Ants, like other social insects, have evolved cooperative societies based on kinship. Colonies headed by a single breeding queen (monogyny) was the ancestral state but today ca. half of the ant species live in multi-queen societies (polygyny), which can sometimes reach extreme sizes (supercolony). Multi-queen breeding requires both social and life-history adaptations from individuals to decrease intra-colony conflicts and to ensure that sterile workers receive inclusive fitness benefits despite lowered relatedness. However, it remains unclear exactly what ecological and life-history covariates could make one social nest type favorable over another. This thesis takes on this challenge by identifying social syndromes (genetic and life-history traits) that consistently differ between monogynous, polygynous and supercolony social nest types.
First, by reviewing existing literature we argue that only ants, not the other obligatorily social insects were able to decrease social and sexual conflicts sufficiently to make polygyny reach obligate form in some species. This can be explained by general ant biology, such as perennial lifehistories, foraging on foot instead of wings and having one mating event in life instead of ongoing events between pairs.
Second, by empirical studies on the native ant species Myrmica rubra we were able to demonstrate that the three social syndromes can co-exist within populations, but with possible overlap in certain traits. Genetic and morphology results suggest that polygynous and supercolony nests have very limited dispersal while virgin queens reared in monogynous nests engage in population-wide dispersal. This likely influences the frequency at which mating among related individuals occurs, as it created inbreeding differences across the three social nest types. High inbreeding also resulted in high genetic relatedness, which could imply substantial indirect fitness benefits since obviously negative fitness effects were not produced such as suboptimal body size, significant fluctuating asymmetry in reproductively relevant traits, or diploid male production.
Finally, we aimed to gain insights into how native supercolonies emerge and compare to other nests of the same population. We found that supercolony nests reached higher density, had very low relatedness and lower genetic differentiation compared to non-supercolony (monogynous, polygynous) nests. Both genetic (microsatellites, mtDNA) and behavioral studies indicate that supercolonies of M. rubra can maintain gene flow with other colonies in the same population and that they could emerge both by clonal growth and fusion of nests, latter being rarely reported. Ants in supercolonies and polygynous nests had similar morphology, which suggests that once colonies start to adopt additional queens, they also gain the potential to ultimately become supercolonial without further adaptations, in case the habitat allows colony expansion.
Hopefully this thesis will inspire similar comparative studies on other ant species to better understand the social syndromes and how supercolonies function. Foremost, this would help to manage invasive supercolonies that harm humans and biodiversity, but could also provide contribution to our general understanding on how ecology, especially demography impacts upon social evolution.