My 3 aren't so into the tail wagging thing either, Rachel. Although they are exuberant when we come home, Boo's about the only one who'll wag his tail at that time. But that's not to say they don't use them to express their feelings. They'll stick them straight up in the air when they are excited. Dropped down when sad or worried. Half-mast when meeting someone or something new and not too sure. Gracie Ruth does a slow lazy swish when she's content. My kids are all very vocal, so perhaps they don't think to use 'the other end' to express themselves.
