Morphological variants of silent bared‐teeth displays have different social interaction outcomes in crested macaques (Macaca nigra)

Abstract

Objectives: While it has been demonstrated that even subtle variation in human facial expressions can lead to significant changes in the meaning and function of expressions, relatively few studies have examined primate facial expressions using similarly objective and rigorous analysis. Construction of primate facial expression repertoires may, therefore, be oversimplified, with expressions often arbitrarily pooled and/or split into subjective pigeonholes. Our objective is to assess whether subtle variation in primate facial expressions is linked to variation in function, and hence to inform future attempts to quantify complexity of facial communication. Materials and Methods: We used Macaque Facial Action Coding System, an anatomically based and hence more objective tool, to quantify "silent bared-teeth" (SBT) expressions produced by wild crested macaques engaging in spontaneous behavior, and utilized discriminant analysis and bootstrapping analysis to look for morphological differences between SBT produced in four different contexts, defined by the outcome of interactions: Affiliation, Copulation, Play, and Submission. Results: We found that SBT produced in these contexts could be distinguished at significantly above-chance rates, indicating that the expressions produced in these four contexts differ morphologically. We identified the specific facial movements that were typically used in each context, and found that the variability and intensity of facial movements also varied between contexts. Discussion: These results indicate that nonhuman primate facial expressions share the human characteristic of exhibiting meaningful subtle differences. Complexity of facial communication may not be accurately represented simply by building repertoires of distinct expressions, so further work should attempt to take this subtle variability into account.

Extending the MaqFACS to measure facial movement in Japanese macaques (Macaca fuscata) reveals a wide repertoire potential

“…This work thus aims at being used as a MaqFACS Extension for Japanese macaques, supported by video examples for this species. This approach was used before in two studies [15,16], where authors reported that the MaqFACS developed for rhesus macaques [14] could be used with Barbary (Macaca sylvanus) and crested macaques (M. nigra), which are phylogenetically more distant from rhesus macaques than Japanese macaques [30,62].…”

“…Skin on the forehead and temples appears stretched.' [16]. Despite the muscular basis being well studied in M. fuscata, we here apply this code for Japanese macaques as well since the movement does not appear to be an independent action (i.e.…”

“…Several macaque species display Lipsmacking-AD181 in varied contexts [16,77,78,80], and with variable presentation, where visual and auditory cues are combined. For example, in rhesus macaques [14], main appearance changes included lips being rapidly pressed together and relaxed.…”

“…In Barbary macaques [15], lipsmacking was described as having the same main appearance changes as in rhesus macaques, but with the addition of a bared-teeth/teeth chattering display, where the teeth are visible and accompany the lip movement, producing a sound. For the crested macaque [16], additional ADs for teeth chattering (AD182) and tongue chattering (AD183) help distinguishing these from lipsmacking (AD181). In Japanese macaques, we observed lipsmacking with and without AU18 and AU25, but further comprehensive studies on the combination and presentation of these movements and sounds are needed to fully characterise lipsmacking in macaques.…”

“…In the last decade, researchers have been adapting FACS to several species, including nonhuman primates: chimpanzees [12], orangutans [13], rhesus [14], Barbary [15] and crested macaques [16], gibbons and siamangs [17], and domestic species: dogs [18], cats [19], and horses [20]. The development of FACS for other species has, for example, revealed unique aspects of cognition in orangutans using OrangFACS (i.e.…”