RESEARCH ARTICLE Multicolonial population structure and nestmate recognition in an extremely dense population of the European ant Lasius flavus C. Steinmeyer • P. S. Pennings • S. Foitzik Received: 15 April 2011 / Revised: 22 May 2012 / Accepted: 26 May 2012 Ó International Union for the Study of Social Insects (IUSSI) 2012 Abstract Under optimal ecological conditions ant popu- lations can reach high nest densities, leading to strong com- petition between colonies and affecting ant behaviour and sociogenetic structure. Extremely high population densities are commonly found in invasive ant species in which all colonies of a population act as a single super-colony with no behavioural or genetic boundaries between nests. In contrast, the typical and ancestral ant population structure consists of discrete genetic units, i.e. colonies with a single or few queens and highly related workers. The subterranean mound building ant Lasius flavus, which is native to Central Europe, reaches nest densities of up to 0.5 colonies per m 2 in our study population in Southern Germany. We investigated the sociogenetic structure and its temporal stability in this dense population using genomic microsatellites. Despite close proximity, neighbouring nests were generally genetically distinct. Most mounds were inhabited by a single colony, but we also found a few polydomous colonies spreading over two or more mounds. About half of all colonies were monogynous, the others either polygynous and/or polyan- drous. We re-sampled mounds after 8 months and found mound occupancy to be predominantly stable over time. Behavioural experiments revealed low aggression between workers from different nests, albeit these ants recognised their nestmates. Our study demonstrates that native ant species can reach high densities despite a multi-colonial population structure and that intense competition does not invariably lead to high aggression between nests. Keywords Unicoloniality Á Nestmate recognition Á Competition Á Territoriality Á Aggression Á Social insects Introduction The densities of populations often vary in time and space and population growth has been shown to be limited either by predators or parasites (top-down control) or through competition for limited resources (bottom-up regulation) (Carpenter et al., 1987; Center et al., 2005; Sinclair and Krebs, 2002; Snyder and Ives, 2003; Stadler, 2004). Under low density conditions, individuals often have the problem of finding conspecific mates, while at the opposite side of the density spectrum, conspecifics intensely compete for resources. Population density affects the behaviour of ani- mals (Wallin and Raffa, 2002). In particular, high densities select for exploitative, territorial or aggressive behavioural phenotypes (Warner and Hoffman, 1980). With increasing density and competition, the level of aggression towards conspecifics often grows. Depending on the resource dis- tribution, animals will establish and maintain territories to defend resources against competitors (Maher and Lott, 1995). The establishment of territories—areas that are exclusively occupied and defended by individuals or groups (Maher and Lott, 1995)—can reduce aggressive conflicts when territory boundaries are accepted by conspecifics (Langen et al., 2000; Temeles, 1994). On the other hand, territoriality can also enhance aggression, because encounters C. Steinmeyer (&) Max Planck Institute for Ornithology, Eberhard-Gwinner-Straße, 82319 Seewiesen, Germany e-mail: steinmeyer@orn.mpg.de P. S. Pennings Biological Laboratories, Harvard University, Cambridge, MA 02138, USA S. Foitzik Institute for Zoology, Johannes Gutenberg University Mainz, 55099 Mainz, Germany Insect. Soc. DOI 10.1007/s00040-012-0244-8 Insectes Sociaux 123