Colony size evolution in ants: macroevolutionary trends

Abstract

Colony size is an incredibly important factor in social insect ecology: it affects everything from foraging strategies to colony defense to mating systems to the degree of polymorphism. However, colony sizes vary dramatically among ant species (Formicidae): sizes range from several workers living together to super-colonies that stretch for hundreds of kilometers. Although the origins of eusociality and colonial life have been extensively theorized, little work has been done describing the evolution of colony size that followed after. Our study provides the first large-scale investigation into such issues, incorporating colony size data from 118 genera and recently published, nearly complete genus-level molecular phylogenies. We find that colony size change exhibits a bifurcation pattern similar to the feedback loop theory posited by Bourke 1999. Once colony sizes become sufficiently large, they rarely undergo radical decreases in size on a macroevolutionary scale. Additionally, the magnitude of colony size changes seem relatively small: rarely do colony sizes jump from small to large sizes without first transitioning through an intermediate size. Lastly, we echo many previous authors in advocating for the release of unpublished sociometric data and a push toward its further acquisition.