If the monkey’s vocalizations carry a significant amount of information referring to objects, in addition to their emotional content, can close observation find other similarities with the human ability to use language? Marc D. Hauser, of Harvard University, analyzed videotapes of the Cayo Santiago rhesus monkeys frame by frame, looking for differences in the right and left sides of the face. In humans, the right side of the brain controls the left side of the face, and appears to concern itself with emotion, while the left side is concerned with language.
Hauser analyzed four expressions of emotion, a fear grimace, which a subordinate monkey gives when being attacked or intimidated by a higher ranking group member; a copulation grimace, similar to the fear grimace, but more fleeting; and two expressions given by a dominant animal toward a lower ranking one: an open mouth threat with lips in an O shape and an ear-flap threat, with the ears flat against the head. Hauser found that in most monkeys, the left side of the face tends to move earlier, takes on a more extreme expression and retains the expression longer than the right side.
These results reveal that in rhesus monkeys, as in humans, the right hemisphere of the brain dominates emotional expression.
Adding Hauser’s evidence to previous studies of Japanese macaques, which showed the left side of the brain to be involved in perception of vocal signals, leads to the suggestion that both humans and monkeys have the same pattern of brain asymmetries related to referential vocalization and facial expression of emotions.
Hauser cautions, however, that this conclusion rests on the assumption that monkey vocalizations carry more symbolic information than emotional. Secondly, he points out that rats and chickens also show some brain asymmetries associated with communication, so these asymmetries may not indicate an increased ability to communicate symbolic information.