cues from other sensory modalities, such as vibrational or tactile stimuli. These kinds of accessory signals usually serve in modulatory communication, lowering the response threshold in the recipient for the actual releasing stimulus. Comparative studies suggest that modulatory signals evolved through ritualization from actions originally not related to the same behavioral context, and modulatory signals may further evolve to become independent releasing signals.

I thank J. Heinze, M. Lindauer, C. Peeters, F. Roces, and J. Tautz for reading the manuscript. Special thanks are due to my former collaborator N. F. Carlin, with whom several of the concepts concerning anonymity and specificity of chemical signals have been developed in a joint paper (3). Some of the author's work presented in this essay has been made possible by grants from the National Science Foundation and the Leibniz-Prize from the Deutsche Forschungsgemeinschaft.

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