Nonshivering thermogenesis is due to

Non-shivering thermogenesis occurs in brown adipose tissue (brown fat) that is present in all eutherians (swine being the only exception currently known). Brown adipose tissue has a unique uncoupling protein (thermogenin, also known as uncoupling protein 1) that allows the uncoupling of protons moving down their mitochondrial gradient from the synthesis of ATP, thus allowing the energy to be dissipated as heat. It is due to norepinephrine. Ref: guyton and hall textbook of medical physiology 12 edition page number:885,886,887