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It goes back a long time...if you look at certain members of the
animal kingdom, they will often flash their teeth or growl to establish
their identity, to show equality with someone else. You have to step
outside a human perspective and ask how an alien would show respect,
which may not be how we would logically do so. (Heck, in many cultures
here on earth, a perfectly normal habit -- crossing your legs and the
bottom of your foot showing -- is taken as a grave insult in some
cultures. Certain customs are deeply rooted in cultural backgrounds or
incidents that are often incomprehensible to us.)

jms