“Hackles” refer to the hair along an animal’s neck and back that stands erect, typically in response to a threat or cold. This involuntary bristling of hair is a survival reflex seen across the mammalian kingdom. Humans lack a dense coat, but we share the exact biological mechanism responsible for this reaction, which is better known to us as goosebumps.
The Function of Hackles in Animals
In mammals with thick fur, the raising of hackles serves two primary, interrelated functions: thermoregulation and threat display. When an animal is exposed to cold, the underlying muscles contract to fluff up the coat. This action traps a layer of still air close to the skin, which provides an insulating barrier to help the animal conserve body heat. Beyond keeping warm, the raising of hair is a visual signal used in self-defense and social communication. By causing the fur to stand on end, the animal instantly appears larger and more intimidating to a rival or potential predator, part of the involuntary fight-or-flight response.
Piloerection: The Human Equivalent
Humans possess the same mechanical structure that produces the hackle reflex, a phenomenon scientifically termed piloerection. This reaction is caused by the contraction of a minute, involuntary smooth muscle attached to the base of every hair follicle. This muscle is called the Arrector Pili muscle, and when it contracts, it pulls the hair follicle upright, which creates the small mound or bump on the skin’s surface. The Arrector Pili muscle is innervated by the sympathetic division of the autonomic nervous system, meaning its contraction is entirely outside of conscious control. Although the muscle still functions, the human body’s drastically reduced hair density makes the thermal and defensive effects negligible. Our sparse body hair cannot trap enough air for meaningful insulation, nor does it create a significant “fluffing” effect for a threat display. The visible goosebump is simply the physical evidence of the muscle contracting beneath the skin.
Current Triggers for Goosebumps
In modern humans, piloerection is still triggered by the ancient physiological response to cold, but also by a variety of psychological stimuli. A sudden drop in skin temperature causes the sympathetic nervous system to initiate the reflex to try and conserve heat. Strong emotional responses also activate the same involuntary pathway. Fear, excitement, and awe can all trigger goosebumps, essentially hijacking the body’s original threat response. A common example is “frisson,” or aesthetic chills, experienced when listening to powerful or unexpected moments in music, or watching a moving scene in a movie. These emotional triggers cause a surge of sympathetic arousal, which is the same nervous system response that would cause an animal’s hackles to rise in a confrontation.
The Vestigial Nature of the Reaction
The persistence of the piloerection reflex in humans, despite its lack of practical function, classifies it as a vestigial trait. This means it is an evolutionary remnant that has lost its original purpose in our species. Early hominids and our mammalian ancestors had much denser coats, and for them, the reflex was highly adaptive for both warmth and defense. As humans evolved and lost most of their body hair, the Arrector Pili muscle became functionally useless for insulation or intimidation. Because the reaction is harmless and the underlying mechanism is deeply embedded in the sympathetic nervous system, there was no evolutionary pressure to eliminate the trait completely. Goosebumps today serve as a biological echo, connecting us to our furred, fear-responding mammalian past.