35. Keeps the hair on his back standing on end. Called piloerection, this is sort of like goose bumps. It’s not something a dog can control. Consider that a dog’s hairs have little muscles attached to them called the piloerectile muscles. When his sympathetic nervous system, involved in fight or flight reactions, releases epinephrine, those muscles contract, in turn raising the hairs. It is assumed that nature programmed dogs to raise their hackles when faced with danger in order to make them look bigger and fiercer. A dog’s hair will also stand on end when he is very, very cold. Again, the sympathetic nervous system kicks in, this time to help the dog burn fuel faster, but the muscle-contracting action in the hair takes place, too. If the hair stands up, an insulating layer of air gets trapped between hair shafts, so the cold air cannot get so close to the skin. It works like a down jacket.

Excerpted from Puppy’s First Steps: The Whole-Dog Approach to Raising a Happy, Healthy, Well-Behaved Puppy, Edited by Nicholas Dodman with Lawrence Lindner, and the Faculty of the Cummings School of Veterinary Medicine at Tufts University; Copyright © 2007. Published by Houghton Mifflin Company, used with permission.