Large-scale regional variation in cooperation and conflict among queens of the desert ant Messor pergandei

To determine the extent and geographical distribution of alternative social systems in the desert ant Messor pergandei, we conducted a large-scale study within the speciesʹ range in the United States. We found that that queen social behaviours are extraordinarily diverse, resulting in three distinct colony social systems distributed in different geographical regions. Experiments show that these differences arise from variation in queen behaviours at two distinct times in the colony life cycle, and are likely to result from underlying genetic differences between regions. Results from our population genetics analyses and the biogeography of the Mojave and Sonoran Deserts suggest that the different queen social behaviours may have evolved in previously isolated populations, with their current distributions explained in part by subsequent range expansions rather than local adaptation in situ. However, regions with alternative social forms currently meet in contact zones where gene flow and natural selection on alternative social systems should occur. The degree of geographical variation in social behaviours exhibited by M. pergandei results in an opportunity to study the role that gene flow, local adaptation, genetic drift and past history can play in the evolution of behaviour.