Evolution of social systems

We have been investigating questions about the evolution of kin

sociality in viviparous lizards in the genus Xantusia that inhabit the

deserts of the southwestern U.S. and Mexico. These lizards are

highly, but facultatively, social, allowing for the rare comparison

between social and solitary individuals within the same population.

By combining a five-year mark-recapture study of 2,400 individuals in

440 social groups, a cross-fostering manipulation of kin environment,

and a DNA microsatellite analysis of relatedness, we have tested

both the mechanisms promoting sociality and the evolutionary,

genetic, and ecological consequences of the transition to social

behavior.

In striking convergence with avian, mammalian, and invertebrate

social systems, we found that these lizards are highly sedentary and

that closely related kin groups often form through the delayed

dispersal of offspring, as juveniles were commonly found in

aggregations with at least one parent and/or sibling. Groups containing

nuclear family members were more stable than groups of less-related lizards, as predicted by social theory developed for birds and mammals. We found that kin presence actively promotes both philopatry and aggregation in juvenile lizards, which suggests that kin sociality in this species is not simply a by-product of limited dispersal. We also found that facultative aggregation confers strong reproductive success and survival advantages and that thermal benefits of winter huddling disproportionately benefit small juveniles, which thus favors delayed dispersal of offspring and the formation of kin groups. This research has important implications for social theory by supporting the theory that transitions to group living arise from direct benefits to social individuals, offering a clear mechanism for the origin of kin groups through juvenile philopatry, and highlighting the importance of social plasticity and the role of environment in social evolution.

We are currently using this system to test hypotheses about the relative impact of mate choice, immune response, and parasitism in structuring kin sociality in Xantusia lizards.

Why is it important?

One component of our research examines 1) how social systems first arise in populations of solitary individuals and 2) how and why these nascent social systems may change over time. These questions are important because they help identify common selective factors that promote sociality across taxa, despite vast differences among species in habitat, ecology, and evolutionary history.