Pygmy mice (Genus Baiomys) are the smallest cricetine rodents in North America. They occur from Nicaragua in Central America into the southwestern United States. The principal part of the geographic range of the pygmy mice lies in the Republic of México. They are notably common in central México, but are only locally common to the north and to the south, and then only in certain seasons.
Pygmy mice were first brought to the attention of biologists in 1887 when Oldfield Thomas described a diminutive species of cricetine rodent, Hesperomys (Vesperimus) taylori. The description was based on a specimen obtained by William Taylor from San Diego, Duval County, Texas. C. Hart Merriam (1892:70) described Sitomys musculus on the basis of specimens from Colima [City of], Colima, México. Merriam (loc. cit.) mentioned that the two kinds of mice, Hesperomys taylori and Sitomys musculus, "in general appearance look almost precisely like the common house mouse (Mus musculus) but are still smaller and have shorter tails." He placed the two species in the genus Sitomys. Frederick W. True in 1894 regarded them as composing a distinct subgenus of Sitomys, Baiomys. According to True (1894:758), S. taylori and S. musculus possessed a different combination of characters (ascending ramus of mandible short and erect, condyle terminal, coronoid process well-developed, uncinate, and near the condyle, size small, tail short, plantar tubercles six, soles hairy) than either Vesperimus, or Onychomys (which had been considered as a subgenus of Hesperomys until 1889). In 1907, E. A. Mearns accorded Baiomys generic rank. Osgood (1909:252) treated Baiomys us a subgenus of Peromyscus, whereas, Miller, in 1912, regarded Baiomys as a distinct genus. Most recent students of North American mammals have followed Miller, but usually with reservations. Ellerman (1941:402) emphasized that the taxonomic position of the genus was uncertain, and wrote that Baiomys "… seems to be considerably distinct from Peromyscus, and may perhaps be a northern representative of Hesperomys or one of the small South American genera."
Only two comprehensive analyses of geographic variation and interspecific taxonomic relationships have been made; the first was by Osgood (1909) who had fewer than a fourth of the specimens of Baiomys available to me; the second was by Hooper (1952a:90-97) who contributed importantly to understanding the relationships of the two living species in central México. No attempts heretofore have been made to correlate and understand the relationships of the five fossil species to one another and to the living species assigned to the genus.
Six objectives of the following report are to: (1) list characters taxonomically useful in recognizing species and subspecies; (2) record amount of variation within and between populations; (3) correlate observed variations with known biological principles; (4) show geographic ranges of the two living species; (5) indicate relationships between fossil and living species of the genus; and (6) clarify the systematic position of the genus.
MATERIALS, METHODS AND ACKNOWLEDGMENTS
This report is based on the study of approximately 3,520 museum study skins, skulls, complete skeletons, and entire animals preserved in liquid. Most specimens examined were accompanied by an attached label bearing data on locality and date of capture, name of collector, external measurements, and sex. In addition, 49 fossil specimens referable to Baiomys were studied. Nearly two-thirds of the specimens were assembled at the University of Kansas Museum of Natural History; the remainder were examined in other institutions.
Specimens studied were grouped by geographic origin, sex, age, and season of capture. Individual variation was then measured in several of the larger samples of each living species and in measurable fossil material. External measurements used were those recorded by the collectors on the labels attached to the skins. Twenty cranial measurements employed in the past in the study of Baiomys and closely related cricetine rodents were statistically analyzed. The coefficient of variation was calculated for each of the 20 measurements in order to determine which varied least. In general, measurements having the least coefficient of variation were used in comparing samples from different geographic areas. [Figure 1] shows the points between which measurements were taken.
Occipitonasal length.—From anteriormost projection of nasal bones to posteriormost projection of supraoccipital bone. A to A' Zygomatic breadth.—Greatest distance across zygomatic arches of cranium at right angles to long axis of skull. B to B' Postpalatal length.—From posterior margin of hard palate to anterior margin of foramen magnum. C to C' Least interorbital breadth.—Least distance across top of skull between orbits. D to D' [Pg 585] Length of incisive foramina.—From anteriormost point to posteriormost point of incisive foramina. E to E' Length of rostrum.—The distance in a straight line from the notch that lies lateral to the lacrimal to the tip of the nasal on the same side. F to F' Breadth of braincase.—Greatest distance across braincase, taken at right angles to long axis of skull. G to G' Depth of cranium.—The distance from the dorsalmost part of the braincase to a flat plane touching tips of incisors and ventral border of each auditory bulla. A glass slide one millimeter thick was placed on the ventral side of the skull. One jaw of the caliper was on the lower surface of the slide and the other jaw on the dorsalmost part of the braincase. The depth of the slide was subtracted from the total reading. H to H' Alveolar length of maxillary tooth-row.—From anterior border of alveolus of M1 to posterior alveolus of M3. I to I'