Noun
underfur (usually uncountable, plural underfurs)
The thick, soft undercoat of some mammals, especially those that spend time in the water
Arctocephalus is characterized by ancestral character states such as dense underfur and the presence of double rooted cheek teeth and is thus thought to represent the most “primitive” line. Source: Internet
Old World porcupines (Hystricidae) have quills embedded in clusters, whereas in New World porcupines (Erethizontidae), single quills are interspersed with bristles, underfur, and hair. Source: Internet
Their pelage is sparse and coarse with poorly developed or absent underfur. Source: Internet