Nestmate discrimination and competition in incipient colonies of fire ants

Abstract

Workers from incipient colonies of the fire ant, Solenopsis invicta, are less aggressive towards conspecific non-nestmates than are workers from reproductively mature colonies. This often leads to brood raiding between incipient colonies, where workers take brood from other incipient colonies without opposition. The development of aggressive reactions by incipient colony workers and queens towards non-nestmate workers and queens was examined. Both workers and queens show nestmate discrimination from the earliest stages of colony activity. Workers from the colony’s first cohort vigorously attacked workers from mature colonies. These same workers rarely attacked non-nestmate workers from incipient colonies, although they were more aggressive towards them than towards nestmates. Workers and queens often vigorously attacked non-nestmate queens, and were more aggressive towards unfamiliar than familiar queens. Queens bitten by workers were likely to be expelled or killed within 3 days of introduction. The highly aggressive response of incipient colony workers towards unfamiliar queens suggests that brood-raiding colonies do not consolidate for mutual benefit. Rather, they struggle for possession of brood while attempting to prevent the entry of additional reproductive females into the colony. In addition, heritable cues were neither necessary nor sufficient for worker acceptance of queens. Finally, although incipient colony workers from colonies with four foundresses were slightly less aggressive towards non-nestmates than workers in colonies with single foundresses, they still discriminated against non-nestmate queens. This result suggests that cooperative nest founding alone cannot explain the development of non-territorial, polygyne S. invicta colonies.