The overall goal of the proposed research is to gain knowledge about mechanisms and evolution of signal pattern recognition. The research exploits the advantages of acoustic communication in frogs as a model system for exploring: (1) combinations of physical properties of signals that convey biologically significant information; (2) the operating principles by which nervous systems processes and extract such information; and (3) patterns of evolutionary change in signal pattern recognition. Because frogs do not require acoustic experience to produce or to recognize signals, this system can yield knowledge bearing on analogous problems in the recognition and discrimination of speech sounds by infants. That is, all languages draw from a common set of phonetic segments, and infants with little linguistic experience can discriminate among phonetic segments that do not even occur in the language that they will ultimately learn. This research thus addresses questions about inborn mechanisms of pattern recognition that make possible the development of normal speech, which, in turn, is important for human mental health. The proposed research will use playbacks of computer-synthesized signals that simulate male communication sounds; females respond to these sounds with positive phonotaxis and discriminate subtle differences in one or more properties. The research addresses interactions of discrimination criteria based on the frequency content of the signal and differences in fine-scale temporal properties. These interactions will be quantified in two closely related species; these results will then be used to select tests to be conducted with seven other species that are more or less closely related. These data, in conjunction with an independently derived estimate of the phylogenetic relationships of all nine species, will be used to assess how pattern recognition might have evolved. Mechanistic and environmental factors that could explain evolutionary trends will also be identified. Ultimately, this research will be important for framing testable hypotheses about sensory mechanisms underlying pattern recognition and for guiding the execution of such studies.