Nest mate recognition in ants with complex colonies: within- and between-population variation

The ability to recognize kin is widespread, and especially important in highly social organisms. We studied kin recognition by assessing patterns of aggression within and between nests of the ant Leptothorax longispinosus. Colonies of this species can be fractionated into subunits, a condition called polydomy. The problem of recognizing relatives is therefore more complex when those relatives can live in two or more different places. We hypothesized that spatial subdivision may have resulted in a stronger genetic component to kin recognition than in cases where colonies live in a single location. To test our hypothesis we assessed recognition capabilities for two populations of this ant that differ in the complexity of their colonies. In a New York, USA, population, polydomy is very common, and colonies also can have multiple queens. By contrast, a population in West Virginia, USA, has colonies that typically are monogynous and rarely are polydomous. We conducted introductions of ants between different nests collected in the same neighborhood, with self-introductions and alien introductions as controls. Nests from the two populations showed corresponding differences in their aggression towards intruders. For New York nests, the extent of genetic similarity was the single best predictor of aggression, whereas for West Virginia nests aggression was jointly influenced by genetic similarity and spatial distance. In both populations, we found nest pairs for which aggression was nonreciprocal; these probably reflect recognition errors by one of the nests. After the ants were maintained in the laboratory for 3 months, their aggression scores rose and fewer recognition errors were made. Thus nest-mate and colony-mate recognition in this species are mediated primarily by endogenous cues (genetic similarity); the importance of exogenous cues for nest mate recognition depends on the population's social system.