We’ve seen people angrily tell their dogs to put down their hair when it’s standing on end because they feel their pet is being unnecessarily aggressive. Don’t. A dog can no better make his hair go down than we can decide not to have goose bumps. In fact, the two involuntary bodily reactions are pretty similar.
A dog’s hairs have little muscles attached to them called piloerectile muscles. When he feels he is in a fight-or-flight situation, his sympathetic nervous system will automatically release epinephrine. That makes muscles contract, or tighten, which is what raises the hairs. Nature may have programmed dogs to raise their hackles (erectile hairs) when faced with danger in order to make them look bigger and fiercer. But it’s not a decision our pets make, so getting stern with them for it only increases their anxiety.
A dog’s hair will also stand on end when he is very cold, by the way. The sympathetic nervous system kicks in for that, too — this time, to help him burn fuel faster, like more logs on the fire. But there’s another up side. When the hair stands up straight, an insulating layer of air gets trapped between hair shafts, so cold air cannot get as close to the skin. It’s like nature’s down jacket.