Sex differences in discrimination of vocal communication signals in a songbird

Vocal communication between zebra finches, Taeniopygia guttata, involves a variety of calls, as well as song. One of these, the long call, is exchanged between individuals in specific social contexts. Male long calls differ from the simpler unlearned long calls of females in one or more learned acoustic features: short, stable duration, elevated fundamental frequency and presence of fast frequency modulations. When species-specific long-call signals produced by male and female conspecifics were played back to adult male and female zebra finch subjects, the birds responded by calling. Males consistently responded much more to female long calls than to male long calls. Females also responded more to female long calls, but the effect was weaker. The responses of females had a correlation with stimulus duration that accounted for most of the higher response to female long calls. Males, like females, responded more to longer long calls, but also showed a step-like increment in responses to all female stimuli. Male birds effectively classified the stimuli by gender; females showed much weaker gender discrimination. The contributions of specific acoustic features to these discriminations were then explored by testing the birds with synthetic versions of natural calls and simplified sounds based on mathematical functions in which acoustic parameters could be independently specified. Male birds showed response functions with changes at particular values of fundamental frequency, stimulus duration and initial modulation frequency, suggestive of category boundaries within these parameters. Female response functions showed changes only for duration. When these response functions were used to predict responses to natural stimuli with given parameter values, the observed response patterns for natural stimuli in the two sexes were reproduced. The decisive gender categorization shown by males thus emerges from categorical evaluation of multiple stimulus parameters.