No sign of reproductive cessation in the old workers of a queenless ponerine ant

Abstract

In many social hymenopterans, workers of different ages engage in different tasks; younger workers remain inside the nest as intranidal workers, while older workers go outside the nest as extranidal workers (i.e., age polyethism). Previous studies have shown that ovarian activity is diminished in old, extranidal workers, but it remains unclear whether workers’ reproductive ability persists for life or whether they exhibit post-reproductive lifespans. In this study, we investigated the age-dependence of worker reproductive ability in a monomorphic ponerine ant Diacamma cf. indicum. In Diacamma ants, all females in a colony have reproductive ability, but effective reproduction is limited to one or a few dominant females, and the remaining females act as sterile helpers. Using long-term laboratory rearing, we investigated whether worker reproductive ability lasts throughout a worker’s lifetime. The ages of workers were accurately tracked, and the reproductive ability of young and old workers was examined by creating several gamergate-less sub-colonies. Results showed that at least one individual in each sub-colony developed ovaries, even in the sub-colonies that solely consisted of very old workers (> 252 days old). Interestingly, in the presence of younger workers, old workers rarely showed ovarian development. Besides age, we found a positive correlation between the amount of fat (i.e., nutritional condition) and ovarian development in old workers. Our data suggest that reproductive activity of old workers is low but maintained throughout their life in Diacamma.

Significance statement

Females of social animals, such as cetaceans, are known to stop reproducing before the end of their lifespan. It is suggested that the reproductive ability of ant queens does not decline and is maintained throughout their lives; however, it is unclear whether this is also true in ant workers. We maintained ant colonies for more than 500 days and then tested whether the reproductive ability of ant workers is maintained throughout their lifespan. Even in small groups composed of only very old workers (> 252 days old), at least one individual in each group always reproduced actively. Interestingly, the presence of young workers seemed to suppress the reproduction of old workers. In addition, fat content was positively associated with the individual’s reproductive potential.