Meddling neighbors induce an untimely end for foragers of the Florida harvester ant, Pogonomyrmex badius

Rate and timing of senescence are thought to trace the lifespan imposed on an organism by extrinsic mortality factors. If mortality due to extrinsic forces is low, then resource allocation to the maintenance of durable soma is expected. In contrast, if the risk of externally induced mortality is high, selection for metabolically inexpensive, “disposable soma” is predicted. In the Florida harvester ant (Pogonomyrmex badius), aging workers (the "soma" of the colony) move through a sequence of behavioral castes, beginning with brood care and culminating in risky labor roles outside of the nest, prior to death.

       By wire marking age-cohorts in field colonies, this study determined that birth month governs the age at which workers begin foraging. While workers born in June forage at 43 days of age, those born in autumn are more than 200 days old.  Regardless of birth month, foraging is the final behavioral role of a P. badius worker and foragers survive approximately 1 month after entering this role (Kwapich & Tschinkel 2013). Field-foragers of both worker types were capable of surviving hundreds instead of tens of days when taken into the lab, and when prevented from foraging in field elcosures, they were also found to live significantly longer than in controls. 

     Finally, by manipulating conspecific neighbor density and presenence, a 40% increase in forager survival was induced without a resource-related change in worker fat content. These results indicate that external mortality factors (namley interactions with conspecifics), not age, control death rate in this species. This is significant, because death rate influences caste size and colony growth. Unlike honeybees, Florida harvester ant workers are not victims of programmed death when they enter their final, deadly behavioral role. These findings challenge the idea that foragers in social insects are “disposable soma” and that selection has favored a lifespan that predicts external mortality.