Final answer:
Shivering thermogenesis in birds and mammals is primarily associated with skeletal muscle tissue, not brown adipose tissue, which is involved in nonshivering thermogenesis. Shivering is an involuntary muscle contraction that produces heat, aiding in thermal regulation during cold stress.
Step-by-step explanation:
The shivering thermogenesis of many birds and mammals is associated primarily with skeletal muscle tissue. During periods of severe cold, a shivering reflex is elicited, which is an involuntary contraction of skeletal muscles. This process generates heat as a byproduct and is a crucial aspect of maintaining thermal homeostasis. While brown adipose tissue is involved in nonshivering thermogenesis, especially in newborns and animals adapted to cold environments, it is the shivering mechanism in skeletal muscles that produces heat during cold stress in birds and mammals.
Brown fat contains a large amount of mitochondria and has a unique role in nonshivering thermogenesis thanks to uncoupling proteins that allow the mitochondria to generate heat without producing ATP. However, the specific shivering thermogenesis response to cold is due to the action of skeletal muscles, not brown fat. Therefore, while both brown adipose tissue and skeletal muscle tissue play roles in thermoregulation, with brown fat more specialized for nonshivering thermogenesis, it is the skeletal muscle tissue that contributes to shivering thermogenesis.