Communicative contexts of the LEN facial expression of pigtailed macaques (Macaca nemestrina)

We have investigated the contexts of the LEN (lips forward-ears back-neck extended) or pucker, a species-specific facial expression of Macaca nemestrina, in four breeding groups. Using the behavior sampling method we recorded all LENs, associated behavior, and the identities of the individuals involved. Of 401 LENs initiated, 62% were directed to the observer, who never responded to them in any way. Infants and dams directed more LENs to the observer than did sires or other females. The average intraspecific LEN rate was 0.82 per monkey-hour, including LENs given in reciprocation. Excluding LENs given in reciprocation, sires LENned significantly more often to females and dams than to infants, and significantly more than females and dams LENned to them. Of LENs directed to conspecifics, 21% were reciprocated, 34% were followed by distance reduction, and 16% led to both. Reciprocating LEN responses occurred more often to LENs initiated by sires. Although primarily affiliative, some LENs occurred in agonistic contexts, suggesting a possible appeasing function or rejection of a LEN's social invitation. Aggression preceded 8% of conspecific LENs, and 15% of LENs were responded to by visual aggression. No response followed 47% of LENs. Affirming the conclusions of others, LENs are often directed by males to females before mating. LENs directed at cradled infants suggest that maternal LENs serve more than a summoning function. LENs directed to people by M. nemestrina usually seem to be inviting interaction or attention, but LENs by dams seem to reflect concerns over their infants. We speculate that at the most basic level the LEN emphasizes that the sender's gaze is fixed on the receiver. We propose that the LEN may have evolved in this quiet rainforest species to facilitate coordination of social activity when silence or stealth is advantageous.