“If you want a friend in Washington, get yourself a dog.” Few quotes pack so much emotion into a dozen words. But why a dog, and not a cat or a canary? Why, because a dog is the ultimate symbol of selfless, unquestioning devotion. What more heartwarming sight than the furiously wagging tail when we return home after a bruising day?

So how did this come about? All vertebrates have a tail, even us, though we discarded most of it when we came down from the trees; but few are tail waggers. Then virtually all carnivores have rectal scent glands by which to mark territory or establish identity, but we have no way to detect it, excepting only the skunk, whom we don’t find attractive. It seems that only the wolf, from whom the dog is descended, was smart enough to find out that the glands could be stimulated and the scent more widely distributed by vigorous tail wagging to demonstrate solidarity, goodwill and a sincere desire to be accepted in the pack.