Ants are renowned for the complexity and diversity of their interspecific interactions. Given the ecological importance and dominance of ants in most terrestrial ecosystems it is valuable to understand the dynamics of these interactions and their impact on ant populations and communities. 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 in western New York, two species of slavemaking ants, Formica subintegra and F. pergandei, both parasitize a locally abundant mound-nesting ant species, F. glacialis. Since 2008, in this site over 1000 nest locations of F. glacialis have been mapped, with approximately half of these nests representing currently active colonies. Over a 7-year-period, the raiding activity of 10-14 colonies of the slavemaking species has been monitored to determine the frequency and distance of raids and the identity of the host nests targeted.
Long-term monitoring of slavemaking colonies suggests constraints on meeting their demand for host brood, despite the high density of F. glacialis nests. The raiding behavior of individual colonies varied considerably; some colonies had as many as 20 raids in a season. In the more successful raids, >2000 host pupae were captured in a day. Slavemaking colonies also exhibited frequent relocation: of 14 colonies tracked for at least three of seven years, all but one has moved at least once by invading existing host nests. Nine colonies have moved at least three seasons over this period. Occasionally slavemaking ant colonies split to occupy several host nests. Slavemaking colonies also launched raids on neighboring slavemaker nests. Four such intraspecific raids resulted in the apparent elimination of the raided colony and differed substantially from raids on host colonies, which usually are not destroyed. Colony relocation and antagonism between slavemaking ant colonies are best explained by efforts to improve raiding success by seeking areas of higher host availability and monopolizing access to these host nests. Such strategies are likely an important component of the ecology of slavemaking ants contributing to the dynamic nature of their interaction with the host ant population.