Novel Worker-Like Behavior Observed in Gynes of the Social Parasite Tetramorium microgyna.

Socially parasitic ants increase their own fitness by exploiting the labor and resources of non-kin ant colonies. Here, we report a novel worker-like behavior observed in an African workerless inquiline, Tetramorium microgyna, a parasite of Tetramorium sericeiventre and Tetramorium sepositum. We observed several T. microgyna gynes excavating soil and performing nest maintenance tasks at the entrance of an established T. sericeiventre host colony. We photographed this event in nature, then dissected T. microgyna gynes to establish mating status and reproductive capacity. All T. microgyna gynes that participated in excavation behaviors were unmated, with ~6 ovarioles. We hypothesize nest excavation by inquilines represents an artifact of a non-parasitic past, where gynes that fail to mate and disperse remain in their natal colony and assume a secondary, but still mutually beneficial worker-like role. While nest excavation by socially parasitic foundresses could be an artifact of an ancestral behavioral repertoire associated with independent colony founding, this possibility is unlikely, because the behavior occurred in the presence of a robust worker population. While helping behavior in post-reproductive inquilines does not increase personal fitness, it also does not reduce it, and may be maintained through relaxed selection. Although host and parasite gynes are morphologically distinct, a third possibility is that putative T. microgyna parasites are actually microgynes of their 'host' species. Socially parasitic ants are rarely found and poorly studied compared to their non-parasitic counterparts. Our findings provide insights into how selection may act on developmental and behavioral programs during the evolution of social parasites from non-parasitic ancestors.