Auditory selectivity for the acoustic properties of conspecific mate-attracting signals in lower vertebrates and songbirds

The interplay between conspecific senders and receivers both maintains the usual species specificity of acoustic communication and yet offers the potential for speciation provided that signals and preferences change in a parallel or coupled way. Acoustic signals commonly function in mate attraction and contribute to reproductive success. Such signals are especially prevalent in some lower vertebrates (some fish and many kinds of frogs) and birds in which prospective mates potentially assess variation in acoustic properties among signalers in order to choose an appropriate mate. Selectivity for specific ranges of values of different acoustic properties has been demonstrated by playback experiments, and neural correlates of this selectivity have been documented at various loci in the auditory system. Behavioral studies of temperature effects in lower vertebrates and learning in songbirds provide opportunities to predict short-term changes in neural activity. Demonstrating such “second-order” correlations increases our confidence that the selectivity of neurons or circuits for conspecific acoustic signals is not merely coincidental. With regard to parallel changes in signals and receiver selectivity over evolutionary time, this review describes the role of linkage disequilibria, the potential for genetic coupling between sender and receiver mechanisms, and how changes in cell numbers and size resulting from polyploidy may have facilitated multiple independent occurrences of “instantaneous” speciation.