Dynamics of interactions between slavemaking ants and their hosts: Spatial, temporal, and genetic patterns

Background/Question/Methods

Identifying the outcomes and dynamics of species interactions is often key to explaining the distribution and abundance of organisms from regional to local scales. Among the most unusual of the many interspecific interactions affecting ant populations is the phenomenon of specialized slavemaking behavior. Slavemaking ants raid colonies of their host species and bring back brood to become a work force in their own nests. The captured brood mature in the slavemaker colony, where the enslaved workers care for slavemaker offspring, maintain the nest, and forage for food. In an 8-hectare patch of forest on the SUNY Geneseo campus, two species of slavemaking ants, Formica subintegra and F. pergandei, both parasitize a locally abundant mound-nesting ant species, F. glacialis. In this site we have mapped over 600 F.glacialis nests, of which about 60-70% are active. The host species faces exploitation by ten F. subintegra colonies and four F. pergandei colonies.To describe the spatial and temporal dynamics of this host-parasite interaction, we documented the raiding behavior of 11-14 slavemaker colonies as well as the fate of raided nests over three summers. Our field data are complemented by analyses of genetic variation using microsatellite markers in both the slavemaker and host populations.

Results/Conclusions

We discovered a high rate of activity of slavemaker colonies in this site, with some colonies conducting over 20 raids in a season, targeting host nests up to 50 m away. F. pergandei was significantly less active than F. subintegra in terms of proportion of days spent raiding. Some host colonies suffered multiple raids in a season. Preliminary analyses suggest that a third of raided nests become inactive or exhibit reduced activity in the subsequent season. Based on known and mapped targets, a substantial proportion of active host colonies are raided each season (38%). Slavemaker ant colonies exhibited considerable mobility: eight colonies moved at least once during the three-year period by invading existing host nests. Intracolony relatedness coefficients based on microsatellite genotypes indicate that the host species is socially polymorphic, consisting of both monogynous and polygynous colonies. The slavemaker F. subintegra is highly polygynous in this population. We consider raiding patterns in the context of both spatial distribution and genetic structure of the host population.