Teeth Baring as a Model to Understand Complex Facial Signals in a Tolerant Macaque Species.
Level 5 - mechanism / opinion, no new human data
Animal behavioral study with no human data
PubMed 39550634 · doi:10.1002/ajp.23697
What was done
Researchers used facial action coding (MaqFACS), unsupervised cluster analysis, Levenshtein distance, and network analysis (NetFACS) to quantitatively categorize bared-teeth facial expressions in Tonkean macaques (*Macaca tonkeana*). They examined the distribution and stereotypy of these expressions across peaceful, playful, and agonistic contexts.
What was found
The abstract reports no numerical values, sample sizes, or test statistics. Two distinct signals were identified: bared-teeth (BT) and open mouth bared-teeth (OMBT). Neither variant was context-specific on its own, though BT occurred frequently with low stereotypy in peaceful interactions, and OMBT was highly stereotyped in play. Specific modifier elements conferred context specificity: tongue-protrusion during OMBT occurred exclusively in peaceful contexts, while glabella-lowering during BT and OMBT was specific to agonistic contexts.
Why it matters
It demonstrates that primate facial expressions can convey nuanced social meaning through the inclusion of secondary facial movements rather than solely through distinct overall expressions.
Limits
The abstract does not state the sample size, number of analyzed video frames/events, or quantitative effect sizes. The findings are restricted to a single tolerant macaque species and rely on observational coding.