Northern Pygmy Mouse
Order Rodentia : Family Muridae : Baiomys taylori (Thomas)

Description. Smallest of the muroid mice in Texas, with the exception of two harvest mice, Reithrodontomys humulis and Reithrodontomys montanus, both of which differ in having grooved upper incisors and longer tail. Upperparts grizzled grayish in adults, blackish in juveniles; underparts smoke gray; tail about three times as long as hind foot, sparsely haired and decidedly shorter than head and body. They have a strong, musky odor similar to that of house mice, Mus musculus. External measurements average: total length, 98 mm; tail, 38 mm; hind foot, 14.5 mm. Weight, 7-10 g, averaging 8 g.

Distribution in Texas. Distributed over the central portions of the state; range excludes Trans-Pecos and eastern Texas. This species is extending its range northward and westward.

Habits. This is a southern species, characteristic of the tropical lowlands of Mexico, that reaches its northern distributional limits in Texas. Early records indicate that B. taylori was restricted to the coastal region of eastern Texas and the mesquite-chaparral regions of southern Texas. Since the early twentieth century, the species has consistently extended its range northward and eastward by invading the oak-hickory association, the blackland prairies, the cross-timbers, rolling plains, and the high plains.

These mice have a preference for grassy areas, and they are commonly found in old fields, pastures, and along railroad and highway rights-of-way, where they usually live in close association with cotton rats (Sigmodon hispidus) and harvest mice (Reithrodontomys spp.). If other types of ground cover such as rocks, cactus, and fallen logs are available, the pygmy mouse may be found in areas where grass is relatively sparse.

Pygmy mice live in nests placed in burrows in the ground, beneath fallen logs, among cactus pads, or in thick clumps of grass. The nest is typically a ball of finely shredded grass or cactus fibers with a central cavity and one or two openings. A network of runways or beaten paths leads away from the nest sites beneath the thick mat of dead grass.

The mice feed chiefly on vegetation; seeds are especially well-liked. They are active the year round and do not hibernate; neither do they store food for winter use. Although they are chiefly nocturnal, they have been caught in traps on several occasions in the daytime. They can swim well when necessary, but water is avoided whenever possible because their short fur is easily water-soaked.

The breeding season is nearly year-long — gravid females have been taken from January to October. In captivity, one female gave birth to nine litters in 195 days; another, to eight litters in 221 days. The gestation period is about 20 days. The litters vary in size from one to five, averaging about three. At birth the young are naked, blind, and helpless and weigh about 1 g each. The eyes open in 12-15 days; the mice are weaned in 18-22 days; and sexual maturity is attained at the age of about 60 days.

Photo credit: R.D. Porter.