In Anderson's Fire Time (London, 1977), about his "Ishtarians":
"...legionaries and porters lay thoughtful, only switching their tails in the male sign for 'Thank you.'" (p. 152)
It makes sense that tail movements would be adapted as signs. Each Ishtarian has not only a tail but also a face and six limbs so their bodies can be very expressive and we are in fact informed that their art, including dance, is more subtle and complex than ours.
As quadrupeds, they effortlessly trot or run and can thus spread their activities over a far wider geographical area than human beings. Having imagined a differently shaped body, Anderson carefully works out all the consequent psychological and social differences.