Mesolimbic dopamine (DA) circuits mediate a wide range of goaloriented behavioral processes, and DA strongly influences appetitive and consummatory aspects of male sexual behavior. In both birds and mammals, mesolimbic projections arise primarily from the ventral tegmental area (VTA), with a smaller contribution from the midbrain central gray (CG). Despite the well known importance of the VTA cell group for incentive motivation functions, relationships of VTA subpopulations to specific aspects of social phenotype remain wholly undescribed. We now show that in male zebra finches (Estrildidae: Taeniopygia guttata), Fos activity within a subpopulation of tyrosine hydroxylase-immunoreactive (TH-ir; presumably dopaminergic) neurons in the caudal VTA is significantly correlated with courtship singing and coupled to gonadal state. In addition, the number of TH-ir neurons in this caudal subpopulation dichotomously differentiates courting from noncourting male phenotypes, and evolves in relation to sociality (flocking vs. territorial) across several related finch species. Combined, these findings for the VTA suggest that divergent social phenotypes may arise due to the differential assignment of ''incentive value'' to conspecific stimuli. TH-ir neurons of the CG (a population of unknown function in mammals) exhibit properties that are even more selectively and tightly coupled to the expression of courtship phenotypes (and appetitive courtship singing), both in terms of TH-ir cell number, which correlates significantly with constitutive levels of courtship motivation, and with TH-Fos colocalization, which increases in direct proportion to the phasic expression of song. We propose that these neurons may be core components of social communication circuits across diverse vertebrate taxa.song ͉ evolution ͉ periaqueductal gray ͉ vocalization A ffiliation behaviors such as courtship, pair bonding, grouping, and parental care can vary dramatically across individuals and species. However, despite the fact that we now know a good deal about the neurobiology of affiliation behaviors (1, 2), we still know very little about the phenotypic variations in neural mechanisms that underlie phenotypic differences in behavior. In fact, nonapeptide systems (the vasopressin-and oxytocin-like peptides) arguably provide the only examples from tetrapod vertebrates in which neural mechanisms have been systematically studied in relation to species-specific social structure (e.g., mating system and sociality) and individual differences in affiliation (3, 4).Among the many other neurochemical systems that regulate motivational and behavioral states, mesolimbic dopamine (DA) circuits are perhaps the strongest candidates as generators of phenotypic diversity. DA influences numerous affiliation behaviors such as pair bonding (5), sexual communication (6-8), and copulation (9-11). Of particular interest are DA cells in the ventral tegmental area (VTA), which are well known to regulate incentive and reward-related processes (12, 13). These cells project o...