Pig-tailed macaques (Macaca nemestrina), like other primates, live in complex, hierarchical societies, and communicate with each other frequently, by means of gestures, facial expressions and vocalizations.
One of the expressions used by primates is the silent bared-teeth display. This is a unidirectional signal – it is always emitted by the same monkey in a pair. In a potential or real conflict situation, in which it is thought to have evolved, the expression is produced by a subordinate monkey to signal its submission to a dominant one.
The signal is effectively a way of managing conflict. Through past experience, the subordinate monkey has learnt that it is likely to lose the conflict with this particular individual, and therefore produces the silent bared-teeth display to prevent the conflict from escalating further.
The conflict situation is the context in which the silent bared-teeth display has been studied. But subordinates use also use it in an entirely different context, during what appear to be peaceful situations. Monkeys will, for example, produce the display when they are approaching or passing another, even though there is no aggression, or even a threat of aggression, from the monkey receiving the signal.
The function of the expression in these peaceful situations was unknown. But now, researchers from Emory University show that the meaning of this signal depends on the context in which it is displayed. Over a 5-period in 1998, they observed the behaviour of 84 pig-tailed macaques housed at the Yerkes National Primate Research Center near Lawrenceville in Georgia. The monkeys had been kept together as a group since 1985, and were therefore socially mature. During the study, about 1,600 silent bared-teeth displays were observed – over 1,200 in the peaceful context, and the remainder in actual conflicts.
It was found that pairs of macaques that exchange the peaceful form of the signal frequently fight less often than pairs who exchange the same signal only when in conflict. When those pairs who exchanged the peaceful signal did come into conflict, reconciliation was achieved more quickly; they also groomed each other more frequently. Those pairs exchanging the peaceful variant of the signal appear have closer relationships than pairs which exchange the expression only in conflict situations.
Thus, in peaceful situations, the expression signals subordination, and facilitates positive social interactions between the pair. In using the expression, the sender is telling the receiver that they agree upon the established pattern of behaviour. This reinforces the hierarchical relationship between the pair, reducing uncertainty about the state of the relationship. (The authors call this “relationship state signalling”.) As a result, the amount of aggression needed by the receiver to maintain its dominant status in the relationship is reduced. The signal, as used in the peaceful context, is therefore a means of bringing about social cohesion within the group.
In a conflict, the silent bared-teeth display is signalling the immediate behaviour of the subordinate. In contrast, in the peaceful context, the expression signals the concept of subordination, or how the monkey would behave in a conflict situation. In other words, the subordinate is communicating hypothetically with the receiver of the signal. It was previously thought that the ability to communicate in such abstract terms was limited to humans, and this is the first example to date of animal communication about a future interaction. Two big questions raised by this study are how this type of communication arose, and what role it played in the evolution of social complexity.
Flack, J. C. & de Waal, F. (2007). Context modulates signal meaning in primate communication. PNAS DOI: 10.1073/pnas.0603565104.