The Norway rat, also called the brown rat, is a species in Colorado’s records. Open occurrence databases hold 147 records for it, 37 of them iNaturalist observations. Unlike the native mammals it shares that list with, this rat is not part of Colorado’s wild fauna at all.
An introduced, human-tied animal
Native to Asia and spread worldwide as a stowaway of trade, the Norway rat lives almost entirely alongside people. NatureServe flags it as exotic in Colorado and assigns it a rank of “not applicable,” the designation reserved for non-native species outside the conservation framework. Its records map onto cities rather than wildlands: Denver, plus Jefferson and Boulder counties along the urban Front Range, account for most sightings, concentrated in alleys, basements, storm drains, and waterfront edges.
Why the pattern looks the way it does
The rat’s occurrence curve has little to do with the seasons in the way a wild animal’s does. It breeds year-round wherever food and shelter stay steady, and the modest bumps in its record track human observation and pest-control activity more than any natural rhythm. Where dense human food waste meets cover, brown rats follow.
Status in Colorado
This introduced rat lives closely with people and arrived through commerce rather than by crossing a natural range. It holds no conservation status because it is not a species Colorado sets out to protect. The state treats the brown rat as an urban pest, not as a member of its native community, which puts it in a different category from Colorado’s native rodents.
Full species profile
