The UCM Mammal collection includes over 18,000 specimens with representation from 37 countries, with the majority of specimens collected from Colorado and other western states. Specimens are typically preserved as a set of a study skin and an extracted skull, although large mammals are also prepared as hides, taxidermy, and skeletons. Many people have added to the mammal collection over the years, including the pioneer Colorado naturalist, Edward Royal Warren (3,000+ specimens), Donald A. Spencer, Dallas A. Sutton, Leslie Viereck, and emeritus professor and former Museum Director, David M. Armstrong. E.R. Warren's archive of field notes and photographs are an invaluable resource for documenting distributions and changes of Colorado mammals in the first three decades of the 20th century.