Final answer:
The principal site for nonshivering thermogenesis in mammals is brown adipose tissue, found over the back, chest, and shoulders. It generates heat through a metabolic process involving the breakdown of lipids.
Step-by-step explanation:
The principal site for nonshivering thermogenesis in mammals is the brown adipose tissue, commonly known as brown fat. This specialized form of fat tissue is responsible for the process of breaking down lipids to produce heat in the absence of a shivering response, which is especially important in newborn mammals. Brown fat is rich in mitochondria and is typically found in regions over the back, chest, and shoulders in mammals. Upon exposure to cold, the brain signals for the breakdown of this brown fat through hormonal actions involving thyroid hormone and epinephrine, leading to increased metabolic activity and heat production without muscle contraction.