Piloerection Or Hair Standing On End 

This most often shows fear and is a sign to back away

MetPet.com staff
Piloerection, or the hair standing on end is often seen through the shoulders and down the back of animals.  It is a sign of excitement or arousal and is most often associated with fear.  In addition to the bristling hair, the eyes are usually open wide. 

In dogs, it looks aggressive but it is best to treat this as a sign of fear. The more hair that is standing on end, the more nervous or fearful the dog is.  Piloerection is more visible on dogs with short, stiff hair. Some truly frightened dogs can look like brushes.


Involuntary reaction to fear

Piloerection is a function of the nervous system and is an involuntary reaction to a stimulus such as fear or even cold weather. 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 terms comes from the Latin pilus meaning hair and erection meaning elevated.  Similar displays occur in other animals including cats. The traditional look of a Halloween cat with arched back, wide open eyes and hair that bristles from head to tail is one example.  The idea of a bird or a human having ruffled feathers is another example. 


Walk away to be safe

In addition to being an involuntary reaction, piloerection has the added advantage of making the animal look bigger.  This may cause the source of the fear, another animal or a human, to back away.  When you see this in your dog or cat, you can calm him down so that fear does not escalate into snarling, lunging, biting or fighting.  If you see this in an animal you do not know, break off eye contact and slowly back away.