Animals display remarkable diversity in sociality, provoking fundamental questions about how it has evolved. Although variation in affiliative behavior, such as pair bonded versus solitary living, has independently evolved numerous times across vertebrates, little is known about the underlying neural mechanisms, or how they differ across species or lineages. Using immunohistochemical detection of phosphorylated ribosomes (pS6) as a proxy for neural induction, we are comparing brain region correlates of affiliative variation (pair bonded vs. solitary living) within and across species of five major vertebrate lineages: fishes, amphibians, reptiles, birds, and rodents. We will present our preliminary findings in the earlier lineages, while work on birds and rodents is still ongoing. We expect that a core subset of socially relevant brain regions will be repeatedly linked to social variation across vertebrates, while other brain regions associated with affiliative variation will be species- and lineage-specific. Our findings will be the first to reveal major neuroanatomical themes of affiliative diversity across vertebrates, shedding light onto how it has evolved.