Quantitative experiments are described on spatiotemporal patterns of coherent chemical signaling activity in populations of Dictyostelium discoideum amoebae. We observe competition between spontaneously firing centers and rotating spiral waves that depends strongly on the overall cell density. At low densities, no complete spirals appear and chemotactic aggregation is driven by periodic concentric waves, whereas at high densities the firing centers seen at early times nucleate and are apparently entrained by spiral waves whose cores ultimately serve as aggregation centers. Possible mechanisms for these observations are discussed.

• Received 6 October 1995

DOI:https://doi.org/10.1103/PhysRevLett.76.1174