Dictyostelids ordinarily live as single-celled amoebae in the forest soil, where they hunt, engulf, and kill bacterial prey by phagocytosis. Upon starvation, the cells aggregate together and become a multicellular organism.
Hundreds of thousands of amoebae communicate using extracellular cyclic AMP, and then aggregate together to form a mound and eventually a migrating, multicellular slug. The slug can migrate for some time, moving towards light and heat, but eventually the cells of the slug transform into a structure called a fruiting body.
Stalk Cells Exhibit Altruism. Approximately 20% of the cells in the slug form a rigid stalk and die, while the remaining 80% rise to the top and form spores. Stalk cells are altruistic, in that they give up their lives to life the other cells up and out of the soil, helping them to disperse to a better location. Because multiple different strains can contribute to a single fruiting body, there can be conflicts over which cells will die and form the stalk, and which will live on as spores. Some strains cheat by contributing less to the stalk than other strains.