What is the name of the layer of soft fur closest to an animal's skin that provides insulation?

The layer of soft fur closest to an animal's skin that provides insulation is referred to as underhair. Underhair serves a crucial role in keeping animals warm by trapping air close to the body, which helps to insulate against cold temperatures. This layer is typically finer and denser than guard hair, which is the longer and coarser outer layer that protects against environmental elements such as moisture and dirt.

In various mammals, underhair is especially prominent in species that live in colder climates, as it is essential for thermoregulation. Guard hair, while important for protection, does not provide the same level of insulation as underhair. Similarly, terms like outer fur and down fur may be used in different contexts but do not specifically identify the insulating layer closest to the skin in mammals. This context reinforces the importance of underhair in maintaining body temperature and overall warmth for the animal.