Stable social aggregations in an Australian lizard, Egernia stokesii

Social groups in many animal species contain family members, and are maintained by parental care, and then tolerance of related individuals in the group. Lizards rarely show prolonged parental care, and rarely form stable social aggregations, although cohesive groups have been reported in the Australian skink genus, Egernia. A population of Egernia stokesii was surveyed for six seasons on Camel Hill, South Australia. At this site individuals occupied rock crevices for refuges on an isolated rocky outcrop of about 1.5 ha. The population was divided into 17 stable social groups, each containing 2-17 individual lizards. Surveyed juveniles took more than 5 years to reach mature size, and most juveniles and subadults remained in the social group of their parents for that period and longer. There were 2-8 permanent adult members of each group. Group members shared common crevice refuges, basked close together (and sometimes on top of each other) and defecated in common scat piles. There was low mortality after the first 2 years of life, and low dispersal either into or out of the population. Some individuals were "floaters" that did not belong to a social group. Over the study a number of these became established in groups. The social structure of these lizards resembles the family groups reported in many species of birds and mammals where group members help to raise the offspring of relatives. The low level of parental care in lizards suggests that the evolution of this form of social organisation in lizards has resulted from different processes than in other vertebrate taxa.