Casteless behaviour in social groups of the bee Exoneurella eremophila

Abstract

The comparison of social systems, particularly in closely related taxa, can be highly valuable to the understanding of social evolution. While much research has focused on the formation of hierarchies and eusocial organisation, it needs to be remembered that not all social systems are necessarily based on hierarchies. The allodapine bee Exoneurella tridentata is the only eusocial species within the entire subfamily Xylocopinae (Apidae) with discrete queen and worker morphology. Here, we show that a non-eusocial congener, Exoneurella eremophila, is casteless. Nest collection and dissection data show no evidence of hierarchies, and there were no per capita benefits to group nesting in terms of brood production in any collection period. The casteless behaviour exhibited by E. eremophila appears to be common among very diverse lineages of the bee tribe Allodapini, and as such represents an evolutionarily persistent behavioural strategy. We discuss likely ecological factors that may have driven the evolution of species lacking castes and a species with morphologically distinct castes from within a small monophyletic group—genus Exoneurella.