Social Status Interacts with Badge Size and Neuroendocrine Physiology to Influence Sexual Behavior in Male House Sparrows (Passer domesticus)

Abstract

Reproduction results from a complex interplay among multiple factors including social stimuli, hormones, the brain, and an individual’s physical characteristics. Male house sparrows (Passer domesticus) possess a bib of black feathers, or badge, that is associated with behaviors important for reproduction including courtship behaviors, copulation, and aggression. Such behaviors are controlled by testosterone activity within the central nervous system and are strongly influenced by social status and female behavior. To understand how multiple factors interact to coordinate reproductive activity we explored relationships among social status, badge size, gonad volume, and the volumes of brain regions involved in male courtship and dominance (HVC, robust nucleus of the archistriatum, and the medial preoptic nucleus). A trend toward a U-shaped relationship was observed between dominance status and badge size, with the most dominant and most subordinate males possessing the largest badges. Male vocal expression, copulation, and aggression were positively related to dominance status, but not badge size. In contrast, the volumes of HVC, the medial preoptic nucleus and the gonads related positively to badge size, but not dominance. Females preferentially approached large-badged males regardless of dominance status, but this interest translated into copulation for dominant rather than subordinate males, a finding possibly related to the observation that dominant males vocalized at higher rates than subordinates. Subordinate males that had large badges, attracted female interest, and possessed the neuroendocrine potential to perform courtship behaviors might have been prevented from doing so through social interactions with dominant males within the flock.