Reproduction by ergatoid queens in the Oriental myrmicine ant Acanthomyrmex concavus, from Sabah, Borneo, with notes on their ecology and behavior

Most Acanthomyrmex ants have a granivorous diet, supplemented with other invertebrates and sugars. They are characterized by dimorphic workers, with major workers having substantially larger head width than minor workers. In this study, we investigated the colony composition and behavioral characteristics of both the queen and worker castes in Acanthomyrmex concavus Moffett, 1986, collected in Sabah, Borneo Island, Malaysia. All colonies had one or two ergatoid queens that exhibited an intermediate morphology between major and minor workers. Colony size was small, ranging from nine to thirty-seven minor and one to three major workers. Consistent with other species of Acanthomyrmex, A. concavus fed on tiny seeds (Ficus and Oxalis), and major workers played a crucial role in seed cracking. We discuss the possible selective pressures linking granivory to ergatoid queen evolution in this genus, where more than half of the species have ergatoid queens.