Sex Differences in Forebrain Monoaminergic Response to Song Performance.

In many species, successful reproduction is dependent on the ability to adjust social behavior in response to an ever-changing social environment. Because a sexual signal's value and meaning can differ between females and males, responses to those signals should also differ. One way individuals can modulate social behavior is through experience-dependent modulation of the sensory systems that process social signals. Central monoamines (norepinephrine, dopamine, serotonin) modulate neural sensitivity to social stimuli and are key regulators of experience-dependent neuroplasticity in vertebrate sensory systems. However, few studies have examined how exposure to different sexual signals influences monoaminergic activity in female compared to male sensory systems. We used Lincoln's sparrows (Melospiza lincolnii) to examine sex differences in how variation in the trill performance of song influences central monoaminergic activity in the auditory telencephalon. Trill performance measures the rate at which a song syllable is produced relative to the syllable's frequency bandwidth and is thought to reflect the difficulty with which songs are produced. High-performance trills are more threatening to males but more attractive to females. We found that the effects of trill performance on monoaminergic activity were sex-dependent. Relative to the response to low-performance songs, exposure to high-performance songs decreased noradrenergic activity in the caudomedial nidopallium, and tended to decrease serotoninergic activity in the caudomedial mesopallium and caudomedial nidopallium of the auditory telencephalon in females, but in males, the monoamine measurements were indistinguishable between song treatments. These results suggest that the mechanisms underlying sensory processing of male sexual signals differ between the sexes.