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Those are some interesting observations which make sense. I do not know of any specific studies but I can speak of my own observations dealing with a career worth of dogs (probably thousands) interacting in puppy classes, as adults with each other, and with people:
The majority of the time, it simply is a reflex of feeling threatened. But, it can confuse people sometimes because of mixed messages. I will see plenty of puppies play in a group while hackles are up when they first show up before it slowly smooths out. Their drive to play is obviously overriding the sense of feeling slightly threatened by all the other dogs/people they don’t quite feel ready to completely put their guard down with.
I’ll also see it in early protection training, where we work the dog with an obvious threat. As the dog gets more confident we cant make the dog’s hackles go up.
So we see it more at a younger age in newer situations for sure.
The other time I see piloerection is more concentrated near the back end and with intense focus when about to pounce into a fight or focusing on live prey. I have never witnessed the piloerection during play aggression such as in sport work and tugs so i assume it part of a very high mature type of serious aggression that is balancing on the edge of fight and prey drive:
My hypothesis is that even with certain prey, the dog is anticipation some slight combat before calming down into a more comfortable and confident prey state.
Dogs learn from experience some prey bite back and kick back. The same goes with very dog aggressive dogs that see other dogs as prey.
