Final answer:
Small mammals increase heat production mainly by burning brown fat, which is a specialized tissue for generating heat through non-shivering thermogenesis, rather than growing denser fur, panting, or sweating.
Step-by-step explanation:
Many species of small mammals, especially those that hibernate, increase heat production primarily by burning highly vascular brown fat. This type of adipose tissue is specialized for generating heat through a process called non-shivering thermogenesis. Unlike methods such as growing denser fur, panting, or sweating—none of which are primarily involved in heat production during hibernation—brown fat utilizes metabolic waste heat from muscle contractions.
This results in a release of energy in the form of heat to maintain body temperature in cold environments. Furthermore, small mammals have adaptations like dense fur and a layer of subcutaneous fat that acts as insulation to conserve heat. However, to directly generate heat, especially during hibernation, brown fat is the primary medium.