Just as an animal’s fur stands on end when cold, hair stands on end as a response to threatening noises or sights.
This bristling is thought to occur to make the animal look larger, and therefore more formidable, to an opponent.
Bristling fur as a response to cold keeps the animal warmer by increasing the amount of air between hairs, which traps body heat.
The reflex to threatening stimuli, which humans may have also retained from their furrier days, is part of a more complicated fight or flight reaction.
As the body prepares to respond to a physical threat, among other things, vision sharpens and becomes more focused, the heart rate increases and blood rushes to the muscles to provide them with additional oxygen.
As a result, blood rushes from the capillaries in the skin.
Reduced blood flow in the skin results in less internal body heat being transmitted to the body’s surface, hence a feeling of coldness or chill.
A disquieting sound, such as chalk screeching across a blackboard, can provoke this reflex, if only for a moment.