The vervets’ social system relies on individuals knowing and communicating their social status. And they are not alone among monkeys. Harold Gouzoules and Sarah Gouzoules, of Emory University and the Yerkes Primate Center in Atlanta, have studied alarm calls made by young rhesus macaque and pigtail macaque monkeys, and find that these calls too communicate information. In a well-studied group of rhesus macaques on Cayo Santiago Island off Puerto Rico, the Gouzouleses tested the long-standing assumption that animal alarm calls vary smoothly, indicating primarily the degree of agitation of the caller.
Because the troupe had been studied for so long, the researchers could identify each individual and knew his or her family relationships with all other individuals. They knew which animals held dominant rank and which were submissive. They recorded juvenile monkeys’ screams, making voice prints with a sound spectroanalyzer. Contrary to previous assumptions, the vast majority of voice prints fell into one of five recognizable categories, which the researchers called noisy, arched, tonal, pulsed and undulating screams. The voice prints varied in frequency and complexity, and although the monkeys appeared to distinguish the sounds easily, human researchers had to rely on voice prints at first.
When the researchers matched up sound prints with videotapes of the monkeys’ behavior, they found the monkeys used each scream category in a specific social situation. Noisy screams indicated an aggressor of higher rank had made physical contact. Relatives of the screaming juvenile reacted appropriately to a higher ranking monkey, attempting to distract the aggressor rather than confront him directly. Undulating screams also told of an attack by a higher ranking opponent, but without physical contact. Arched screams indicated a lower ranking aggressor and did not indicate any physical contact. Pulsed and tonal screams tended to indicate a squabble within the immediate family. The screams appeared to function as recruitment calls, communicating specific information about the situation and the location and identity of the calling individual, not just the degree of fear. In addition to telling listeners who was screaming, the juveniles communicated who was attacking. This gave relatives the information they needed to respond appropriately. As with vervets, rhesus macaques react to recorded calls the same way they do when they hear live calls. Harold Gouzoules likens the screams to simple words, functioning as symbolic representations of reality.