Shivering

Shivering (also called shuddering) is a bodily function in response to cold and extreme fear in warm-blooded animals.

When the core body temperature drops, the shivering reflex is triggered to maintain homeostasis.

Skeletal muscles begin to shake in small movements, creating warmth by expending energy.

This is a primary trigger for the mechanism of heat production in mammalian bodies when exposed to cold.

[1] Located in the posterior hypothalamus near the wall of the third ventricle is an area called the primary motor center for shivering.

[3][4] The functional capacity of the thermoregulatory system alters with aging, reducing the resistance of elderly people to extreme external temperatures.

The shiver response may be greatly diminished or even absent in the elderly, resulting in a significant drop in mean deep body temperature upon exposure to cold.