Final answer:
Fur and feathers truly reduce the temperature gradient between the skin and the surrounding medium by trapping air in small pockets, which insulates and limits heat transfer due to air's low thermal conductivity.
Step-by-step explanation:
The statement that fur and feathers tend to reduce the overall temperature gradient between skin and the bulk phase of the surrounding medium is true. Fur and feathers function as insulators, which means they limit the exchange of heat between the body of an animal and its environment. This insulation effect is achieved through the trapping of air in small pockets within the fur or feathers. Air has a low thermal conductivity, making it an excellent insulator. As convection within these pockets is limited due to their small size, the overall rate of heat transfer is reduced. This helps to maintain the animal's body temperature by minimizing heat loss in cold conditions and heat gain in hot conditions.