The cellular slime mold, or social amoeba, exists as a unicellular form that divides and multiplies rapidly. When food is depleted, cells aggregate to form a fruiting body within which cells differentiate into spores and stalks. Some spores succeed in dispersing to a new micro-habitat with plenty of food and in resuming a unicellular phase with fast population growth. In contrast, stalk cells lift spores to aid in their dispersal and then die, see Fig. 1 (left). Becoming a stalk cell is an altruistic behavior, see . This system provides an ideal system for studying the maintenance of altruism.