The dog or cat with his hair standing on end is likely showing fear

Piloerection, or the hair standing on end at the shoulders or from the shoulders down the back is a sign of arousal or excitement associated most often with fear.  In addition to the bristling hair, the dog's pupils are likely dilated, or open wide. 

Although the dog looks aggressive, he is fearful and this is a sign to back off.  The more hair that is standing on end, the more nervous or fearful.  Piloerection is more visible on a dog with short, stiff hair.  A slight raising of the shoulder hairs can progress to hairs raised up and down the back and even down the tail.  Some severely frightened dogs resemble cats with all of their hairs bristling and tails that look like bottle brushes. 

Piloerection is a function of the nervous system and is an involuntary reaction to a stimulus such as fear or cold.  Adrenalin triggers contraction of the arrectores pilorum muscles in the skin.  They contract and push up the hair follicles causing the hair to stand away from the body. 

The term comes from the Latin pilus meaning hair and erection meaning elevated.  Similar displays occur in other animals.  The Halloween cat with his back arched and his hair bristled is one example.  The idea of a bird or a human having their 'feathers ruffled' is another. 

In addition to being an involuntary reaction, piloerection has the added advantage of making the dog look bigger.  This may cause the source of his fear: another dog, another animal or a person, to back away allowing him to relax. 

When you see this in your dog, you can calm him down so that his fear does not escalate into a snarling lunge, a bite or even a fight.  If you see this in other dogs, it is a sign to look away, quietly and slowly back off.
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