The development of social behavior following neonatal amygdala lesions in rhesus monkeys.

We examined the role of the amygdala in the development of nonhuman primate social behavior. Twenty-four rhesus monkeys received bilateral ibotenic acid lesions of either the amygdala or the hippocampus or received a sham surgical procedure at 2 weeks of age. Subjects were reared with their mothers and were provided daily access to social rearing cohorts. The subjects were weaned at 6 months of age and then observed while paired with familiar conspecifics at 6 and 9 months of age and with unfamiliar conspecifics at 1 year of age. The subjects were also observed during daily cohort socialization periods. Neither amygdala nor hippocampus lesions altered fundamental aspects of social behavior development. All subjects, regardless of lesion condition, developed a species-typical repertoire of social behavior and displayed interest in conspecifics during social encounters. The amygdala lesions, however, clearly affected behaviors related to fear processing. The amygdala-lesioned subjects produced more fear behaviors during social encounters than did control or hippocampus-lesioned subjects. Although the heightened fear response of the amygdala-lesioned subjects was consistent across different testing paradigms and was observed with both familiar and novel partners, it did not preclude social interactions. In fact, the amygdala-lesioned subjects displayed particular social behaviors, such as following, cooing, grunting, presenting to be groomed, and presenting to be mounted more frequently than either control or hippocampus-lesioned subjects. These findings are consistent with the view that the amygdala is not needed to develop fundamental aspects of social behavior and may be more related to the detection and avoidance of environmental dangers.