Evolutionary transitions in social organization and the relationship between climate and queen number in ant societies

In about one fourth of ant species, multiple queens reproduce within the same colony (‘polygyny’), reducing relatedness and potentially fuelling evolutionary conflicts among colony members. Polygyny is more common at higher altitudes / latitudes in several species, suggesting that cold climates favour multiple queens. We tested this prediction with phylogenetic analyses comparing 263 ant species and modelled the evolutionary pathways between alternative social structures. Queen number was phylogenetically labile, with bidirectional transitions between single and multi-queen species passing through socially polymorphic species (with both types of colonies). Contrary to our predictions, polygyny was not more common at higher latitudes or altitudes. Socially polymorphic species tended to occur in environments with more uniform precipitation throughout the year, and queen number increased towards the tropics. We discuss how multiple queening may be favoured in warm climates due to competitive advantages of faster colony growth and higher genetic diversity. We also call for more studies in tropical regions, as most studies were performed in the northern hemisphere.