Neuroendocrine correlates of behavioral polymorphism in white-throated sparrows

Interspecific differences in the neuropeptide systems of the lateral septum (LS) often parallel differences in social behavior. In rodents, some closely related species that differ in aggressive behavior also differ according to the level of vasopressin (VP) innervation of the LS. In songbirds, the neuropeptides vasotocin (VT) and vasoactive intestinal peptide (VIP) affect aggression when administered directly to the LS. Here, we tested whether the density of VT or VIP innervation of the LS reflects patterns of intraspecific behavioral polymorphism in male and female white-throated sparrows (Zonotrichia albicollis), in which the "white-stripe" (WS) morph behaves more aggressively than the "tan-stripe" (TS) morph. We found that the WS birds had more VT-immunoreactivity (IR) than the TS birds in the ventrolateral subdivision of the caudal LS (LSc.vl) and in the medial portion of the bed nucleus of the stria terminalis (BSTm). In addition, the TS birds had more densely stained VIP-IR in the LSc.vl than the WS birds. Males had more VT-IR than females in the LSc.vl and BSTm, and more VIP-IR in the LSc.vl. We also report sex and morph differences in VIP-IR in the basal hypothalamus, where VIP is synthesized and released into the portal vasculature. Males had nearly twice as many VIP-immunoreactive (ir) neurons in the infundibular nucleus than did females, and birds of the WS morph had more densely stained VIP-IR in the median eminence than TS birds. Our results support the hypothesis that differences in these neuropeptide systems underlie inter- and intraspecific differences in social behavior across vertebrates.