Experiments by Klüver and Bucy

Chapter 1 endnote 30, from How Emotions are Made: The Secret Life of the Brain by Lisa Feldman Barrett.
Some context is:

The amygdala was first linked to fear in the 1930s when two scientists, Heinrich Klüver and Paul C. Bucy, removed the temporal lobes of rhesus monkeys. Lacking an amygdala, these monkeys approached objects and animals that would normally frighten them, like snakes, unfamiliar monkeys, or others that they’d avoided before the surgery, without hesitation. Klüver and Bucy attributed these deficits to an “absence of fear.”

Adult monkeys with amygdala lesions are more likely to explore novel objects right away, a behavior that is usually interpreted as a “lack of fear.” Klüver and Bucy removed the temporal lobes in monkeys (i.e., the hippocampus and amygdala) and called the behavior they observed “psychic blindness.” The monkeys experienced a variety of other behaviors as well: an inability to recognize objects visually, strong tendencies to put things in their mouths or have intercourse with other species, and other symptoms unrelated to fear.

Later studies of monkeys with amygdala damage have shown clearly that these animals don't have deficits that are specific to fear (see facial configurations in monkeys and apes). For example, for the past decade, Eliza Bliss-Moreau and her colleagues have studied rhesus macaques who received amygdala lesions at 2 weeks of age. These animals were raised normally by their mothers in small social groups. They developed normal behavior in all domains, although they were overly social when young (a behavior that they grew out of), and they are less reactive to evocative events.

Humans also do not necessarily show fear-specific deficits with amygdala lesions—see Patient SM.