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Gestural Communication by a Group of Western Lowland Gorillas

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Table 1: One group of gorillas at the Bronx Zoo as of September, 2007.


Introduction 
As I walk past the window, I hear a loud bang coming from my right. I turn and see a small gorilla pressed against the window, looking at me. I hear a few more bangs and see that three more gorillas have come to the window, eagerly looking to see who has come to visit them. I am standing face to face with some of the most magnificent and intelligent creatures in the world. In the corner, underneath the shade of a tree, lies a baby, held in a warm embrace by his mother. To my left I see another gorilla, who has just finished making what appears to be a nest, clapping. As I take out my video camera, all four gorillas continue to watch me, and a large male, the silverback, joins them. Dian Fossey once said, "I feel more comfortable with gorillas than with people. I can anticipate what a gorilla's going to do, and they're purely motivated." During the summer of 2007, I stepped into the Congo Gorilla Forest exhibit at the Bronx Zoo and soon came to realize that I feel the same way. Without another human being in sight, I knew that this was where I was meant to be.

Background 
It has been shown that western lowland gorillas ( Gorilla gorilla ) use gestures as a means of communication (Tanner 1998, 2004; Tanner and Byrne 1993, 1996; Pika 2007; and Pika, Liebal, and Tomasello 2003). Information is exchanged between communicative partners through auditory, olfactory, tactile, and visual signals. The way the receiver uses this information determines how the primate responds and therefore regulates the action that will follow (Bradbury and Vehrencamp 1998). When communicating through non-vocal gestures, the meaning of the information is constructed through action between social partners (King 2004). The way an action is received by one social partner determines the next action. If one gesture is received falsely, another gesture may be made in order to make the individual's desire or need more understandable. Although the majority of studies of nonhuman primates have focused on vocal communications, very few have concentrated on gestural communications.
The western lowland gorilla is the largest of all primates and is primarily terrestrial. Gorillas live in groups that contain up to 30 individuals and are usually dominated by a single adult male. These groups also contain several females with offspring, who typically remain together while moving and resting. Chest beating is seen in all gorillas and is used by the dominant male, the silverback, to scare away intruders, but is also used during play.

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Figure 1: Shana claps after placing a branch on his neck.


Purpose 

The first goal of the present study was to identify the full gestural repertoire of one group of western lowland gorillas at the Bronx Zoo. There have never been any studies of gestural communication done at the Bronx Zoo, and in order to have a full understanding of the gestural repertoire of a species, the gestures of that particular species must be analyzed. Once the gestural repertoire of this group is known, more inferences about species-typical gestures can be made, as well as changes in gesturing due to age. I hypothesized that younger gorillas would perform more gestures than older gorillas, and that tactile gestures would be performed more frequently than visual and auditory gestures. The second objective of this study was to compare the gestures in this group of gorillas to those seen in studies by J.E. Tanner (1998, 2004), J.E. Tanner and R.W. Byrne (1993, 1996), S. Pika (2007), and S. Pika, K. Liebal, and M. Tomasello (2003), as these are the only studies that have focused on the gestural repertoire of captive gorillas. I hypothesized that the environment affects the development of culture and therefore gestures other than species-typical gestures would be different in studies done at different locations.

Literature Review 
Tanner and Byrne's study (1996) focused on one male and one younger female in a group of captive gorillas at the San Francisco Zoo. Although about 30 different types of gestures were catalogued for this gorilla group, nine of the male gorilla's gestures were focused on. Three degrees of visual attention groups were found: a high-visual-attention group that consisted of silent head and limb gestures, a medium-visual-attention group of audible gestures, and a low-visual-attention group of tactile-close gestures alone. The effectiveness of play faces alone, gestures alone, and gestures with play faces in achieving contact with the female was also examined. The female usually backed away, stayed away, or resisted the approach by the male gorilla. Tanner and Byrne (1993) also observed one gorilla who hid her play face, and when this occurred, play did not usually follow immediately. The investigators stated that the gorilla's behavior implied that she was aware of her facial expressions and the outcome they result in. Tanner and Byrne (1996) found iconic uses of gesture performed by the adult male gorilla. These gestures consist of the gorilla depicting a path of motion, either on the body or in space, in the direction that he wanted the female recipient to move or the actions that he wanted her to perform. Pika, Liebal and Tomasello's (2003) study concentrated on 13 individuals in two different captive groups of gorillas. Thirty-three different gestures were recorded, and individual differences were found, as well as some gestures that were group-specific. This study helped to establish that gorillas have gestural repertoire consisting of many aspects. It is characterized by a great deal of flexibility and is adjusted to communicative circumstances, such as the attentional state of the recipient. The results of this study also provide evidence that the main learning process involved is ontogenetic ritualization, and that a form of social learning may also be accountable for the acquisition of special gestures.

Materials And Methods 
Definition of Gesture 
In this study, the term gesture is used for all expressive movements of the limbs or head that appeared to be communicative through auditory, tactile, visual, or olfactory signals.

Subjects and Setting 
Eleven western lowland gorillas from one troop were observed in the Bronx Zoo. Their ages varied from 1 to 27 years. Information about each of the gorillas is given in Table I. One silverback, Zuri, was present in the troop. The Bronx Zoo's outdoor gorilla enclosure, the Congo Gorilla Forest, is large and replicates the natural habitat of the western lowland gorilla. It contains natural, artificial, and deadfall trees, vines, a small pond, climbable rocks, and also includes one rock cave (the inside is blocked from view of visitors). At the time of my observations, rabbits and ducks were also present. The entire exhibit is enclosed by a windowed area for human viewing where the gorillas and human visitors are face to face.

Study Methods 

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Figure 2: Number of times gestures of each type were seen.


My observations for this study were made between June 2007 and September 2007 for approximately six hours a day over eight days, for a total of 47 hours. Videotaping was done continuously, recording all social interaction. The individuals that were 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 the cave blocked some individuals from view. All video was taken with a Sony Handycam DCR-SR82 camcorder. All recorded gestures, during all social activities, were catalogued in a database on Microsoft Office Excel.

Results 

A total of 44 distinct gestures were seen, and these gestures were performed a total of 1,001 times. Tactile gestures occurred the greatest number of times (72.8%), followed by visual gestures (13.6%). Auditory gestures occurred the least number of times (13.3% ).

Age differences 
The younger gorillas performed more gestures than the older gorillas, and the youngest gorillas used nearly all (90.9%) of the gestures in this group's repertoire. Shana, age 4, also performed nearly every single gesture in the group's repertoire. The gestures that he did not perform were body beat , hit intention , and prod (although he did perform self-prod ). Baraka and Johari, both age 1, performed all gestures except body throw , head nod , object push , and pick up . These gestures were only performed by one individual, Shana.

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Figure 3: Baraka reaches towards Imani.


Comparison 
A small number of gestures in the current study seem to be specific to this Bronx Zoo gorilla group, as they were not seen in the studies of Tanner (1998, 2004), Tanner and Byrne (1996), Pika (2007), and Pika, Liebal and Tomasello (2003). The gestures that were seen only in the current study are body throw and splash display . Gestures that were seen in all three studies are printed in bold on Table 3.

Discussion 
Gestures are used as a form of communication between individuals in one group of western lowland gorillas at the Bronx Zoo. These gestures included many that are species-typical, as they were seen in other studies, as well as gestures that may not be species-typical. Gestures were performed by every gorilla in this group, whose ages ranged from 1 to 27 years.
Many gestures seen in this study were also seen in the studies of Tanner (1998, 2004), Tanner and Byrne (1996), Pika (2007), and Pika, Liebal, and Tomasello (2003). These gestures may be considered species-typical, as they were seen in four different groups of gorillas in four different locations. Other gestures, such asarm cross and chuck up , may not be species-typical, as they were only seen in two or three different groups, and not all four. The only two gestures that were exclusive to the current study are body throw and splash display . Splash display was seen in the wild during the studies of R.J. Parnell and H.M. Buchanan-Smith (2001). In the wild and in the current captive study, this gesture was used solely by males. It is believed thatsplash display is used to intimidate potential rivals for female acquisition (Parnell and Buchanan-Smith 2001). In the current study, this gesture was displayed by younger males (ages 1 to 6), who appeared to want to frighten ducks and rabbits. The lack of a body of water is a probable cause of the gesture splash display not occurring in other captive studies.

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Figure 4: Number of times gestures were performed by each individual.


Although every individual performed gestures during the time of study, the younger gorillas performed gestures more often than older gorillas did. A cause of this may be that the amount of play seemed to decline as the age of the gorilla increased; the older gorillas did not interact with each other nearly as much as the younger gorillas did. Interestingly, by age 1, about 90.9% of the gestural repertoire was seen, and by age 4, 100% of the repertoire was seen. In the studies of Tanner and Byrne, the full gestural repertoire was not seen until age 6. This may also be because of the differences in the levels of interaction between the older gorillas.

Environment could be a factor in determining gestures that are not species-typical. As with humans, culture is greatly influenced by the environment in which a group of gorillas live. As culture begins and continues to develop, there are changes in styles of communication, and therefore the gestural repertoire changes. Each group of gorillas living in a different environment has a different culture, and their gestural repertoires are therefore different.

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Table 2: Percentage of individuals who performed each gesture.


Conclusion 

The first goal of this study was to identify the full gestural repertoire of one group of western lowland gorillas at the Bronx Zoo, as no study of gestural communication had ever been done on either gorilla group. The second goal of this study was to compare the gestures seen in this troop of gorillas to those seen in studies by Tanner (1998, 2004), Tanner and Byrne (1993, 1996), Pika (2007), and Pika, Liebal and Tomasello (2003), as these are the only studies that have focused on the gestural repertoire of captive gorillas. This group of gorillas has a repertoire of 44 distinct gestures, and these gestures were performed 1,001 times during the period of study. While many of the gestures performed seem to be species-specific, a few were not seen in every group of captive gorillas that has been studied, and two have not been seen in any other group. Most of the gestures performed by the individuals in this group were tactile gestures.
Age had a large impact on how often gestures were performed. The younger gorillas, up to age 6, performed many more gestures than the older gorillas. The youngest gorillas, Baraka and Johari, both age 1, performed nearly all of the gestures in this group's repertoire, and Shana, age 4, performed all but three gestures. In the studies of Pika (2007), and Pika, Liebal, and Tomasello (2003), the full gestural repertoire was seen at about age 2, but in the studies of Tanner and Byrne, the full repertoire was not seen until age 6.
In order to have a full understanding of the gestural communication of western lowland gorillas, more studies in different environments need to be done. It is impossible to know exactly how environment affects gestures, and how gestures differ between groups, until studies have been done on enough groups living in different environments.

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Table 3: Auditory, Tactile and Visual gestures seen in each study.


Appendix 1: Ethogram Of Gestures 

Auditory gestures 
Body beat Animal slaps repetitively on own body part except chest with knuckles or open hands
Body slam Animal forcefully throws whole body into an object
Clap Palms of hands strike each other in front of body
Chest beat Animal slaps repetitively on own chest with alternating knuckles or open hands
Slap ground Animal hits ground or rock with the palm of the hand
Slap object Animal hits object with palm of the hand
Splash display Subject stands near or goes into water and splashes
Window bang Subject hits window with one hand or both one or more times
Tap Subject taps an object one or more times with one or both hands

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Figure 5: Pierrepont embraces Baraka.


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
Bite hold Subject bites recipient while holding the part of the body that he is biting
Body throw Animal forcefully throws whole body into another animal
Embrace Animal wraps one or both arms and/or legs around the body of another animal
Grab 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 on Subject jumps on or over another animal
Long touch Gentle contact (>5 seconds) with body part, flat hands, knuckles, or feet
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 their palm
Pull Subject grasps a body part of another animal and pulls the animal forcefully
Pull down Subject puts both hands around another animal and forcefully pulls down
Punch Animal performs a forward or downward push on or against another animal with the knuckles or finger
Push Subject uses 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 body part to push a body part of another up
Self-prod Subject pats lightly and repetitively upon their own body with their palm
Touch Gentle and brief (<5 seconds) contact with body part, flat hands, knuckles, or feet

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Figure 6: Shana bangs on the window with both hands.


Visual Gestures 
Arm cross Subject crosses both arms over chest
Arm raise Subject puts one arm up in the air
Chuck up One or both arms are thrown upward
Gallop Run in an exaggerated manner towards another animal
Head nod Subject moves head vertically
Hit intention Subject performs an arm movement as if to hit but without making physical contact
Ice Skating Makes a pirouette with hands either on the ground or in the air
Peer Animal sits or stands very close and brings face very close to another animal
Reach Subject extends one or both arms or feet out towards another animal
Object push Subject pushes an object (not necessarily towards another)
Object shake Push an object rabidly back and forth or from side to side
Somersault Animal makes a flip
Stick tongue out Tongue is stuck out with an open mouth
Throw An object is tossed at or close to another animal

Acknowledgements 

First and foremost, I would like to thank the staff of the Bronx Zoo and the Wildlife Conservation Society for their extraordinary Congo Gorilla Forest exhibit. Second, I would like to thank Dr. Joanne Tanner for all of her assistance and input. My thanks are also due to Nancy Flanagan, my research teacher, Dr. Simone Pika, Dr. Norm Rosen, Mrs. Jill Simonson, and Mrs. Karen Suchy for their help and encouragement during my research. Last but not least, I would like to thank my parents, Robyn and Richard Draiss, for all of their incredible support and guidance.

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