Eighteen species of bats are known to occur in Colorado, representing two families and 10 genera. Sixteen of these are in the cosmopolitan family Vespertilionidae -- the so-called common bats, which (with over 300 species in more than 30 genera) is the largest family of bats in the world. Nearly all members of this family are insect-eaters, and most are cave-dwellers.
The other family of bats in Colorado, the Molossidae, has 12 genera and over 80 species worldwide, generally in tropical and subtropical regions. The tails of these bats extend beyond the trailing edge of the uropatagium, hence the name free-tailed bats. Members of the family have short, velvety hair. Some free-tailed bats congregate in groups of millions, probably the largest gatherings of any mammal except for a few human cities.
In addition to the species described beyond, we expect that at least two other vespertilionid bats may eventually be found in Colorado. The cave myotis (Myotis velifer) is known from about 10 miles south of the Colorado border in the Oklahoma Panhandle and may well occur on Colorado's Mesa de Maya. It differs from other species of Myotis (the so-called mouse-eared bats') by its short, rather coarse dorsal hair and its large size (its forearm is greater than 44 mm.).
Allen's big-eared bat (Idionycteris phyllotis) is known from southeastern Utah within about 30 miles of Colorado, in pi-on-juniper habitat that is generally continuous between the two states. The species is similar in size and general form to Townsend's big-eared bat but has two prominent flat lappets on the face.
Accounts of species of Colorado bats follow. Families and genera are in phylogenetic order, and species are treated alphabetically within genera. Maps of Colorado show records of occurrence of each species. These are from a technical publication by Armstrong, Adams and Freeman, Distribution and Ecology of Bats of Colorado (1994), by permission of the University of Colorado Museum. Red dots indicate actual specimens examined by them, and blue dots indicate additional records from the literature. Orange indicates localities no more specific than county. The map for the western pipistrelle has one yellow dot representing a single record of an eastern pipistrelle.