The winter fur is generally brighter in colour, due to the more prominent underfur.
From the en.wikipedia.org
Their pelage is sparse and coarse with poorly developed or absent underfur.
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In typical erythristic individuals, the underfur is usually bright reddish.
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To survive the harsh Arctic winters it grows a layer of fine underfur.
From the newscientist.com
The skins and underfur of rodent prey are used to line the nest chamber.
From the en.wikipedia.org
The fur is long and dense and consists of thin, short, woolly underfur and long, thick overfur.
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They are marked by their dense underfur, which made them a long-time object of commercial hunting.
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It protects the underfur from outside factors, such as rain, and is thus often water-repellent.
From the en.wikipedia.org
They have a very soft, insulated underfur which is protected by an outer layer of long guard hair.
From the en.wikipedia.org
More examples
Undercoat: thick soft fur lying beneath the longer and coarser guard hair
Fur is a synonym for hair, used more in reference to non-human animals, usually mammals; particularly those with extensive body hair coverage. The term is sometimes used to refer to the body hair of an animal as a complete coat, also known as the "pelage". ...
The thick, soft undercoat of some mammals, especially those that spend time in the water
Pale, short fine hairs in the coat that help keep the animal warm. The underfur has an oily substance that makes them water-resistant.
The short hairs close to a polar bear's skin; the fur with the purpose of keeping a polar bear's body heat next to its skin
The underhair covered by guard hairs. This is the layer of fur that acts as insulation to provide warmth.