When you think of an animal's fur standing on end, a cat may first spring to mind, although the action certainly isn't exclusive to the feline species. Dogs occasionally use their coats as a means to express emotion, as well. For the most part, spiked fur isn't a positive thing.
If a dog's hair is pointing straight up to the sky the whole way down his back, it may be because he's feeling a mixture of excitement and fear, with an emphasis on the latter. Something may have piqued his interest -- or anxiety -- and he's totally alert as a result of it. The doggie may be totally aware of his surroundings, and nothing will likely get by him when he's in this heightened state. According to the ASPCA, this communication method is known as "piloerection." Casually, it also is called "raising the hackles."
In some cases, a dog's hair standing up can point to the desire to instill intimidation in another. A dog may be attempting to appear imposing when his hair stands straight up. He may be preparing to attack physically, so watch out. Other key body language and vocalization cues that may correspond with the standing hair are growling, loud barking, rigid posture, puckered lips and extended staring -- yikes.
Similarly, standing fur -- especially over the spine and in the rear region -- may also signify a dog who is feeling defensive and frightened. The doggie may be in protective mode, so look out for other telling defensive body language hints, such as hunched posture, tail curled below, exposed teeth and ears pushed back.
With piloerection, you may notice standing hair all over a dog's body, including the back, tail, rear area, shoulders and neck. However, in some cases the specific location of the bristling hair may be especially indicative of the doggie's emotional state. The Caring Hands Humane Society notes that standing hair on a canine's shoulder region is often a sign of aggression mixed with abject terror. In these cases, a dog may dread the notion of fighting, but feel like he has absolutely no choice but to do so.
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