Davis says that while about 20 lizard species are thought to form family groups, only two of those lay eggs. "Viviparity provides the opportunity for prolonged interaction between the mother and offspring, which predisposes the animal to form a family group," she writes. "The importance of parent-offspring interaction fits with what is currently understood about evolution of family groups and cooperative behaviors in birds and mammals."

The researchers found that the lizards huddle together in kin-related groups. "This is remarkable, given the fact that in most species of lizards, individuals actively avoid each other," Davis said. The groups remain together for years after young lizards are born, and in some cases are multi-generational.

In 1995, Stephen Emlen, the Jacob Gould Schurman Professor of Behavioral Ecology in the Department of Neurobiology and Behavior at Cornell University, published An Evolutionary Theory of the Family (PNAS), in which he described the evolution of family groups in birds and mammals, drawing out similarities across different groups of species. But he did not extend his study to lizards. David says that her results show their groups to have similar characteristics. "Biologically, lizards are very different from both mammals and birds, yet a few species of lizards have evolved a social system around nuclear family members that is nearly identical to what we see in ground squirrels, primates, and woodpeckers," she says.

Davis's co-author Barry Sinervo, Professor of Ecology and Evolutionary Biology at UCSC, says: "Establishing a common pattern for how kin-based groups and cooperative behaviors evolve across different taxa gives us an invaluable tool. It helps us to predict where similar group behaviors may be found in other species," he said.

In the current study, the researchers did not attempt to identify or measure the survival advantages of the lizards' groupish behaviour, but they plan to do so in future. "Determining the fitness consequences of kin-based social groups in this species will be an important next step," Sinervo says.

It's not perhaps very surprising that there are general rules that would apply to cross-species groupish behaviour. Apart from obvious advantages such as preserving warmth, the proximity of conspecifics would seem likely to encourage cooperation and communication. These things are much easier to study in larger animals, evidently. How do you set about identifying cooperation or communication among small bugs that live inside dead logs, for instance? So the fact that groupish behaviour has mostly been described among 'advanced' animals such as mammals and birds doesn't argue for its absence among lower forms of life. And the link with viviparity may be a red herring; obviously there is a causative factor at work in the case of the lizards, but one should not conclude that viviparity is a necessary pre-condition for groupish or kin-related behaviours. Ants and bees disprove that immediately.