Why does a cat arch its back when it sees a strange dog?

If a cat feels threatened by a large dog, it pulls itself up on fully stretched legs and at the same time arches its back in the shape of an inverted U. The function of this display is clearly to make the cat look as big as possible, in an attempt to convince the dog that it is confronting a daunting opponent. To understand the origin of the display it is necessary to look at what happens when cats are threatening one another. If one cat is intensely hostile towards another and feels little fear, it approaches on stiffly stretched legs and with a straight back. If its rival is extremely frightened and feels no hostility, it arches its back and crouches low on the ground.

In the case of the cat approached by a dog, there is both intense aggression and intense fear.

It is this conflicting, double mood that gives rise to the special display. The cat borrows the most conspicuous element of its anger reaction – the stiff legs – and the most conspicuous element of its fear reaction – the arched back – and combines them to produce an 'enlarged cat' display. If it had borrowed the other elements – the straight back of anger and the low crouch of fear – the result would have been far from impressive.

Aiding its 'transformation display' is the fact that the animal, while stretching its legs and arching its back, also erects its fur and stands broadside-on to the dog. Together these four elements make up a compound display of maximum size increase. Even if the cat retreats a little, or advances towards the dog, it carefully keeps its broadside-on position, spreading its body in front of the dog like a bullfighter's cloak.

During the arched-back display the cat hisses ominously, like a snake, but this hissing turns to growling if it risks an attack. Then, when it actually lashes out at the dog, it adds an explosive 'spit' to its display. Experienced cats soon learn that the best policy when faced with a hostile dog is to go into the attack rather than run away, but it takes some nerve to do this when the dog is several times the cat's weight. The alternative of 'running for it' is much riskier, however, because once the cat is fleeing it triggers off the dog's hunting urges.

To a dog a 'fleeing object' means only one thing – food – and it is hard to shift the canine hunting mood once it has been aroused. Even if the fleeing cat halts and makes a brave stand, it has little hope, because the dog's blood is up and it goes straight for the kill, arched back or no arched back. But if the cat makes a stand right from the first moment of the encounter with the dog, it has a good chance of defeating the larger animal, simply because by attacking it, the cat gives off none of the usual 'prey signals'. The dog, with sharp claws slashing at its sensitive nose, is much more likely then to beat a dignified retreat, and leave the hissing fury to its own devices. So, where dogs are concerned, the bolder the cat, the safer it is.