Nest site limitation and colony takeover in the ant Leptothorax nylanderi

Ecological constraints on the success of independent colonies are thought to strongly shape the organization of ant societies. One of the most important factors is probably the availability of suitable empty nest sites. By population censuses, laboratory experiments, and microsatellite analyses, we investigated the colony and population structure of the small, myrmicine ant Leptothorax (Myrafant) nylanderi in a deciduous forest near Wurzburg, Germany, where nest sites appear to be strongly limited, especially in late summer. Colonies of L. nylanderi inhabit cavities in rotting branches, hollow acorns, grass stems, etc. After hibernation, a temporary overabundance of empty nest sites facilitates the fragmentation of larger colonies into smaller buds, which, because the species is monogynous, are in part queenless. Nest sites become scarce in summer due to rapid decay, and both established colonies and young founding queens face a severe shortage of suitable nest sites. This leads to the fusion of established, unrelated colonies, which after initial fighting permanently merge and live together. Typically only one queen survives after fusion. Similarly, young mated queens may seek adoption in alien nests instead of founding their own colonies solitarily, and here again only a single queen survives. This temporary intraspecific parasitism may be an important first step in the evolution of obligatory permanent parasitism, which is widespread in the genus Leptothorax.