Reproductive division of labour related to age in the Japanese queenless ant, Pristomyrmex pungens

Abstract

In the ant Pristomyrmex pungens, only intranidal workers (which stay inside the nest) reproduce. This paper reports laboratory investigations into the factors differentiating intranidal and extranidal (foraging) workers. All workers begin life as intranidal workers, and later become extranidal workers. Taken over the whole lifespan of individuals, there is no non-reproductive caste in this species. All intranidal workers have active ovaries, but extranidal workers have resorbed ovaries. Thus, in P. pungens, reproductive division of labour has been reduced and is now related only to age rather than caste. Extranidal workers collected from the field, isolated from intranidal workers and fed to satiation in the laboratory, failed to recover ovarian activity, indicating that once a worker has become extranidal, she cannot return to laying eggs. All individuals thus both work and reproduce in this species in cooperative societies. Parthenogenetic, laying of diploid eggs by workers may be an evolutionary response to the risk of losing a queen in nomadic species such as P. pungens.