Cool Critters: Cute, all-American western deer mouse plays big role in our ecosystem

If a truly snowy winter ever arrives to our region, you might see footprints resembling those left by some tiny, whimsical rabbit. In reality, they likely belong to the western deer mouse. As the animal hops across the snow, its larger hind paws land in front of the two smaller front paws. Following at the rear is a track left by its long, thin tail.

Scientifically known as Peromyscus sonoriensis, the western deer mouse weighs less than one ounce. Nonetheless, it is a mouse that roars.

In Washington state alone, there are more than 40 rodent species, including squirrels, voles, chipmunks, beavers and muskrats. But for Carl Barrentine, a naturalist and retired biology professor in Spokane, the deer mouse stands out.

“Quite candidly, in my opinion, deer mice will always be the darlings of the rodent world,” he said. With its big rounded ears, shiny black eyes and soft fur, it gives “the “charming illusion of petiteness, femininity, unrivaled cuteness.”

Walt Disney, who built an entertainment industry on an animated mouse named Mickey, perhaps agreed. Legend has it that Disney drew inspiration for the iconic character from the “field mice” that sometimes wandered into his Kansas City animation studio, according to researchers at Harvard University who published a 2015 paper in the journal eLife. (Field mice is a colloquial term for deer mice.)

Yet unlike Mickey Mouse, whose physical presence is mostly limited to the walkways, parades and stages of Disney amusement parks, deer mice are everywhere.

“The deer mouse (genus Peromyscus) is the most abundant mammal in North America and it occupies almost every type of terrestrial habitat,” according to the paper’s authors.

Yes, you read that sentence correctly – there are more deer mice on our continent than any other mammal that lives here. It is also the most widespread mammal geographically, from our own region’s woodlands and wheatfields to the tropics of Mexico and the snowy landscapes of the Arctic Circle.

While it might be hard to see the benefits of a tiny deer mouse nibbling on a wildflower or wheat shaft, the species actually plays an important role in the natural world, according to Bryan McLean, assistant professor of biology at the University of North Carolina Greensboro, who has published studies on deer mice.

For starters, they are a major food source for predators, including owls, hawks, snakes, bobcats and foxes, he explained. What’s more, they disperse seeds to sites where they later sprout, and the same goes for fungi spores. Additionally, their burrowing and feeding activities cycle nutrients in the soil.

“They are major ecosystem contributors,” McClean said.

Deer mice may be cute and beneficial, but in rare instances they are deadly.

The species is the most common carrier of hantavirus, a pathogen that causes a rare but sometimes fatal pneumonialike infection in humans. The virus is shed in the urine, droppings and saliva of deer mice, according to the Washington Department of Health, and is most commonly contracted when those particles get stirred up to become airborne in enclosed spaces such as sheds, barns and cabins.

The species commonly seen inside people’s homes is the house mouse, which doesn’t carry the hantavirus pathogen. At first glance, it resembles a deer mouse. There are key differences, however. House mice are Old World rodents originally from Europe and Asia, while deer mice originated right here on North American soil, McClean said.

How to tell the two apart?

First of all, deer mice prefer the outdoors and typically aren’t drawn to urban and residential areas unless fields, forests and other suitable habitats are nearby. Appearance-wise, the deer mouse has a white underbelly, where the house mouse is a solid-colored brown or gray. Also, note the little animal’s tail. If it’s covered in fuzz, it’s a deer mouse; if scaly and bare, you’re looking at a house mouse.

And one last thing – the western deer mouse that resides in our region was identified as a separate species only recently. In 2019, scientists determined that the deer mouse west of the Rockies was genetically different than the deer mouse to the east. Instead of lumping the two together as the North American deer mouse, the western deer mouse came to be after a 4-million-year identity crisis.