In addition to barking, dogs also use their bodies to communicate about social and emotional matters. A dog’s tail, eyes, ears, and mouth all speak to us, and whole body postures add further information, serving to modify the message given.
Tail wagging can come about simply as a sign of excitement, the degree of which is indicated by the vigor or speed of the wag. In judging excitement, you should attend to the speed of wagging independent of the size of the actual movement. A sporting dog with a full,
flowing tail might seem to move it much more than a terrier moves its carrot-shaped tail (where a furious wag may seem like nothing more than a tremor). Yet in both cases, high-speed movements simply mean “I’m excited.” The relative size of any single dog’s tail wag does convey other information, however.
Tail wagging is a completely social gesture. In some ways, it serves the same functions as a human smile. Humans seem to reserve most of their smiles for when somebody is around to see them or when they are thinking about somebody or something special. For dogs, the tail wag seems to have the same properties. A dog will wag its tail for a person or another dog. It may wag its tail for a cat, a horse, or a mouse. But when a dog is by itself, it will not wag its tail to anything it perceives as lifeless.
A dog will wag its tail to express its gratitude to you as you put its food bowl down, but should the dog walk into the room and find the bowl full, it will approach and eat the food just as happily, but with no tail wagging other than, perhaps, a slight excitement tremor. This is one indication that tail wagging is meant as communication or language. In the same way we don’t talk to walls, dogs do not wag their tails to things that are not apparently alive and socially responsive.
For most breeds of dog, the tail will tend to lighten toward the tip, and on many breeds there is a characteristic white tip to the tail. It is also quite visible in jackals, foxes, wild dogs, and dingoes. Some evolutionary biologists have suggested that the purpose of this light area is to make tail signals more visible. For some wolves, the tail is marked with a dark tip, which, of course, can serve much the same function of making it easier to see the tail position and motion.