Northern Pygmy Mouse

The average size of an adult male is approximately 7.5 grams (0.26 oz), while adult females can weigh up to 9.4 grams (0.33 oz). The average length of an adult (across all subspecies) is 64.1 millimetres (2.52 in) for body length and 42 millimetres (1.7 in) for tail length. Keeping with Bergmann's rule, northern populations and high-altitude populations are larger than southern and low-altitude populations. B. t. allex, the southernmost subspecies, is the smallest subspecies, with an average total body length (including the tail) of 100 millimetres (3.9 in). The largest subspecies, B. t. analogus and B. t. fuliginatus, have average total body lengths of 109 millimetres (4.3 in) and 105.5 millimetres (4.15 in), respectively. Species range from light brown to reddish brown to an almost black color. Their undersides range from white to grayish. Darker specimens tend to be from high-humidity and high-altitude habitats, while paler mice are from low-humidity and low-altitude habitats. analogus and fuliginatus, as well as eastern taylori, are darker, canutus, allex, and southern and eastern taylori are pale, and ater and paulus are reddish brown to match the soil they live on. Juveniles have a distinctly gray fur before shedding into their adult coloration. Both albinism and melanism have been recorded in this species.

The pygmy mouse has litters of 1 to 5 pups, with an average of 3. The gestation time for pregnant females is less than 20 days. Unlike many other rodents, father pygmy mice will care for offspring, and groom and huddle over young. These species breed year-round, but breeding is reduced in winter and early spring months. Nests typically occur underneath logs or cacti. The nest is ball-like and made of cactus fibers, corn silk, and grasses. Both males and females care for the young. A 1976 study showed that B. t. ater raised by the house mouse Mus musculus had a greater tendency to have positive interactions with their "foster" species, and reacted more negatively to open spaces. This indicates that the behavior of these species are influenced to some degree by learned behavior as young. The M. musculus raised by B. taylori "foster parents" were not significantly impacted behaviorally.