This time of year, the birds who migrated here to breed are starting to think about—or have already started—heading south. But the wintering species that moved further north to breed have yet to make their re-appearance. The skies have grown quiet as the summer’s fledglings hone their foraging skills and begin to disperse from their family groups. In this avian transition month, let’s do a change-up of our own here and explore a local mammal—specifically, a rodent species.
About 40% of all of the mammal species in the world are rodents, which include rats, mice, squirrels, porcupines, and beavers. The defining characteristic of a rodent is the 2 pairs of curved incisors—one pair on the top jaw and one pair on the bottom. These teeth are always growing, so a rodent needs to chew constantly to keep them trimmed. Hard enamel coats the front of the incisors, while softer exposed dentin makes up the back of the tooth. The enamel on the front wears down more slowly than does the dentin on the back. As a result, the back of the incisors wears faster than the front, resulting in a perpetually chisel-sharp cutting edge.
The most common rodents in our area are deer mice (genus Peromyscus). (Note that Colorado also has harvest mice and non-native house mice—but these each fall in a different genus than that of deer mice.) Not only are deer mice the most common rodent but the combined species of deer mice are the most common native North American mammals. A number of species fall into the deer mouse genus, including Pinyon Mice (P. truei) and (just to be super-confusing) Deer Mice (P. maniculatus). (In the photos below, both free from Wikimedia, a Pinyon Mouse appears on the left; a Deer Mouse, on the right.) Deer Mice (P. maniculatus) are by far the most common species here; however, in my opinion, Pinyon Mice are by far the cutest species. Pinyon Mice are grayish to yellowish-brown above and whitish below, with light-colored feet. The tail is bicolored, dark on top and light below, covered with hair. And their ears! The distinctive ears are the longest of any deer mouse species in Colorado, usually exceeding ¾”. That may not sound all that big. But since the Pinyon Mouse’s head is perhaps 1” long, its ears are nearly as big as its head! (You can see the difference in relative ear size in the 2 photos below.)
The Pinyon Mouse is an agile climber. In fact, they are likely the most arboreal of the deer mouse species and often forages in trees for the pinyon nuts and juniper seeds. They also eat other seeds, leafy plants, berries, and insects and will store seeds and nuts. This mouse lives in hollow juniper trunks, or under rocks, in a nest made of grasses, leaves, and shredded juniper bark. They can often be found among rocks or on rocky slopes in a wide variety of habitats, but they prefer pinyon/juniper woodlands.
Primarily nocturnal, Pinyon Mice are active throughout the year. A female Pinyon Mouse is sexually mature and can reproduce 50 days after being born. (Wow!) They reproduce from mid-February through mid-November, giving birth to litters of 3 – 6 blind, hairless young. The young have fur by the time they are 2 weeks old. They nurse for 3 – 4 weeks; sometimes a female becomes pregnant while she is still nursing a litter. (Wow again!) In our area, females produce 3 or 4 litters per year.
The Pinyon Mouse’s scientific name, Peromyscus truei, features some interesting components. In Greek, pero means “boots” and mys means “mouse.” So the genus name, literally, means “mouse with boots,” referring to their white feet. The species name, truei, honors Frederick W. True, a North American biologist who was the first curator of biology at an early precursor to the Smithsonian Institution. In case you were wondering, the Deer Mouse’s species name of maniculatus derived from the diminutive form of manicula—little hand. So they are mice with boots and tiny hands. An interesting image, wouldn’t you say?
You can read more about Pinyon Mice—including a link to see photos of Pinyon Mice in our nestboxes and to compare Pinyon and Deer Mouse nests—click here.
A cautionary tale—Hantavirus and the deer mouse
Deer mice have gained notoriety as the primary culprit in spreading a hantavirus called “Sin Nombre virus” (SNV)—meaning the virus with no name. Humans who contract SNV can developed a serious—and at times deadly—illness called hantavirus pulmonary syndrome. Transmitted by inhaling dust contaminated with the virus in a mouse-infested area, hantavirus pulmonary syndrome is diagnosed in 4 – 5 cases a year. (As of July, 2011, Colorado has had 76 cases diagnosed since 1993.) Researchers use the defenses the mouse has against SNV—antibodies—as a marker of which mice carry SNV. The Deer Mouse species (P. maniculatus) is more often found to harbor these antibodies than are other species of deer mice. For example, one study found the antibodies in 9.5% of P. maniculatus (Deer Mouse species) and only 3.3% of P. truei (Pinyon Mouse). However, one should always take care when working in areas with evidence of mice, to avoid stirring up the dust that can carry this virus. Always work in well-ventilated areas; if you’re removing nesting or bedding material, spray it thoroughly with a mixture of 9 parts water to 1 part bleach. You can read more guidelines from the Colorado Department of Public Health and Environment about preventing SNV infection here.