We talked about body language and gestures. It's important at the start to recognize the difference between habitual gestures, and iconic gestures - iconic gestures having a particular and specific assigned meaning, similar to what you get in sign language, which is fully iconic.
There are a lot of cross-cultural differences in both gestures and sign language. Greetings differ widely across the world. Do people bow? Do they shake hands? Often you will find that a gesture like shaking hands has a long history. Shaking hands was a way to indicate that one's weapon hand was empty. Bowing is something that we see even across species, and it may have to do with the instinctive link between height and status - as Brian said, "how much you can oversee." In this day and age, it can be very finely tuned to social meaning. In Japan (at least, and possibly in other countries as well) you tune your bow to be lower than the bow of a person whose status is higher than yours (someone older, or a year ahead of you in school, your boss, etc.).
Gestures work in concert with auditory language, and are particularly useful over long distances, which is why we see things like semaphore.
Obscene gestures are a type of iconic gesture that always comes up! However, these vary across cultures as well. A gesture like "thumbs up" which is innocuous to Americans may not be so innocuous to Greeks, for example. Then there is also the kind of hand sign that can be used in a friendly way in one community, yet be interpreted as a gang sign by another community (due largely to racism in this case). There is an example of cross-species misinterpretation of body language in one of Piers Anthony's Xanth books, where the protagonist is communicating with a spider through a piece of talking spiderweb on his shoulder, and the spider interprets his smile as aggressive, and raising his fists as friendly.
Thumbs up, hook 'em horns, read between the lines, middle finger, finger dragged across the throat, bite the thumb, thumb from chin or nose...
Body language is connected to emotion, but it can be ambiguous.
Reggie mentioned how feather-fluffing in parrots can either indicate comfort or alarm (pretending to be big!), and it's just a matter of degree to tell the difference. Cats puff up, show their bodies side-on to look bigger, and hop with their front legs splayed out when they are alarmed. By contrast, the slow closing of eyes can be a sign of affection. In both dogs and cats, direct eye contact is interpreted as aggressive, and this can be the case for humans also.
I mentioned how I researched canine gestures of various sorts (puppy hunger gestures, play gestures, surprise gestures, etc.) for use with my canine-inspired protagonist, Rulii, in the story "Cold Words." [Note: this story is going to be reprinted soon in Forever Magazine, editor Neil Clarke. So look for it!]
Morgan said that it would be interesting to have a creature that looked like one kind of animal but behaved like another.
If you were to look at shapeshifters, would their gestural language follow their form, or would they be stuck in between human and animal communications systems? Would some of their gestures be carried across from one form to another?
Inside jokes, gestures, and secret handshakes are another good example of gestural communication. Kicking someone under the table, finger against the nose.
"Nudge nudge wink wink" is an odd one because people have stopped actually doing it in favor of just saying the words for it.
Air quotes are interesting in that they function to change the meaning of words that are being said.
Sign language has regional variation. It also has slang. Across the world, different countries typically have distinctly different sign languages. I loved this video of Swedish sign:
Dogs and wolves do not use quite the same gestures for communication.
Che had the idea of creating intelligent velociraptors, and imagining gestural communications for them based on toe claw position, etc. How would reptile gestural/expressive communication differ given that they have so many fewer facial muscles? How would cephalopods' incredible muscularity and flexibility influence a form of gestural communication?
Thanks to everyone who came and participated! I'm sorry that I'm unable to hold a hangout this week (4/22) but I will keep in touch about next week. Our next author guest will be Malon Edwards, who will join us toward the beginning of May (date still to be confirmed).
Here's the video: