Speciation and Evolution of the Pygmy Mice, Genus Baiomys

by Robert L. Packard

[Peromyscus] musculus, Trouessart, Cat. Mamm., 1:518, 1898.

[Peromyscus] musculus [musculus], Elliot, Field Columb. Mus. Publ., 95(4):175, July 15, 1904.

Baiomys musculus musculus, Miller, Bull. U. S. Nat. Mus., 79:137, December 31, 1912 (part); Miller, Bull. U. S. Nat. Mus., 128:318, April 29, 1924 (part); Ellerman, The Families and Genera of Living Rodents, 2:402, March 21, 1941; Poole and Schantz, Bull. U. S. Nat. Mus., 178:258, March 6, 1942; Davis, Jour. Mamm., 25:394, December 12, 1944 (part); Hooper, Jour. Mamm., 28:50, February 15, 1947 (part); Hall and Villa-R., Univ. Kansas Publs., Mus. Nat. Hist., 1:460, December 27, 1949 (part); Hall and Villa-R., Anal. del Inst. Biol., 21:196, September 28, 1950 (part); Goldman, Smith. Miscl. Coll., 115:336, July 31, 1951 (part); Miller and Kellogg, Bull. U. S. Nat. Mus., 205:512, March 3, 1955 (part); Hooper, Occas. Papers Mus. Zool. Univ. Michigan, 565:13, March 31, 1955; Hall and Kelson, The Mammals of North America, 2:661, March 31, 1959 (part).

B. [aiomys] m. [usculus] musculus, Hooper, Jour. Mamm., 33:97, February 18, 1952 (part); Packard, Univ. Kansas Publs., Mus. Nat. Hist., 9:400; December 19, 1958.

Baiomys taylori allex, Hall and Kelson, The Mammals of North America, 2:659, March 31, 1959 (part).

Type.—Adult female, skin and skull; No. 33437/45460 U. S. Nat. Mus. (Biol. Surv. Coll.); Colima (City), Colima, Republic of México, obtained on March 9, 1892, by E. W. Nelson, original number 2055.

Range.—Southwestern Nayarit and northwestern Jalisco, south into Colima, thence eastward into Michoacán. Zonal range: part of arid Lower Tropical Subzone of Goldman (1951:330); approximates part of the Nayarit-Guerrero Biotic Province of Goldman and Moore (1945:349). Occurs from near sea level in Colima up to 5800 feet in Jalisco.

Diagnosis.—Size large for the species; dorsum Olive-Brown in darkest series to Buffy Brown with tones of Fawn Color in the palest series; guard [Pg 621] hairs of dorsum black-tipped, gray basally (in some specimens, guard hairs gray-tipped with subterminal black band, and gray base); underfur of dorsum black-tipped with subterminal band of fawn to buff, Neutral Gray basally; face and head paler than back because of greater number of fawn-colored and buff-colored hairs; hairs on throat and chin white to base; venter and flanks Pale Olive-Buff in palest series to Gray (Pale Gull Gray) in darkest series; individual hairs of venter tipped with white to buff, basally Gray (Dark Gull Gray); forefeet and hind feet white to gray with flesh-colored undertones; tail faintly bicolored, individual hairs above black, below white; nasals flared anteriorly; zygoma and zygomatic plate thick. Average and extreme external and cranial measurements for 8 adults from Armeria, Colima, are as follows: total length, 125.5 (115-135); length of tail vertebrae, 47.5 (42-54); length of body, 75.6 (68-81); length of hind foot, 16.5 (16-17); occipitonasal length, 20.3 (19.8-20.7); zygomatic breadth, 10.7 (10.3-11.1); postpalatal length, 7.4 (7.1-7.7); least interorbital breadth, 4.0 (3.9-4.1); length of incisive foramina, 4.3 (4.1-4.5); length of rostrum, 7.3 (6.9-7.6); breadth of braincase, 9.8 (9.4-10.0); depth of cranium, 7.1 (6.7-7.2); alveolar length of maxillary tooth-row, 3.4 (3.3-3.6); for photographs of skull, see [Plate 1e], and [Plate 3e].

Comparisons.—For comparisons with B. m. brunneus, B. m. infernatis, and B. m. pallidus, see accounts of those subspecies. From B. m. nigrescens, B. m. musculus differs in: dorsum paler throughout (less of blackish brown); region of face and ears paler, more buff and fawn-colored hairs rather than blackish-brown to grayish hairs; vibrissae paler; venter paler, less dark gray and less of sooty-colored undertones, tips of hairs whitish to pale Olive-Buff rather than light gray at tips becoming darker basally; forefeet and hind feet paler, whitish to pale buff-color with flesh-colored undertones, not sooty-colored to dark brown; tail paler below; nasals flaring outward, not tapering toward midline at anteriormost point; zygoma more massive; larger in external and cranial dimensions.

Remarks.—Merriam (1892:170) described Sitomys [= Baiomys] musculus on the basis of 23 specimens (from Colima City, Colima; Armeria, Colima; Plantinar, and Zapotlán, Jalisco). According to the original description, B. musculus resembled a small house mouse and was smaller than any known species of Sitomys except S. taylori [= Baiomys taylori]. From taylori, musculus differed in being larger [in size of body], and in having longer ears and tail, and larger hind feet. When Allen and Chapman (1897:203) described Peromyscus [= Baiomys] musculus brunneus from Jalapa, Veracruz, the specimens described by Merriam from Colima and Jalisco became representative of the nominal subspecies B. m. musculus. Osgood (1909:258) assigned specimens from Colima, Guerrero, Jalisco, Michoacán, Morelos, Oaxaca, Puebla, Sinaloa, Veracruz, and Zacatecas to the subspecies musculus. Subsequently, Russell (1952:21) named the subspecies pallidus from the arid lowlands of Morelos; Hooper (1952:96) described the subspecies infernatis from northern Oaxaca and southeastern Puebla; and Goodwin (1959:1) described a new subspecies nebulosus from the Oaxaca highlands. Each of the subspecies mentioned immediately above was described from within the geographic range assigned to B. m. musculus by Osgood (loc. cit.). Hall and Kelson (1959:661) mapped the range of B. m. musculus so as to include Colima, parts of Jalisco, Michoacán, Guerrero, Oaxaca, and Veracruz. Lukens (1955:159), in a study of the mammals of Guerrero, has shown that the characters attributed to B. m. pallidus are not significantly different from those of pygmy mice studied from Guerrero. He (loc. cit.) concluded that: (1) if the specimens of pygmy mice from central Guerrero were typical of the subspecies musculus, then pallidus did not deserve subspecific recognition, or; (2) the name B. m. musculus should be restricted to the larger pygmy mice inhabiting the lowlands immediately adjacent to the Pacific Coast and the area to the north. My data (see [Figure 12]) show pygmy mice from southwestern Nayarit, northwestern and central Jalisco, Colima, and parts of Michoacán to be significantly larger in certain cranial and external measurements than pygmy mice from Guerrero, Oaxaca, Morelos, and parts of Puebla. This finding essentially corroborates Hooper's (1952a:96) findings. It seems advisable, therefore, to restrict the range of B. musculus musculus to the large mice inhabiting west-central México and the coastal lowlands of Colima and Michoacán. The name pallidus is applicable to the smaller mice occupying Morelos, southwestern Puebla, Guerrero, Oaxaca, and southwestern Chiapas.