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The neural basis of affiliative behavior was examined in the prairie vole, a rodent that exhibits high levels of social contact and paternal behavior. In the first study, the axon-sparing excitotoxin N-methyl-D, L-aspartic acid (NMA) produced lesions in the basolateral nucleus of the amygdala or the corticomedial amygdala. Males with corticomedial lesions showed significantly less contact with a familiar adult female and a pup when compared with males with lesions of the basolateral nucleus or controls. This behavioral change was not associated with changes in exploratory behavior, motor function, performance in an olfactory task, fearfulness, physical well-being, or body temperature. In a second study, NMA lesions restricted to the medial nucleus also decreased paternal behavior. Neurons in the medial nucleus of the amygdala appear to be essential for the normal expression of paternal care in this species.