Abstract
Relaxed open-mouth display serves important social functions in relation to submission, reconciliation, affiliation and reassurance among non-human primate societies; however, quantitative evidence on this behavior remains insufficient among multi-level social groups. From July to November 2016, we examined four potential functions of the relaxed open-mouth display during pairwise, intra-unit social interactions among 18 free-ranging adult and sub-adult golden snub-nosed monkeys (Rhinopithecus roxellana) who belonged to three one-male, multi-female units (OMU) at Dalongtan, Shennongjia National Park, China. Results showed that: compared with no relaxed open-mouth display, (1) the occurrence of displacement by a dominant individual approaching a subordinate was lower and the distance of the subordinate to the approaching dominant was shorter when the subordinate showed open-mouth display; (2) relaxed open-mouth display reduced the probability of continued attack for victims of aggression and allowed victims to achieve closer proximity to the aggressor during post-conflict periods; (3) relaxed open-mouth display by dominant individuals allowed them to achieve closer proximity to subordinates; and (4) the exchange of relaxed open-mouth display had a greater impact on the outcome of interactions than one individual alone giving this signal. These findings suggest that relaxed open-mouth display serves important functions regarding submission, reconciliation, affiliation and reassurance in coordinating social interactions within OMUs in golden snub-nosed monkeys.
Highlights
Open-mouth display is a common social behavior in non-human primates
We predicted that: (1) for submission, the occurrence of displacement by a dominant individual approaching a subordinate should be less frequent when the subordinate shows relaxed open-mouth display; and the risk of continued aggression by an aggressor should be reduced if the victim shows relaxed open-mouth display; (2) for affiliation, the distance from the subordinate to the approaching dominant individual should be shorter when the subordinate shows relaxed open-mouth display; (3) for reconciliation, the distance between the aggressor and victim should be closer if the victim shows relaxed open-mouth display; and (4) for reassurance, a dominant individual should be able to approach a subordinate more closely when showing relaxed open-mouth display
Reconciliation: any affiliative contact between former opponents within 10 min after a conflict ceased (Ren et al, (4) Dominance index: we evaluated the dominance index by averaging, for each individual, the percentage scores of aggression given and submission received with all other individuals in the group
Summary
Open-mouth display is a common social behavior in non-human primates. It comprises individuals opening their mouths and presenting their teeth or canines and can be associated with different body postures and behaviors in either hostile or relaxed scenarios (Altmann, 1962; Hinde & Rowell, 1962; Yang et al, 2013). In addition to the common function of aggression, researchers have suggested that relaxed open-mouth display in R. roxellana may function in submission, reconciliation, affiliation or reassurance during normal, calm or post-conflict periods (Li et al, 2006; Ren et al, 1990b; Yan et al, 2006; Yang et al, 2013). These conclusions are primarily based on qualitative observations, with no current research providing quantitative evidence regarding the functions of relaxed open-mouth display in golden snub-nosed monkeys during normal, relaxed or post-conflict periods. We predicted that: (1) for submission, the occurrence of displacement by a dominant individual approaching a subordinate should be less frequent when the subordinate shows relaxed open-mouth display; and the risk of continued aggression by an aggressor should be reduced if the victim shows relaxed open-mouth display; (2) for affiliation, the distance from the subordinate to the approaching dominant individual should be shorter when the subordinate shows relaxed open-mouth display; (3) for reconciliation, the distance between the aggressor and victim should be closer if the victim shows relaxed open-mouth display; and (4) for reassurance, a dominant individual should be able to approach a subordinate more closely when showing relaxed open-mouth display
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