This is a very intriguing and entertaining video uploaded to YouTube two days ago. In it we can see a man at the Louisville Zoo showing pictures of other gorillas to a live gorilla (Jelani). It would have been better, of course, to have seen the initial encounter between man and ape and how it progressed. And although this is a cross-species interaction, we can certainly glean some valuable nonverbal lessons very applicable in human-to-human communication.
Initially (at least with respect to this edited video) Jelani keeps his body oriented roughly perpendicular to the man showing him the pics.
If we had been there and seen this encounter a few moments earlier, their faces, torsos, hips and probably even their feet - would have most likely been more parallel (pointing more directly at each other - roughly mirror images).
As the Jelani's emotional comfort increases, he orients his body in a more relaxed manner - getting closer (further into the man's personals space [intimate space] and oriented perpendicular to the man).
As his emotional comfort increase still further, Jelani leans his torso as well as his head-neck unit towards the man. The man of course is doing the same thing - they are mirroring each other. Again, who initiated this? - It would be valuable to know. With respect to human behavior, this touching of the heads and shoulders (of course this is not quite occurring here because of the thick glass) during similar moments will amplify any rapport already established (thus it's one example of what is known as a "Rapport Amplifier").
It's of profound importance that at least some modestly significant level of rapport must have existed prior to this moment - otherwise if one person attempts such nonverbal behavior it will be interpreted as a form of aggression or at the very least "false intimacy" (asymmetrical intimacy) by most everyone (and certainly animals).
Head-neck tilting (particularly toward each other during a shared experience) indicates a level of friendly, emotional-vulnerability (e.g., "I'm don't feel threatened by you.", "I trust you", "I feel comfortable with you", "I'm letting you into my world."). And while the thick glass certainly facilitates feelings of safety and protection for both primates, such head-neck tilting towards another never occurs when a person/animal feels threatened.
If you watch Jelani's left hand/arm, he appears to gesture several times. Some people have interpreted this movement (including a woman heard describing this to another onlooker) as a signal for the man to advance to the next photo. This may indeed be true, however we would need to see a longer duration sample (with a better view) as well as other examples of Jelani's behavior to be sure.
See also:
Nonverbal Communication Analysis No. 3302: Hillary Clinton, the Recent MSNBC Interview, Low Likability and Diminished Trustworthiness
Nonverbal Communication Analysis No. 3159: "When a Silverback attacks" - Gorillas, Cross-species Body Language, False Distancing, Fear and Surprise
Nonverbal Communication Analysis No. 3245: Gov. Scott Walker Jumps into the 2016 Presidential Race - Body Language Amplifiers and Misdirection
Nonverbal Communication Analysis No. 2341: Angela Bassett Interview - Rationalization Rapport Empathy Expression R2E2, Body Language & "Olympus has Fallen"
Nonverbal Communication Analysis No. 2016: Hillary Clinton, Roosevelt Island, Building Rapport and Body Language
Nonverbal Communication Analysis No. 2508: Anne Frank, Sincere Smiles and Emotional Comfort
Nonverbal Communication Analysis No. 2276: Michelle Obama's Eye Roll .... What was she thinking? and What Did Boehner's & Barack's Body Language Say?
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