Figure 1. Mechanisms that act to maintain cooperation.

A. Quorum Sensing. The cooperative behavior is induced only when a sufficient amount of signal has accumulated (left). B. Spatial Structure. When cells are able to assort with kin in space, particularly in the case of biofilm formation (bottom left), dispersal of cells and diffusion of public goods are limited and promote the maintenance of cooperative behavior. C. Policing. This mechanism may act through directed harm (left) or restraint of benefits (right). D. Metabolic Constraint. Producers of a cooperative behavior such as a public good also produce an individually retained private good that is beneficial or required for survival and growth in the focal environment. E. Metabolic Prudence. Cells detect nutrients and other cues in their environment to determine whether it is cost effective to cooperate.