Facial Complexity in Sun Bears: Exact Facial Mimicry and Social Sensitivity

Facial mimicry is a central feature of human social interactions. Although it has been evidenced in other mammals, no study has yet shown that this phenomenon can reach the level of precision seem in humans and gorillas. Here, we studied the facial complexity of group-housed sun bears, a typically solitary species, with special focus on testing for exact facial mimicry. Our results provided evidence that the bears have the ability to mimic the expressions of their conspecifics and that they do so by matching the exact facial variants they interact with. In addition, the data showed the bears produced the open-mouth faces predominantly when they received the recipient’s attention, suggesting a degree of social sensitivity. Our finding questions the relationship between communicative complexity and social complexity, and suggests the possibility that the capacity for complex facial communication is phylogenetically more widespread than previously thought.


Recording Equipment
Two video cameras (Sony HandyCam DCR-TRV19E (Sony Electronics Oradell, NJ, USA) and JVC Everio GZ-MS110BEK (JVC Electronics, Wayne, USA) were used inter-changeably dependent on the individual collecting the recordings and other logistical issues. The recordings were obtained by altogether six recordists.

Behavioural coding
Definitions on open-mouth variants as well as face-to-face, play intensities and play actions are presented in Tables S2-S4, respectively. The coding was conducted by one coder, separately for each bear for the facial expressions, facial orientations and play intensities; play actions were also identified by this coder in order to determine play intensities and play bouts.
Coding the behavior of each bear separately was particularly important for the facial expressions; since the coder could not know if and when the facial expressions of the other bear in the play bout was coded, and therefore if facial expressions of play partners were frequently followed by facial expressions in the subject, this could not but attributed to the expectations of the coder.  WUI expression Open-mouth expression of play that show upper incisors. The upper incisors are exposed by raising the nose, which also causes a corresponding wrinkling of the upper muzzle bridge.

Face-to-Face
Facial orientation when the playing bears were within 45 degree head rotation of one another. Table S3. Definitions of play intensities.

Play intensity Definition
Gentle play Play with slow sequences of play actions, including shoving, grappling, and leavingfollowing sequences.

Rough play
Play with fast sequences of play actions, which included all play actions produced during gentle play but faster, in addition to hitting, biting, and flailing.

Leavingfollowing sequences
A behavioral sequence consisting of one play partner leaving and the other following their play partner, often many times consecutively.

Hitting
Raising the paw and striking the play partner.

Biting
Biting the play partner, most often on the nape.

Flailing
Rapidly swinging the head, torso and/or arms from one side to another.

Facial behavior and play duration
Correlations were used to explore whether facial behavior (expression production, mimicry, and exact mimicry) was associated with longer play duration as an indication that this behavior may help to regulate play. Rates of expression production, mimicry and exact mimicry were calculated by dividing the total number of observations of each behavior per subject by the total time spent playing per subject. We then calculated spearman rank correlations between each form of facial behavior and play duration.

NUI and WUI occurrences and play intensity
To ensure that exact facial matching was not an artifact of particular facial variants being produced mostly in one type of play, patterns of expression production Rates of NUI and WUI expression production per subject were calculated by dividing the total number of expressions produced per subject, by the total amount of time spent playing in each play intensity (gentle and rough), which therefore gave 4 rates per subject; the rate of NUI expression production during gentle play, the rate of NUI expression production during rough play, the rate of WUI expression production during gentle play, and the rate of WUI expression production during rough play. Rates of NUI versus WUI expressions were compared during gentle play, and during rough play, using Mann-Whitney U tests. Rates of NUI variants in gentle versus rough play were compared within-subjects using Wilcoxon Signed-Ranks tests. Rates of WUI variants were also compared within-subjects in gentle versus rough play using Wilcoxon Signed-Ranks tests.  (1974). Social play in the American black bear: its similarity to canid social play and an examination of its identifying characteristics. Am. Zool. 14(1), 371-389. Table S5 shows the percentage of expressions that were mimicked out of all the expressions perceived in a play partner by the subject.