Habitat and numbers.—The habitat occupied by the northern pygmy mouse ranges from sparse grassy areas along rock walls in central México (see Davis, 1944:394), and mesquite-cactus associations in southern Texas (Blair, 1952:242) to heavy stands of grasses such as Bouteloua sp., Andropogon sp., Hilaria sp., and sacaton grass intermixed with Yucca glauca in New Mexico, Arizona (see Hoffmeister 1956:281), and Chihuahua. Baker (1951:213) reports the species from 2 km. W El Carrizo, Tamaulipas, in dense grass and weeds at the edge of a cornfield. Hooper (1953:7) recorded the northern pygmy mouse in a cultivated field overgrown with herbaceous vegetation at Pano Ayuctle, Tamaulipas. In the State of Sinaloa, Hooper (1955b:13) obtained specimens in grass and among shrubs and vines bordering a fallow field. The northern pygmy mouse, in general, lives in situations more xerophytic and more grassy than does the southern pygmy mouse.
The northern pygmy mouse, as the southern pygmy mouse, is locally abundant in its geographic range. Stickel and Stickel (op. cit.: 145) pointed out that on the third night of live-trapping in Bexar County, Texas, there was a sudden increase in unmarked pygmy mice trapped. This increase in numbers, after the resident population was seemingly marked, followed a one-half inch rainfall. Collectors from the University of Kansas, myself included, have had similar experiences in trapping these mice. In the Mexican states of Guanajuato, Querétaro, and Jalisco, B. taylori is one of the commonest small mammals. In New Mexico and Arizona and the Mexican states of Sonora and Sinaloa, nevertheless, these mice are rare.
Stickel and Stickel (loc. cit.) thought that the home range normal for B. taylori in a grassy habitat was less than 100 square feet, but Blair (1953:10) thought that a complete home range had not been recorded by Stickel and Stickel.
Behavior.—The northern pygmy mouse is crepuscular to nocturnal and where I trapped in northern Mexico was one of the first small rodents to appear in my traps in the evening. Hall and Villa-R (1949:460) recorded this habit in Michoacán. Observations of wild-taken B. taylori held in captivity, lend support to its being crepuscular. Captives were rarely active in bright lights, but in diffuse or dim lights the same mice were active.
Blair (1941:381) pointed out that captive B. t. subater were much more tolerant of one another than mice of the genus Peromyscus. He pointed out also that males aided in care of young. In one litter born in captivity in the course of my study, the female killed the male when the young were four days old. In another instance, the female and two eight-day-old young were killed by the male. Until that time, the male, female, and young had lived together peacefully. In other litters born in captivity, adult males did not harm the other mice.
I have noted, as Blair (loc. cit.) did, that B. taylori utters high-pitched squeals in a "singing" posture resembling that of the coyote, yet remains silent when being handled.
The northern pygmy mouse makes runways in the grass, in miniature resembling those of Microtus, and often uses runways constructed by Sigmodon. A small firm nest of finely shredded plant material (mostly grasses) is constructed in burrows or under logs, rocks, or fallen cactus plants. Thomas (1888:447) recorded nests of fine curly grass and cornsilk. Secondary refuge nests are not uncommon. Thomas (loc. cit.) states, "If other mice live in the same place, the individuals of Baiomys watch till others disappear, then suddenly steal part of the other nest and run to their own with it."
Enemies and food.—Little is recorded of the animals that prey upon the northern pygmy mouse. Twente and Baker (1951:120) found remains of B. taylori in 16 per cent of barn owl pellets (Tyto alba pratincola) collected 21 mi. SW Guadalajara, Jalisco. Presumably most of the crepuscular and early nocturnal raptorial birds and carnivorous mammals feed on these mice.
Food of B. taylori consists in part of grass seeds and leaves, prickly pear (Opuntia sp.) and the softer exposed parts of roots of vegetation among which the mice reside.
Reproduction.—The northern pygmy mouse breeds throughout the year. The only months in which I have not recorded pregnant females or females with young are June and October. Forty-one records of embryos or young per litter average 2.48 (less than in B. musculus), and range from as few as one to as many as four per litter.