Baiomys taylori [ater], Justice, Jour. Mamm., 38:520, November 20, 1957.
Type.—Adult male, skin and skull; No. 85425, University of Michigan, Museum of Zoology; 7 mi. W Hereford, Cochise County, Arizona, obtained on March 25, 1941, by Philip M. Blossom, original number 2195.
Range.—Southeastern Arizona, north to Graham County, thence east to the Animas Valley, Hidalgo County, New Mexico; south to northern Chihuahua and northwest to the southern border of Cochise County, Arizona, see [Figure 11]. Zonal range: largely lower Sonoran (Apachian Biotic Province of Dice, 1943:56). Occurs from 4300 feet in Chihuahua up to 6200 feet in New Mexico.
Diagnosis.—Size medium for the species; dorsum between Mummy Brown and Prouts Brown; individual tips of hairs intermixture of black and Ochraceous-Tawny, bases of all hairs slate-gray; sides of body and face, Buffy Brown to Cinnamon Brown; belly Cinnamon Buff, proximal half of individual hairs Deep Neutral Gray, distal half white; in region of throat, proximal [Pg 641] fourth of individual hairs gray, distal three-fourths white; dorsal vibrissae black to base, ventral vibrissae white to base; tail brownish above, gray below; dorsal and ventral surface of forefeet and hind feet buffy to gray; interparietal somewhat compressed anteroposteriorly. Average and extreme cranial measurements of 15 adults from 91/2 mi. W New Mexico State Line, 51/2 mi. N Mexican border, Cochise County, Arizona, are as follows: occipitonasal length, 18.0 (17.5-18.6); zygomatic breadth, 9.5 (9.2-9.9); postpalatal length, 6.6 (6.0-7.1); least interorbital breadth, 3.6 (3.4-3.8); length of incisive foramina, 4.0 (3.8-4.2); length of rostrum, 6.1 (5.7-6.4); breadth of braincase, 8.6 (8.4-9.1); depth of cranium, 6.5 (6.3-6.9); alveolar length of maxillary tooth-row, 3.2 (3.1-3.4). Average and extreme external measurements for six adults from 9 mi. W Hereford, Cochise County, are as follows: total length, 106.3 (98-115); length of tail vertebrae, 42.3 (39-46); length of body, 64 (59-69); length of hind foot, 13.6 (13-14.2); length of ear from notch, 11.1 (10.5-11.5); for photographs of skull, see [Plate 2b], and [Plate 4c].
Comparisons.—For comparisons with B. t. canutus, see account of that subspecies. From B. t. paulus, the subspecies to the southeast, B. t. ater differs in: dorsum darker brown; tail less strikingly bicolored; belly buffy rather than whitish to white-gray; forefeet and hind feet darker dorsally and ventrally; posterior margin of basioccipital bowed anteriorly in a broad U-shape with a secondary small median anteriorly directed U-shaped curve, rather than bowed anteriorly in a simple U-shape; interparietal more compressed anteroposteriorly; coronoid process of mandible so acutely recurved that tip of coronoid points posteroventrally and appears sickle-shaped.
Remarks.—Blossom and Burt (1942:1) described B. t. ater as the darkest of the known subspecies. It is dark, but specimens from some parts of the ranges of B. t. analogous, B. t. fuliginatus, and B. t. subater exceed in melanins the darkest individuals of ater. Blair and Blossom (1948:5) also concluded by the use of an Ives tint photometer that B. t. subater was significantly darker than B. t. ater.
When paratypes of ater and specimens of B. t. paulus are compared, the darkest individuals of ater exceed but slightly the darkest of paulus. The darkest specimens of paulus occur in southern Zacatecas, and northern Jalisco, and the palest of the series are in northern Durango and southern Chihuahua. When paratypes of ater and paulus are compared, the difference in color is readily distinguishable. Specimens from 11/2 mi. N San Francisco, in northern Chihuahua, appear to be intermediate in color between ater and paulus except for a faint tinge of buff ventrally. In characters of the crania, these specimens resemble ater and are referred to that subspecies. A slightly different pattern of color is present in pygmy mice from the Peloncillo Mountains and the Animas Valley of New Mexico; the upper parts resemble those of paratypes of ater, but the venter has only the faintest suggestion of the buffy wash. Crania of these specimens from New Mexico are inseparable from those of paratypes of ater, and the specimens are, therefore, referred to ater.
When specimens are arranged by localities from Arizona east into southern New Mexico, thence south into Chihuahua and Durango, gradual intergradation in color is evident from dark in the north to pale browns in the south, whereas, size and shape of interparietal and size and shape of coronoid process of the lower jaw divide quite distinctly into two morphological types in central Chihuahua.
Cranial variation in size and proportion among adults is slight throughout the range of ater compared to variation detected in other subspecies of Baiomys taylori. Perhaps such a relatively stable pattern of characters of the crania reflects the homogeneity of the gene pool, with respect to these characters, of the populations sampled. The fact that the color of the pelage of this subspecies varies considerable throughout its known range and that the crania do not is perhaps a clue to the mode of inheritance of characters in these mice. Seemingly, color of pelage is inherited independently of characters of the cranium. The relative lack of variability in the crania of ater may result from uniform environmental conditions, which have served to select for uniform characters in the populations. All of the other wide-ranging subspecies of B. taylori occupy more diverse habitats than ater. Secondly, the rather abrupt change in the cline of measured characters of the crania between ater and paulus in central Chihuahua suggests a secondary zone of intergradation. The probable cessation of gene flow in the past between these two subspecies, allowing ater to be isolated for a time, may also, in part, account for the relative lack of variability in the crania of ater.