Two Eurasian rodents came to Colorado with Euroamerican settlers, the house mouse and the brown rat. The house mouse is all too familiar to most Coloradans. These unsavory immigrants are the only mice many people know, and they have given all mice a bad name. House mice are about the size of a deer mouse, six to seven inches long and about three-fourths of an ounce. However, the scaly, naked tail is as long as the body. The mice are grayish as adults, and the belly is grayish to buffy, never white.
House mice live mostly around buildings, moving into weedy fields and borrow pits in summer and into buildings (homes, barns, poultry houses) in winter. They feed on stored grain, carrion and just about anything in the pantry that is not canned or bottled. House cats, owls, weasels and spotted skunks are good mousers and should be encouraged in the task.
The brown (or Norway) rat is a large (14-18 inches in total length, with a six to eight inch tail), heavy-bodied (seven to 14 ounces) rat. Unlike native woodrats, which it may equal in size, it has a naked, scaly tail, dark brown to blackish fur and a gray (rather than white) belly.
Brown rats arrived in Denver in the 1880s and occur mostly in urban areas, around livestock feedlots and under outbuildings. Like the house mouse, they may breed throughout the year, bearing several litters of two to 14 young. Large house cats and dogs kill rats, and so do owls and a variety of native mammalian carnivores. Where brown rats are present, they may need to be trapped or even poisoned. They are bold and aggressive animals and are known to bite children.
A third immigrant rodent, the black rat (also called roof rat or ship rat), may occur in Colorado only as wild populations of albinos escaped from laboratories. These animals are slimmer than the brown rat, and their naked tails are longer than the head and body.
No other introduced non-game mammals are known to have persistent populations in Colorado. Among species that people have tried to establish was the European hedgehog (Erinaceus europaeus), which was released near Colorado Springs to help control rattlesnakes! Mouse opossums (Marmosa) escaped to the wild from 19th Century fruit markets, where they had arrived from Central America as stowaways on bananas.
Today, people sometimes tire of their pets – gerbils, hamsters, guineapigs, domestic ferrets, and even exotics like sugar gliders – and release them to the "wild." This is inhumane because animals raised in captivity nearly always perish in natural habitats. It is also unlawful. A responsible pet owner always takes unwanted animals to the local humane society where they can be placed for adoption.
By David M. Armstrong
Department of Ecology & Evolutionary Biology
Environmental Studies Program, University Museum of Natural History
University of Colorado-Bouldermausmann@aol.com