Loss of intraspecific, intercolonial aggression

Aggression between ants of the same species but different colonies (intraspecific, intercolonial) is a behaviour currently regarded as obligatory in ant species with one queen per nest (monogynous). In Lasius austriacus, a monogynous species, which lives nearly exclusively beneath soil surface (hypogaeic), however, the complete absence of this aggression was found between colonies of the same population and between colonies of separated populations (Steiner et al. in press). Five hypotheses are posed as possible explanations, taking into account current knowledge of social organisation of ants, population genetics and cuticular hydrocarbons as chemical signature of kin recognition in ants. They include partial inadequacy of aggression tests, as currently performed, due to rapid fading of the nest odour in Lasius austriacus or for hypogaeic ant species in general, reduction of intraspecific aggressivity while the ability of colony recognition persists, loss of the capacity of perceiving intraspecific differences in cuticular hydrocarbons and loss of colony and / or population specificity of hydrocarbon profiles. While two of the hypotheses would indicate the need to redesign behavioural assays, the other three would indicate a hitherto unknown situation in monogynous ants. An interdisciplinary approach is proposed to clarify the situation. This involves a combination of six different intraspecific and interspecific behavioural assays inlcuding aggression tests, field assays and tolerance tests, semiochemical analyses (gas chromatography-mass spectrometry of cuticular hydrocarbons), electrophysiological analyses (gas chromatography-electroantennographic detection) and determination of the queen number, the mating frequency and the degree of relatedness between colonies and populations (determination of the frequency of microsatellite alleles and mtDNA haplotypes).

The project reveals that the ant Lasius austriacus maintains a strong genetic boundary between colonies and discriminates between self and nonself, but completely lacks intraspecific aggression. This results in occasional integration of nonself workers into colonies and challenges a current paradigm of sociobiology. It underlines the importance of the cost-benefit ratio in determining whether ants opt for aggression or integration, frequently but not always resulting in a correlation of high relatedness within and aggression between colonies. Besides, critical steps during insect microsatellite marker development were identified, and a combination procedure was successfully established to single out those substances of the ants` "nest odour" that are at the bottom of nestmate recognition. - Conspecific societies of closely related individuals with a high degree of reproductive division of labour are predicted to be mutually hostile because a distinct self-nonself boundary between them preserves high relatedness between the sterile individuals and the genes this society transmits. It has hence been a paradigm of sociobiology that the degree of intracolonial relatedness decides on aggression or integration at the self-nonself boundary. In the course of this project one of the existing but hitherto underplayed observations of contrary cases was investigated in terms of genetic relatedness, semiochemistry, self-nonself discrimination and interactions within self and across the self-nonself boundary. Our finding clearly challenges the paradigm. Furthermore, the L. austriacus case indicates that the lack of aggression in ant supercolonies, which is an important attribute of many invasive ant species, may at least sometimes have evolved through neither reduced self-discrimination nor through relatedness reduction within colonies. - We applied an enrichment based protocol for microsatellite marker isolation to L. austriacus and compared the satisfactory results with the low yield of suitable microsatellite loci through the same protocol in the beetle Pityogenes calcographus, resulting in the identification of critical steps during marker development. - Body surface hydrocarbons have a pivotal role in the nestmate recognition of social insects. Due to methodological constraints it is still largely unknown how many and which substances out of the bouquet of body surface hydrocarbons encode nest identity in the course of nestmate recognition by ants. We applied a combination procedure to analyse hydrocarbon datasets of various ant species including L. austriacus and revealed that frequently three to five substances, frequently comprising low- and high-weight and thus volatile and nonvalite molecules, encode nest identity.