Part 14

External and cranial size is less in B. t. allex, the southernmost subspecies, and progressively more in B. t. paulus, B. t. taylori, B. t. ater, B. t. subater, B. t. fuliginatus, B. t. canutus, and B. t. analogous. Size is largest in subspecies that occur at higher altitudes. Those subspecies are B. t. analogous and B. t. fuliginatus. The correlation with Bergman's Rule is less exact in B. taylori than in B. musculus. It is noteworthy that the smallest subspecies, B. t. allex, occurs in the area where the two species are sympatric.

There is close correlation in B. taylori, as also in B. musculus, of darker pelages with zones of high relative humidity. The subspecies having dark pelages are: analogous, fuliginatus, and subater. The two first-mentioned subspecies occur at high altitudes, and the other, subater, occurs in the humid coastal region of Texas. The paler subspecies, taylori, canutus, and allex, occur at lower altitudes. Two subspecies that occur at relatively high altitudes, ater and paulus, are reddish-brown. The color of pelage in these subspecies resembles the color of soil upon which they live. Blair and Blossom (1948:5) demonstrated close correlation of color of soil with color of pelage in B. t. ater by use of an Ives tint photometer.

1. B. t. allex 2. B. t. analogous 3. B. t. ater 4. B. t. canutus 5. B. t. fuliginatus 6. B. t. paulus 7. B. t. subater 8. B. t. taylori]

Natural History

Habitat and numbers.--The habitat occupied by the northern pygmy mouse ranges from sparse grassy areas along rock walls in central Mexico (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, Queretaro, 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 Michoacan. 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.

=Baiomys taylori allex= (Osgood)

Peromyscus allex Osgood, Proc. Biol. Soc. Washington, 17:76-77, March 21, 1904; Elliot, Field Columb. Mus. Publ., 105(6):135, July 1, 1905; Lyon and Osgood, Bull. U. S. Nat. Mus., 62:124, January 15, 1909.

Baiomys taylori allex, Packard, Proc. Biol. Soc. Washington, 71:17, April 11, 1958; Hall and Kelson, The Mammals of North America, 2:659, March 31, 1959 (part).

Peromyscus taylori paulus, Osgood, N. Amer. Fauna, 28:255, April 17, 1909 (part).

Baiomys taylori paulus, Miller, Bull. U. S. Nat. Mus., 79:137, December 31, 1912 (part); Miller, Bull. U. S. Nat. Mus., 128:317, April 29, 1924 (part); Ellerman, The Families and Genera of Living Rodents, British Mus. Nat. Hist., 2:402, March 21, 1941 (part); Poole and Schantz, Bull. U. S. Nat. Mus., 178:259, March 6, 1942; Goldman, Smith. Miscl. Coll., 115:373, July 31, 1951 (part); Miller and Kellogg, Bull. U. S. Nat. Mus., 205:512, March 3, 1955 (part); Hall and Kelson, The Mammals of North America, 2:659, March 31, 1959 (part).

Baiomys taylori analogous, Hall and Kelson, Univ. Kansas Publs., Mus. Nat. Hist., 5:367, December 15, 1952 (part).

Type.--Adult male, skin and skull; No. 33429/45452 U. S. Nat. Mus. (Biol. Surv. Coll.); Colima (City), Colima, Republic of Mexico, obtained on March 7, 1892, by E. W. Nelson, original number 2029.

Range.--Colima, western lowlands of Michoacan and Jalisco, thence north into southern half of Nayarit, see Figure 11. Zonal range: arid lower tropical, approximates northern half of the Nayarit-Guerrero Biotic Province of Goldman and Moore (1945:349). Occurs from near sea level in Nayarit, up to 4000 feet in Jalisco.

Diagnosis.--Size small for the species; dorsal ground color pale grayish-brown, near Isabella color; mid-dorsal region washed with blackish, individual guard hairs black to base, other hairs black-tipped with subterminal light olive bands, Neutral Gray at base; laterally, black-tipped hairs less abundant, hairs grayish-white to base; venter Pale Gull Gray to whitish, distal half of individual hairs white, proximal half Neutral Gray; hairs in regions of throat and chin white to base; facial region colored like dorsum, becoming paler below eye; in region of mouth, hairs white to base; dorsalmost vibrissae black to base, others white to base; ears flesh-colored, sparsely haired; tail unicolored, sparsely haired for the species; dark blotches on tail of some series (particularly the paratypical series); dorsal and ventral parts of forefeet and hind feet flesh-colored, whitish to gray in some series. Slightly smaller in most cranial dimensions. Maxillary part of zygoma forming almost a right angle with rostrum, rather than tapering at less than a right angle to rostrum; supraoccipital rounded posteriorly rather than indented on each side of foramen magnum; cranium, relative to length of rostrum, more nearly square; interparietal large relative to size of cranium. Average and extreme measurements of five adults from 2 mi. SSE Autlan are as follows: total length, 100.0 (93-107); length of tail vertebrae, 40.0 (37-44); length of body, 60.0 (56-63); length of hind foot, 14.0 (14); length of ear from notch, 10.5 (10-11); occipitonasal length, 17.3 (16.8-17.9); zygomatic breadth, 9.1 (8.7-9.4); postpalatal length, 6.3 (6.0-6.6); least interorbital breadth, 3.4 (3.3-3.5); length of incisive foramina, 3.9 (3.8-4.0); length of rostrum, 5.5 (5.2-5.8); breadth of braincase, 8.6 (8.0-8.9); depth of cranium, 6.4 (6.0-6.7); alveolar length of maxillary tooth-row, 3.0 (2.8-3.1); for photographs of skull, see Plate 1i and Plate 4a.

Comparisons.--For comparisons with B. t. canutus, see account of that subspecies. From B. t. analogous, B. t. allex differs in: external and cranial dimensions less; dorsal coloration paler; tail and ears paler and less hairy; dorsum and belly paler; dorsal and ventral parts of forefeet and hind feet paler; median parts of incisive foramina less constricted on either side of midline and wider open laterally; interparietal larger in relation to skull; interorbital breadth greater relative to occipitonasal length.

B. t. allex differs from B. t. paulus as follows: dorsum gray with yellowish-brown wash rather than fawn to buff; tail unicolored in most series, less hairy; hind feet flesh-colored to light sooty, rather than whitish; rostrum slightly longer relative to occipitonasal length; incisive foramina differ from those of paulus in much the same way as from analogous.

Remarks.--Osgood (1909:255-256) dismissed as taxonomically unimportant the differences in color of pelage and size of cranium that he observed between the specimens from Colima (City), Colima, representative of allex and those representing paulus and chose to synonomize allex with paulus. The differences that Osgood (loc. cit.) deemed "... scarcely worthy of recognition ...," are, in fact, not only worthy of recognition, but also important in an understanding of the evolution of Baiomys taylori (see speciation p. 659). Recently, I (1958b:17-18) studied ten specimens from Colima (City), Colima, and chose to regard Peromyscus [= Baiomys] allex as a subspecies. I suggested (loc. cit.) that the geographic range of B. t. allex might encompass the southern part of Nayarit, and western Jalisco. Subsequent study of specimens from these areas reveals that the populations there are referable to allex. Most of the specimens obtained from these areas, however, merit special comment.

In color of pelage, those populations from south of the Rio Grande de Santiago and northwest of Guadalajara (4 mi. SE Ahuacatlan; 1 mi. E Ixtlan; Etzatlan) show evidence of intergradation with paulus to the east and south (Magdalena, Tequila, and Tala, Jalisco), and with populations more closely adjacent to the south bank of that river. Intergradation is indeed complex in this area. Specimens from some localities seem to be intergrades between allex and paulus; from other localities, some specimens are referable to allex, and the others to paulus; from still other localities, all specimens are referable to allex.

A series of 39 specimens from 1 mi. SSE Ameca, 4000 ft., Jalisco, are uniformly grayish-brown. This series averages grayer than paratypes of allex. There is little, if any, difference between the series from 1 mi. SSE Ameca and paratypes of allex in external size of body, hind foot, length of ear, and size and conformation of the cranium. Populations from Ameca and vicinity might be expected to average considerably larger inasmuch as they occur at higher altitudes (see Bergman's Rule, p. 609) then the material from the lower coastal plains to the south in Colima and Michoacan, and at lower elevations in the west in Jalisco and Nayarit. The means of external and cranial measurements are not significantly different between the specimens from the highlands and those from the lowlands. In the area of Ameca where the two species B. musculus and B. taylori occur together, interspecific competition seems to have limited, perhaps even reduced, size of external and cranial parts of taylori (see p. 660).