One great question and one that I hear from clients regularly is, “His hackles went up. What does that mean?” When the hair on a dog’s back goes up (technically called piloerection), it’s usually a sign that the dog is aroused, fearful, startled, stimulated or excited in some way. It is an involuntary reaction, just like the goose bumps we humans get, so it’s important not to have any expectation of a dog being able to control it. While sometimes aggressive dogs do exhibit piloerection, it is not true that it’s necessarily a sign of aggression. It's important to observe a dog's other body language including posture and ear position in addition to the surrounding environmental context to make the best determination at what is causing their hackles to raise.
Data are limited on the piloerection phenomenon, but as an ethologist trained to observe animals and their behavior, I have noticed some things about it. Based on my experience with many dogs over the years, it seems that different patterns of piloerection are associated with different dog behaviors, probably because they are associated with different internal emotional states. It's important to observe the dog's body language and the situation around them to make the best determination at what is causing their hackles to raise.
One example I have seen in the past is some dogs exhibit a thin line (at most a few inches wide) of hair all along their back to the base of the tail. I associate this pattern of piloerection with a high level of confidence and in my experience, these dogs are more likely to go on offense and behave in an aggressive way than other dogs.
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Another common pattern of raised hackles in dogs is a broad patch of fur (up to 8 or so inches wide) across the shoulders, which does not run more than one-quarter or one-third of the way down the back. I associate this pattern of piloerection with low confidence and I often find that these dogs are somewhat fearful.
I find the most confusing pattern is when a dog exhibits a patch of hair that is raised at the shoulders and another raised patch at the base of the tail. When the hair in between the two, along the back is not raised. This pattern of piloerection often occurs in dogs who are in an ambivalent emotional state and feeling conflicted. Many of the dogs who show this pattern are somewhat unpredictable in their behavior and inclined to be more reactive than other dogs.
Of course, there are many exceptions, but these generalizations apply to the majority of dogs that I see. What have you observed about dogs and piloerection?