Facial Expression and its Relationship to Gesture in Western Lowland Gorillas

Introduction

It is early in the morning, and as I near the building ahead, I pull out my video camera and walk anxiously to the exhibit. Baracca approaches the window when he sees me and sticks out his tongue as he bangs on the glass. Shana is sitting on rocks near the pond, his lips pulled back and his teeth exposed. He raises his arms and begins to clap.

For my entire life, I have been interested in the abilities of great apes. I have always wondered what their behavior actually means, and how intelligent they truly are. During the summer of 2008, I was able to take what has become my passion a step further and study the communication of western lowland gorillas firsthand. However, I did not expect to discover what I did.

Although I have recorded many gestures before, I was intrigued by the pairing of a gesture with a facial expression. While Shana’s expression may have looked like a smile, it was actually a fear grimace, or bared-teeth display. Although Shana pairs his seemingly happy expression with a gesture that is an attention-getter, it soon becomes obvious to me that he fears the duck that is approaching him. Irving Stone wrote, “All of the character that is necessary to show can be done through the face … and perhaps the hands.” I had never realized how much one could learn from simple movements that most do not notice.

Jane Goodall once said, “Chimpanzees have given me so much. The long hours spent with them in the forest have enriched my life beyond measure. What I have learned from them has shaped my understanding of human behavior, of our place in nature.” During the summer of 2008, I stepped into the Congo Gorilla Forest and began to truly understand the meaning behind this revelation.

Background

Primates regulate their social interactions and group dynamics through several kinds of nonverbal communication, including auditory, olfactory, tactile, and visual displays. There is general agreement that both facial expressions and gestures reveal a variety of information about an individual’s emotions, motivations, and intentions (Van Hooff 1967, Ekman 1997, Parr et al. 2002).

The majority of studies of nonhuman primates have focused on vocal communication, yet it is crucial to understand the use of nonvocal communication. No study of gorilla (Gorilla gorilla) facial expression has been completed to date, although it has been shown in previous studies that gorillas use gestures of the limbs as a means of communication (Tanner 1998, 2004; Tanner and Byrne 1993, 1996; Pika 2007; and Pika, Liebal and Tomasello 2003). A handful of studies have been completed on chimpanzee (Pan troglodytes) facial expressions and have shown the existence of an extensive repertoire (Parr, Cohen, and De Waal 2005). Subjects in these studies used facial displays differently depending on the behavioral context (Parr, Cohen, and De Waal 2005), and therefore the social situations in which facial expressions are used need to be carefully observed. In order to fully understand why facial expressions have evolved to convey a specific meaning, similar facial expressions between closely related species must be studied (Preuschoft and Van Hooff 1997).

It is nearly impossible to suppress facial expressions while experiencing certain emotions, but some gestures of the limbs and body may be more consciously produced, and therefore allow individuals to voluntarily express a mixture of thoughts and feelings (Tanner and Byrne 1993). A facial expression communicates an emotional state of being, which, when viewed, gives the receiver an immediate impression of what the other is feeling. As early as 1872, Darwin recognized that the actions that regularly accompany a state of mind are recognized as expressions, and that facial movements reveal the state of mind. In order to have a complete understanding of nonvocal communication, both its voluntary and involuntary aspects must be examined.

Despite the importance of both gestures and facial expressions in communication, no studies have been attempted that demonstrate the relationship between the two, even in humans. Ekman (1993) states that we know virtually nothing about the type of information that people typically derive from a facial expression when the expression is seen in situ, accompanied as it usually is by speech, gestural, and postural behaviors. The use of play faces in concurrence with gestures was surveyed by Tanner and Byrne (1996). While some gestures had a low association with the play face, others had a very high association. Although there was no significant difference between the consequences of a play face alone and gesture + play face, there was a significant difference between gesture alone and gesture + play face, with gesture + play face more associated with subsequent contact with the receiver. This warrants further study; if a gesture + play face has a greater impact on the receiver than a gesture alone, the effects of other facial expressions may also be important and should be studied.

Hence, this study has three main goals. The first goal is to classify the facial expressions of a single group of western lowland gorillas living at the Bronx Zoo in New York. Secondly, it focuses on the important but overlooked relationship between gestures and facial expressions and the resulting actions of the onlooker. I hypothesized that western lowland gorillas would display a variety of facial expressions that change with the onset of a gesture. Lastly, the third goal is to compare the facial expressions of two species of great apes: chimpanzees and western lowland gorillas. I hypothesized that western lowland gorillas would display just as many categories of facial expressions as chimpanzees.

Methods And Materials

Definition of Gesture

In this study, the term gesture is used for all expressive movements of the limbs and the head as a whole that appear to be communicative through auditory, tactile, or visual signals.

Subjects and Setting

Ten western lowland gorillas from one group were observed at the Bronx Zoo. There were five females and five males, ranging in age from 2 years old to 28 years old. One silverback, Zuri, was present in the group. Information about each of the gorillas is given in Table 1.

The Bronx Zoo’s outdoor gorilla enclosure, the Congo Gorilla Forest, is large and imitates the natural habitat of the western lowland gorilla. It contains natural, artificial, and deadfall trees, a small pond, climbable rocks, and a rock cave whose interior is blocked from the view of visitors. At the time of observation, rabbits, ducks, and chipmunks were also present in the enclosure. The entire exhibit is enclosed by a windowed area for human viewing, which allows visitors to be face to face with the gorillas.

Study Methods

Observations for this study were made between June 2008 and August 2008, on an average of one day per week for approximately six hours each visit. Videotaping was continuous, recording all social interactions between individuals. The individuals being filmed depended on the social situation, as gestures were more likely to occur during communal interactions. Not every gorilla could be filmed each day, as some were blocked from the view of visitors. All video was taken with a Sony Handycam DCR-SR82 camcorder. The film was then played back in slow-motion, and each filmed instance of a gesture and/or facial expression was catalogued in a database made in Microsoft Excel.

Results

Facial Expressions

Figure 2. Shana displays a pout face to Zuri.

Figure 3. While engaging in play, both Baracca and Johari display play faces.

Facial expressions were performed a total of 370 times during the period of study. These facial expressions were placed into 10 categories based on the descriptions of expressions used by Parr et al. (2007) for chimpanzees. The gorillas in this study displayed the same number of categories as have been documented for chimpanzees. Gorillas do not display the pant-hoot facial expression, as this vocalization is unique to chimpanzees (King 2004). The rolled-lip face seen in this study has not been seen in chimpanzees. Although included as an expression category, the neutral expression was used as a basis for no facial expression during analysis. The percentage that each category occurred was as follows:

• play face = 53.5%
• relaxed open-mouth face = 23.2%
• ambiguous = 4.3%
• bared-teeth display = 3.5%
• pout = 3.5%
• alert face = 3.2%
• whimper = 3.0%
• rolled-lip face = 2.7%
• scream = 0.8%

Descriptions of each facial expression category can be seen in Appendix 2.

Facial expressions were performed without an accompanying gesture a total of 89 times, and resulted in an immediate visible body movement (within two seconds) from the receiver only 12 times. Facial expressions were performed while the individual was solitary 51 times.

Gestures

Gestures were performed a total of 1,531 times during the period of study. Thirty-nine different gestures were observed, and these are shown in Appendix 1. Gestures were used alone 1,301 total times, and resulted in an immediate visible body movement (within two seconds) from the receiver 587 times. The 39 types of gestures were split into three categories: auditory, tactile, and visual. Auditory gestures involve a sound not produced by the vocal cords; tactile gestures involve physical contact between the performer and the receiver, as opposed to visual gestures, which involve no physical contact but rather a visible movement. Auditory gestures were performed alone a total of 128 times and resulted in an immediate visible body movement 28 times; tactile gestures were performed alone a total of 783 times and resulted in an immediate visible body movement 465 times; and visual gestures were performed alone a total of 390 times and resulted in an immediate visible body movement 94 times.

The type of gesture and the rate of the resulting visible body movement were not independent; this effect was highly significant, as shown by a chi square test (χ2 (2) = 129.513, p < 0.0001). Tactile gestures were most likely to be followed by a resulting visible body movement (59.4% of the time), then visual gestures, which were followed by a resulting visible body movement 24.1% of the time, and last auditory gestures, which were followed by a resulting visible body movement 21.9% of the time. (Figure 4.)

Gesture + Facial Expression

Gestures and facial expressions were performed simultaneously 229 times during the period of study. Gesture + facial expression resulted in an immediate visible body movement (within two seconds) from the receiver 154

times. Facial expressions occurred most frequently in concurrence with tactile gestures. The facial expression
changed with the onset of a gesture 91 times.

The Relationship Between Gesture and Facial Expression

Gestures alone and facial expressions alone have significantly different effects, as shown by a chi square analysis (χ2 (1) = 33.930, p < 0.0001). A gesture used alone is more likely to produce an immediate visible body movement (within two seconds) from the receiver, although gestures used alone and facial expressions used alone produced no resulting visible body movement from the receiver more frequently than they produced a resulting visible body movement. (Figure 6.)

Gestures alone and gesture + facial expression have significantly different effects, as shown by a chi square analysis (χ2 (1) = 38.308, p < 0.0001). When a gesture and facial expression are performed simultaneously, a resulting visible body movement is more likely to be seen immediately from the receiver.

Facial expressions alone and gesture + facial expression also have significantly different effects, as shown by a chi square analysis (χ2 (1) = 74.253, p < 0.0001). When a gesture accompanies a facial expression, an immediate resulting visible body movement from the receiver is more likely. (Figure 7.)

Facial Expression with the Onset of a Gesture

The facial expression of the performer of a gesture changed 79 times with the onset of his or her gesture, out of a total of 229 occurrences of gesture + facial expression. The change of a facial expression with the onset of a gesture did not have a significant effect on whether there was a result of immediate visible body movement from the receiver, as shown by a chi square analysis (χ2 (1) = 3.293, p > 0.05).

Discussion

The current study examined the use of gestures and facial expressions in a single group of western lowland gorillas. There were three main goals. The first goal was to classify the facial expressions of the group, while the second goal was to examine the relationship between gestures and facial expressions. Finally, the third goal was to compare the facial expressions of two species of great apes: chimpanzees (Pan troglodytes) and western lowland gorillas (Gorilla gorilla).

Both gestures and facial expressions are used as a form of nonvocal communication in this group of gorillas. Gestures are used very frequently and were seen much more often than facial expressions during the period of study. Gestures tend to display either a need or a desire to another gorilla, and facilitate the social dynamics of the group.

This group of western lowland gorillas displayed 10 different categories of facial expressions. In the past it has been assumed that gorillas have few to no facial expressions, yet they are used often as a system of nonvocal communication. While a handful of studies have been completed on chimpanzee facial expression (Parr, Cohen, and De Waal 2005), there has not previously been a study focused on gorilla facial expression. This study demonstrates that gorillas display just as many facial expressions as chimpanzees do, and future studies may find that other groups of gorillas display even more facial expressions than the group observed in this study.

Interestingly, 51.5% of the facial expressions that did not accompany a gesture were performed while an individual was solitary and not within view of another gorilla. Fridland (1997) states that there are several ways in which people can be socially alone, with their facial behavior implicitly social. They

argue that when we are alone, we treat ourselves as interactants. We talk to ourselves and punish ourselves, and deploy facial expressions in the course of these acts. When others are not present, we often imagine that they are. In our imagination, we engage in interactions with others who are not there, and in these situations we often display facial expressions. Although these faces are made while we are alone, they are highly communicative. Fridland (1997) also states that faces made in solitude are no more or less “social” than those observed in the presence of others, because people are always implicitly social even when schematically alone. In past studies, facial expressions were found to be emotion-specific. In humans, emotions typically occur in response to an event, usually a social event, real, remembered, anticipated, or imagined (Ekman 1993). As facial expressions are outward manifestations of changes that have occurred and are occurring internally (Ekman 1997), a gorilla exhibiting a facial expression while alone may be experiencing an emotion. There were no other gorillas near at the times when this occurred; something must have been occurring internally, something that was mentally driven.

Gestures used alone resulted in an immediate visible body movement from the receiver more frequently than facial expressions alone. Facial expressions may not have provoked a response because they convey emotion, and one may not feel the need to respond to a display of emotion. Both gestures alone and facial expressions alone had no outcome more often than they had an outcome. This may be because the receiver was not looking directly at the performer, did not have enough information to respond, or simply did not feel the need to respond.

Gestures and facial expressions, when performed simultaneously, resulted in an immediate visible body movement from the receiver more frequently than either gesture alone or facial expression alone. When facial expressions accompany gestures, the receiver is being provided not only with the performer’s need or desire, but also with the emotion that the performer is experiencing. This may allow the receiver to better understand the performer and facilitate communication between individuals. The receiver has more knowledge when both a gesture and a facial expression are performed, and can respond accordingly.

The facial expression of an individual gorilla sometimes changed with the onset of the performer’s gesture, yet this change did not have a significant effect on the receiver’s immediate action. This might illustrate a relationship between involuntary and voluntary modes of communication. The perhaps involuntary facial expression displaying an emotion often occurred simultaneously with the perhaps more voluntary use of a gesture, but the message upon which the receiver’s visible response was contingent may have been conveyed by the gesture. Gestures may be linked more closely to the social situations in which they are used, while facial expressions seem to be linked to the emotional state of being.

Conclusion

The current study provides many new findings significant to the study of nonvocal communication. Contrary to the belief that gorillas perform few, if any facial expressions, this group displayed 10 distinct categories of facial expression, the same number displayed by chimpanzees. Previously, no studies had surveyed the relationship between gestures and facial expressions, although they are used simultaneously quite frequently.

In the future, many more studies are needed. Other groups of gorillas must be examined in order to determine the full repertoire of facial expressions of this species. The social situations in which the expressions occur are critical, as they provide clues to determining the meaning and emotions expressed through the face. With the development of a Facial Action Coding System (FACS) for gorillas like that of Parr, Waller, and Vick for chimpanzees (2005), and its model, Ekman and Friesen (1978) for humans, facial expressions could be categorized very accurately based on the movements of the facial muscles. This categorization would allow for a direct comparison of gorilla and chimpanzee expressions, as well as gorilla and human expressions, which would allow us to further understand the evolution of facial expressions.

The relationship between gestures and facial expressions must also be examined further. Since gestures and facial expressions used simultaneously seem to have the greatest impact on the receiver in this species, we must study the relationship between voluntary and involuntary forms of communication in other species. This area of study has been overlooked in the past, yet in order to fully understand nonvocal communication, all modes must be taken into account.

Bibliography

Bradbury, J.W., and S.L. Vehrencamp. Principles of Animal Communication. Sunderland, Massachusetts: Sinauer Associates, 1998.

Burrows, A., B.M. Waller, L.A. Parr, and C.J. Bonar. “Muscles of facial expression in the chimpanzee (Pan troglodytes): Descriptive, comparative, and phylogenetic contexts.” Journal of Anatomy 208.2 (2006): 153-168.

Byrne, R.W., and J.E. Tanner. “Gestural Imitation by a Gorilla: Evidence and Nature of the Capacity.” International Journal of Psychology and Psychological Therapy 6 (2006): 215-231.

Dixson, A.F. The Natural History of the Gorilla. New York: Columbia University Press, 1981.

Ekman, P. (1993). “Facial Expression and Emotion.” American Psychologist 48 (1993): 384-392.

Ekman, P. “Should We Call It Expression or Communication?” Expressions in Social Science Research 10.4 (1997): 333-344.

Fridland, A.J. “The new ethology of human facial expressions.” In: James A. Russelland J.M. Fernández Dols, eds. The Psychology of Facial Expression. Cambridge: Cambridge University Press, 1997, 103-132.

Hostetter, A.B., M. Cantero, and W.D. Hopkins. “Differential use of vocal and gestural communication by chimpanzees (Pan troglodytes) in response to the attentional status of a human (Homo sapiens).” Journal of Comparative Psychology 115.4 (2001): 337-343.

King, Barbara. The Dynamic Dance: Nonvocal Communication in African Great Apes. Cambridge, MA: Harvard University Press, 2004.

Leavens, D.A., W.D. Hopkins, and K.A. Bard. “Understanding the Point of Chimpanzee Pointing.” Current Directions in Psychological Science 14.4 (2005): 185-189.

Leavens, David A., A.B. Hostetter, M.J. Wesley, and W.D. Hopkins. “Tactical use of unimodal and bimodal communication by chimpanzees, Pan troglodytes.” Animal Behaviour 67 (2004), 467-476.

Liebal, K. “The gestural communication of orangutans.” In: M. Tomasello and J. Call, eds. The Gestural Communication of Monkeys and Apes, Mahwah, NY: Lawrence Erlbaum Associates, 2007.

Liebal, K., J. Call, and M. Tomasello. ”Use of gesture sequences in chimpanzees.” American Journal of Primatology 64.4 (2004): 377-396.

Parr, L.A., B.M. Waller, S.J. Vick, and K.A. Bard. “Classifying chimpanzee facial expressions using muscle action.” Emotion 7.1 (2007): 172-181.

Parr, L.A., B.M. Waller, and S.J. Vick. “New Developments in Understanding Emotional Facial Signals in Chimpanzees.” Current Directions in Psychological Science 16.3 (2007): 117-122.

Parr, L.A., and B.M. Waller. “Understanding chimpanzee facial expression: Insights into the evolution of communication.” Social Cognitive and Affective Neuroscience 1 (2006): 221-228.

Parr, L.A., M. Cohen, and F. de Waal. “The influence of social context on the use of blended and graded facial displays in chimpanzees (Pan troglodytes).” International Journal of Primatology 26 (2005): 73–103.

Pika, S. “The gestural communication of gorillas.” In: M. Tomasello and J. Call, eds. The Gestural Communication of Monkeys and Apes. Mahwah, NY: Lawrence Erlbaum Associates, 2007.

Pika, S. ”The gestural communication of bonobos.” In: M. Tomasello and J. Call, eds. The Gestural Communication of Monkeys and Apes. Mahwah, NY: Lawrence Erlbaum Associates, 2007.

Pika, S., K. Liebal, and M. Tomasello. “Gestural communication in subadult bonobos (Pan paniscus): Repertoire and use.” American Journal of Primatology 65.1 (2005): 39-61.

Pika, S., K. Liebal, and M. Tomasello. ”Gestural communication in young gorillas (Gorilla gorilla): Gestural repertoire, learning and use.” American Journal of Primatology 60.3 (2003): 95-111.

Pollick, A.S., and F. de Waal. “Ape gestures and language evolution.” Proceedings of the National Academy of Sciences of the United States of America 104.19 (2007): 8,184-8,189.

Poss, S.R., C. Kuhar, T.S. Stoinski, and W.D. Hopkins. “Differential use of attentional and visual communicative signaling by orangutans (Pongo pygmaeus) and gorillas (Gorilla gorilla) in response to the attentional state of a human.” American Journal of Primatology 68.10 (2006): 978-992.

Preuschoft, S. “Primate Faces and Facial Expressions.” Social Research 67.1 (2000): 245-271.

Preuschoft, S., and J. van Hooff. “The social function of smile and laughter: Variations across primate species and societies.” In: U. Segersträle & P. Molná, eds. Nonverbal Communication: Where Nature Meets Culture. Mahwah, NY: Lawrence Erlbaum Asociates, 1997: 171-191.

Russell, J.A., J. Bachorowski, & J.M. Fernandez-Dols. “Facial and Vocal Expressions of Emotion.” Annual Review of Psychology 54 (2003): 329-349.

Russon, A., K. Bard, and S.T. Parker. Reaching into Thought: The Minds of the Great Apes. New York: Cambridge University Press, 1996.

Schmidt, K., and J. Cohn. “Human facial expressions as adaptations: Evolutionary questions in facial expression.” Yearbook of Physical Anthropology 44 (2002): 3-24. Tanner, J.E., F.G. Patterson, and R.W. Byrne. “Development of spontaneous gestures in zoo-living gorillas and sign-taught gorillas: From action and location to object representation.” Journal of Developmental Processes 1 (2006): 69-103.

Tanner, J.E. (2004). “Gestural phrases and gestural exchanges by a pair of zoo-living lowland gorillas.” Gesture 4 (2004): 25-42.

Tanner, J.E., and R.W. Byrne. “The development of spontaneous gestural communication in a group of zoo-living lowland gorillas.” In: S. Taylor Parker, R.W. Mitchell, and H.L. Miles, eds. The Mentalities of Gorillas and Orangutans: Comparative Perspectives. Cambridge: Cambridge University Press, 1999, 211-239.

Tanner, J.E., and R.W. Byrne. “Representation of action through iconic gesture in a captive lowland gorilla.” Current Anthropology 37 (1996): 162-173.

Tanner, J.E., and R.W. Byrne. “Concealing facial evidence of mood: Perspective-taking in a captive gorilla?” Primates 34 (1993): 451-457.

Tomasello, M., and J. Call. The Gestural Communication of Monkeys and Apes. Mahwah, NY: Lawrence Erlbaum Associates, 2007.

Van Hooff, J. (1967). “The facial displays of the catarrhine monkeys and apes.” In: D. Morris, ed. Primate Ethology. London: Weidenfeld and Nicolson, 1967.

Vick, S.J., B.M. Waller, L.A. Parr, M.C. Smith Pasqualini, and K.A. Bard. “A Cross-Species Comparison of Facial Morphology and Movement in Humans and Chimpanzees Using the Facial Action Coding System (FACS).” Journal of Nonverbal Behavior 31 (2007): 1-20.

Waller, B.M., K.A. Bard, S.J. Vick, and M.C. Smith Pasqualini. “Perceived differences between chimpanzee and human facial expressions are related to emotional interpretation.” Journal of Comparative Psychology 121.4 (2007): 398-404.

Appendix 1: Ethogram of Gestures

Auditory gestures

Clap: Subject’s palms of hands strike each other in front of body

Chest beat: Subject slaps repetitively on own chest with alternating knuckles or open hands

Slap object: Subject hits object or ground with palm of the hand

Window bang: Subject hits window with one or both hands one or more times

Tap: Subject taps an object one or more times with one or both hands

Tactile gestures

Arm on: Subject approaches recipient with an extended arm and places arm on recipient’s neck or back

Bite: Subject bites recipient on neck, back, leg, arm, or belly

Embrace: Subject wraps one or both arms and/or legs around the body of another animal

Grab: Subject grasps another animal with the whole hand; fingers are bent

Grab-push-pull: Subject grabs another animal and either pulls or pushes it during the same movement

Hand on: Subject places palm of hand on the body of another animal

Hit: Subject hits a body part of the other with hands, feet, or other body part

Jump: Subject jumps on or over another animal

Kick: Subject lifts one leg and hits receiver with it

Pick up: Subject grabs and holds up body part (leg, arm, hand etc.) of another animal

Prod: Subject pats lightly and repetitively upon the body of another animal with its palm

Pull down: Subject puts both hands around another animal and forcefully pulls down

Punch: Subject performs a forward or downward push on or against another animal with its knuckles or finger

Push: Subject use one or both hands or feet and pushes another away (full body or body part)

Push down: Subject pushes another down by the head or back (pushes down full body or body part)

Push up: Subject uses one of its body parts to push a body part of another up

Self-prod: Subject pats lightly and repetitively upon its own body with its palm

Touch: Subject makes gentle and brief (<5 sec) contact with another animal using a body part, flat hands, knuckles, or feet

Visual Gestures

Arm cross: Subject crosses both arms over its chest

Chuck up: Subjects throws one or both arms upward

Flail limbs: Subject flails arms and/or feet

Gallop: Subject runs in an exaggerated manner towards another animal

Head nod: Subject moves head vertically

Hide face: Subject covers face with one or both hands

Hit intention: Subject performs an arm movement as if to hit but without making physical contact

Ice Skating: Subject makes a pirouette with hands either on the ground or in the air

Appendix 2: Ethogram of Facial Expressions

Neutral: The neutral face consists of a relaxed upper face with the mouth closed and lips together.

Bared-Teeth Displays: The bared-teeth display consists of a relaxed upper face, the mouth opened with lips parted, a raised upper lip, and retracted lip corners. The teeth are either together or slightly parted. This expression was used by individuals of all ages.

Play Faces: The play face consists of a relaxed upper face and lip corners stretched with mouth wide open, allowing all teeth to be visible. This expression was seen almost solely in the social context of play and was mostly used by juvenile and subadult individuals.

Ambiguous Facial Displays: Ambiguous facial displays consist of funny faces and yawns. There is no specific description for this category, but common facial movements included mouth open, tongue protrusion, upper lip raised, lower lip depressed, lips pressed together, and lips pulled to one side.

Screams: Screams are described as a raised upper lip, with lip corners pulled back, exposing the upper teeth, lower lip depressed, exposing the lower teeth, and mouth stretched wide open with lips parted. This facial expression was only performed by one individual in the group, Shana.

Alert Faces: The alert face consists of a relaxed upper face, depressed lower lip, and lips slightly parted, with mouth opened.

Pouts: Pouts are described as a relaxed upper face, with lips funneled and parted. This expression was performed only by the younger individuals in the group (ages 2-5 years).

Whimpers: Whimpers consist of a relaxed upper face, with lip corners retracted and lips slightly parted. The whimper was only seen in two gorillas, Shana and Baracca.

Relaxed Open-Mouth Faces: The relaxed open-mouth face is described as a relaxed upper face, with lips slightly parted and mouth open.

Rolled-Lip Faces: The rolled-lip face consists of a relaxed upper face with either the upper or lower lip, or both, folded in over the teeth. This allows either the upper teeth or no teeth at all to be exposed.

Acknowledgements

First and foremost, I would like to thank my mentor, Dr. Joanne Tanner, for all of her assistance and input. I would also like to thank my research teachers, Nancy Flanagan and Jon Decker, for their help and encouragement during my research. My thanks are also due to my mother, Robyn Draiss, for her support and guidance during my study. Lastly, I would like to thank Marcus Perlman for his assistance in the statistical analysis, and Janet Pollizzi-Kulka for her assistance in proofreading this paper.