Dominance hierarchies are linear but shallow in the social amoeba Dictyostelium discoideum .

Social groups often form dominance hierarchies, and these hierarchies are almost always linear. However, why linear dominance hierarchies emerge is not well-understood. In the social amoeba Dictyostelium discoideum , cells form a multicellular fruiting body when starved, which consists of a ball of viable spores held aloft by a stalk of dead cells. In genetically mixed (chimeric) fruiting bodies, conflicts can arise over the equitable sacrifice of cells to the dead stalk, and some strains predictably dominate others in the spores. Using pairwise mixes of strains that co-occurred in small soil samples, we determined the dominance hierarchies in four natural populations of Dictyostelium . These hierarchies were significantly linear in two of four populations, but also extremely shallow, indicating that co-occurring strains are competitively similar. We used quantitative genetic analyses to assess the causes of social dominance. Each strain's solo spore production was a significant predictor of its performance in pairs. However, we detected additional genetic contributions of both the focal and partner strain, indicating additional cryptic traits that mediate social competitiveness. In contrast to earlier studies showing strong fitness differences among strains collected over a larger spatial scale, we show that co-occurring strains are remarkably competitively equivalent, resulting in linear yet shallow hierarchies. Our results underscore the importance of biologically relevant spatial scales in assessing fitness interactions among microbes. They also explain why social trait diversity might be observed despite dominance hierarchies that should eliminate this variation.