of herbivory increasing with body size and during ontogeny in larger species. Most smaller species are either insectivorous or omnivorous. Species of Egernia have a wide range of reptilian, avian, and mammalian predators. Several larger species possess several behavioral and morphological features to prevent their extraction from rock crevices, including highly modified keeled scales and numerous defensive behaviors. Color pattern polymorphism is present in five Egernia species.
Potential ecological correlates of sociality and monogamy are discussed. The life-history hypothesis predicts long-lived, late-maturing species should evolve complex sociality. The habitat availability hypothesis relies on the assumption that refugia may be limited in some ecological settings, and group formation is a consequence of co-habitation of available refugia. These hypotheses are not mutually exclusive, and testable predictions are formulated and discussed. Specific future research directions are outlined to take advantage of Egernia as a model system for comparative research on a lineage that represents an independent origin of social organization comparable to that found in birds and mammals.