Social bonds, such as pair bonds, are one of the best predictors of mental health and longevity in humans. Species from all vertebrate lineages exhibit pair bonding behavior, but there is variation in related behaviors such as parental care, which make understanding the neural basis of pair bonding difficult. By performing a molecular and neural network analysis across behaviorally divergent pair bonding species, I will use the power of comparative analysis to reveal core mechanisms that regulate pair bonding. Using an interdisciplinary approach that combines behavioral ecology, evolutionary neuroscience, and molecular genetics, I will test the hypothesis that a core set of neural components promotes pair bonding across vertebrates. I will use comparative model systems for five vertebrate lineages where pair bonding has independently evolved: Chaetodon fishes, Ranitomeya amphibians, Tiliqua reptiles, Coturnix birds, and Microtus mammals. In each lineage, I will identify the neural basis of pair bonding by comparing males of a pair bonding species to a solitary sister species in a standardized behavior assay that quantitively measures pair bond strength. Using this behavior assay, I will determine the brain regions (Aim 1) and cell types (Aim 2) important for pair bonding in each vertebrate lineage. After identifying lineage-specific patterns, I will compare them across vertebrates to determine if there is a conserved neural circuit that promotes pair bonding across all species or if there are multiple mechanistic solutions to forming a pair bond. Understanding the core features of pair-bonding is important because this behavior is important for human health. Given the neuronal cell types and brain regions that regulate social behavior are broadly conserved across vertebrates, my research into the fundamental mechanisms of affiliative behaviors will shed new light into evolutionary mechanisms that enable these behaviors and also provide biomedically relevant insights into human social interactions.