Mexican Vole
Order Rodentia : Family Muridae : Microtus mexicanus (Saussure)

Description. A small mouse with short tail, brown color, and only four mammary glands; tail usually less than 35 mm in length, less than twice as long as hind foot; pelage long and fluffy; upperparts dull umber brown, underparts buffy gray, feet and tail brownish gray. External measurements average: total length, 141 mm; tail, 32 mm; hind foot, 21 mm. Weight, 29-48 g.

Distribution in Texas. Restricted in Texas to the higher parts of the Guadalupe Mountains in Culberson County.

Habits. In the Guadalupe Mountains of western Texas these mice live in colonies in the grassy openings of the yellow pine forest, especially in the vicinity of old logs that are partly decayed and well-bedded in the soil. Their numerous, well-defined runways meander through the tall grass, radiating chiefly from the logs under which the mice live and rear their families. They also occur on open ridges where their runways wind about among stones, under shinnery oaks, and even into the edge of dry woods.

Their globular nests of dried grasses and herbs are placed in dense clumps of vegetation above ground, in hollowed-out places under logs, or in special underground chambers off their burrows. One located under a log was cup-shaped, rather than globular, about 10 cm in diameter, and contained four small mice.

Trapping records indicate that these mice are more active in the daytime than are most small mammals, especially in places where adequate ground cover offers concealment. More than 90% of a series trapped in the Guadalupe Mountains were caught in the daytime although the traps were kept set day and night.

Their food is almost entirely vegetation — the green parts of grasses and herbs in summer and the basal portions, roots, bulbs, and bark in winter. There is no evidence that they store food other than the small piles of cut vegetation seen along their trails and at their feeding stations.

Breeding probably continues through most of the year with an interval of about 30-40 days between litters. Gravid females have been trapped in every month from May to October, inclusive. The size of litters, based on embryo counts, ranges from two to five, averaging three. At birth the young are nearly naked, blind, and helpless. They develop rapidly as indicated by the records of young females in the "black" juvenile pelage, weighing slightly more than 20 g, that were sexually mature and gravid. Such mice were probably not more than 6 weeks old.

In Texas, these mice are restricted to the high parts of the Guadalupe Mountains and are of no economic importance except as for food for fur-bearers and other flesh-eaters. Their remains have been identified in droppings of gray fox, bobcat, badger, coyote, and skunk.

Photo credit: John L. Tveten.