Why does a cat wag its tail?

Most people imagine that if a cat wags its tail it must be angry, but this is only a partial truth. The real answer is that the cat is in a state of conflict. It wants to do two things at once, but each impulse blocks the other. For example, if a cat cries to be let out at night and the door is opened to reveal a downpour of heavy rain, the animal's tail may start to wag. If it rushes out into the night and stands there defiantly for a moment, getting drenched, its tail wags even more furiously. Then it makes a decision and either rushes back in to the comforting shelter of the house, or bravely sets off to patrol its territory, despite the weather conditions. As soon as it has resolved its conflict, one way or the other, its tail immediately stops wagging.

In such a case it is inappropriate to describe the mood as one of anger.

Anger implies a frustrated urge to attack, but the cat in the rainstorm is not aggressive. What is being frustrated there is the urge to explore, which in turn is frustrating the powerful feline desire to keep snug and dry. When the two urges momentarily balance one another, the cat can obey neither. Pulled in two different directions at once, it stands still and wags its tail. Any two opposing urges would produce the same reaction, and only when one of these was the urge to attack frustrated by fear or some other competing mood – could we say that the cat was wagging its tail because it was angry.

If tail-wagging in cats represents a state of acute conflict, how did such a movement originate? To understand this, watch a cat trying to balance on a narrow ledge. If it feels itself tipping over, its tail quickly swings sideways, acting as a balancing organ. If you hold a cat on your lap and then tip it slightly to the left and then to the right, alternating these movements, you can see its tail swinging rhythmically from side to side as if it is wagging it in slow motion.

This is how the tail-wagging movement used in mood-conflicts began. As the two competing urges pulled the cat in opposite directions, the tail responded as if the animal's body were being tipped over first one way and then the other. During evolution this lashing of the tail from side to side became a useful signal in the body language of cats and was greatly speeded up in a way that made it more conspicuous and instantly recognizable. Today it is so much faster and more rhythmic than any ordinary balancing movement of the tail that it is easy to tell at a glance that the connflict the animal is experiencing is emotional rather than purely physical.