Evolution of advanced social traits in phylogenetically basal ants: striking worker polymorphism and large queens in Amblyopone australis

Abstract

The ecological success of ants results notably from their ability to derive several phenotypes from the same genotype. In higher ants, polyphenism can be extreme: high queen–worker size dimorphism and worker polymorphism allow for better success at colony founding by lone queens, better division of tasks, and ultimately access to new ecological niches. However in basal ants, polyphenism is much more limited, restricting them to narrow niches. It is often assumed that basal ants lack the ability to produce a pronounced polyphenism. In Amblyopone, most species nest and hunt centipedes in soil and leaf litter, and polyphenism is weak, e.g. A. pallipes. Here we studied A. australis, which forages and nests in rotten logs. Using analyses of morphometry and allometry, we showed that queen–worker dimorphism and worker polymorphism are much higher in A. australis than in A. pallipes. Workers of A. australis exhibit various sizes and shapes, and large individuals could be better adapted for digging galleries in rotten logs. Moreover, larger queens are probably more efficient during non-claustral colony foundation. We conclude that the evolution of advanced social traits typical of higher ants is also possible in basal ants, but it is not often selected for.