Novel worker-like behaviour observed in gynes of the social parasite Tetramorium microgyna

Socially parasitic ants increase their own fitness by exploiting the labour and resources of non-kin ant colonies. Here, we report a novel worker-like behaviour observed in an African workerless inquiline, Tetramorium microgyna, a parasite of Tetramorium sericeiventre. 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 worker-like behaviours were unmated, with ~6 ovarioles and an estimated capacity to store 12,000 sperm cells and fertilize 3,700 eggs. We hypothesize nest excavation by inquilines represents an artefact of a non-parasitic past, where gynes that fail to mate remain in their natal colony and assume a secondary, but still mutually beneficial worker-like role. Alternatively, nest excavation by socially parasitic foundresses, could be an artefact of an ancestral behavioural repertoire associated with independent colony founding. While helping behaviour in post-reproductive inquilines does not increase personal fitness, it also does not reduce it, and may be maintained through relaxed selection. 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.