Speciation and Evolution of the Pygmy Mice, Genus Baiomys Part 9

1. _B. t. allex_ 2. _B. t. a.n.a.logous_ 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 spa.r.s.e gra.s.sy areas along rock walls in central Mexico (see Davis, 1944:394), and mesquite-cactus a.s.sociations in southern Texas (Blair, 1952:242) to heavy stands of gra.s.ses such as _Bouteloua_ sp., _Andropogon_ sp., _Hilaria_ sp., and sacaton gra.s.s 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 gra.s.s 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 gra.s.s and among shrubs and vines bordering a fallow field. The northern pygmy mouse, in general, lives in situations more xerophytic and more gra.s.sy 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 gra.s.sy 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 gra.s.s, in miniature resembling those of _Microtus_, and often uses runways constructed by _Sigmodon_. A small firm nest of finely shredded plant material (mostly gra.s.ses) is constructed in burrows or under logs, rocks, or fallen cactus plants. Thomas (1888:447) recorded nests of fine curly gra.s.s 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 gra.s.s seeds and leaves, p.r.i.c.kly 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. Was.h.i.+ngton, 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. Was.h.i.+ngton, 71:17, April 11, 1958; Hall and Kelson, The Mammals of North America, 2:659, March 31, 1959 (part).

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

_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 a.n.a.logous_, 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, spa.r.s.ely haired; tail unicolored, spa.r.s.ely 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 1_i_ and Plate 4_a_.

_Comparisons._--For comparisons with _B. t. canutus_, see account of that subspecies. From _B. t. a.n.a.logous_, _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 _a.n.a.logous_.

_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 encompa.s.s 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 alt.i.tudes (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 compet.i.tion seems to have limited, perhaps even reduced, size of external and cranial parts of _taylori_ (see p. 660).

In color, specimens from the northern part of the valley of the Rio Tepalcatepec (10 mi. S, 1 mi. W Apatzingan) in Michoacan resemble paratypes of _allex_. Intergradation probably occurs to the north with _a.n.a.logous_.

In the eight specimens from 13 mi. E and 1 mi. N Talpa de Allende, the skull, as reflected in occipitonasal length and zygomatic breadth relative to length of body, is larger than in other specimens here a.s.signed to _allex_. The median part of the belly of the eight specimens is buff-colored rather than whitish-gray as in typical _allex_; the mid-dorsal region also averages darker than in any other specimens referred to _allex_. Additional specimens are needed from this and closely adjacent areas, especially to the west on the coastal plain, in order to determine more accurately the taxonomic status of the mice there. At present, it seems best to refer them to _allex_. Possibly the population represented by the eight specimens is an incipient subspecies.

There is no evidence of hybridization or intergradation of populations of _B. t. allex_ with any population of _B. musculus_ where the two species occur together.

_Specimens examined._--Total 233, all from the Republic of Mexico, distributed as follows: NAYARIT: 3 mi. SE Mirador, 7; _2 mi. S.

Compostela_, 2900 ft., 5; _4 mi. N Santa Isabel_, 3800 ft., 2[25]; _2 mi.

N Santa Isabel_, 3800 ft., 22[25]; _4 mi SE Ahuacatlan_, 5200 ft., 2[26]; _1 mi. E Ixtlan_, 4000 ft., 13[25]; 1 mi. E Ixtlan del Rio, 3700 ft., 1; 2 mi. WNW Valle de Banderas, near sea level, 1. JALISCO: Arroyo de Gavalan, 16[28]; Etzatlan, 6[27]; _Mascota_, 3900 ft., 6[27]; _7 mi W Ameca_, 15[25]; _6 mi. W Ameca_, 15[25]; _3 mi. W Ameca_, 5[25]; Ameca, 4000 ft., 11[27]; _1 mi. SSE Ameca_, 4000 ft., 38; 2 mi. N Resolana, 1500 ft., 28[25]; 13 mi. E, 1 mi. N Talpa de Allende, 8; 2 mi. SSE Autlan, 5; 1 mi. N San Gabriel, 4000 ft., 1[25]; Las Canoas, l[28]. COLIMA: Type locality, 10[27] (including the type). MICHOACaN: 9 mi. S Lombardia, 1500 ft., 1; _3 mi. W Apatzingan_, 1000 ft, 1; Apatzingan, 3[25]; 10 mi. S, 1 mi. W Apatzingan, 800 ft., 10.

_Marginal records._--NAYARIT: 3 mi. SE Mirador; 1 mi. E Ixtlan del Rio.

JALISCO: Etzatlan; Ameca; 2 mi. N Resolana; Las Canoas. MICHOACaN: 9 mi.

S Lombardia; 10 mi. S, 1 mi. W Apatzingan. COLIMA: type locality.

NAYARIT: Valle de Banderas.

[25] Univ. Michigan, Museum of Zoology.

[26] California Academy of Sciences.

[27] U. S. Nat. Museum (Biol. Surv. Coll.).

[28] American Museum of Natural History.

=Baiomys taylori a.n.a.logous= (Osgood)

_Peromyscus taylori a.n.a.logous_ Osgood, N. Amer. Fauna, 28:256, April 17, 1909 (part); Elliott, Check-List Mamm., N. Amer. Cont., West Indies and Neighboring Seas, Suppl., Amer. Mus. Nat. Hist., p. 44, January 8, 1917.

_Baiomys taylori a.n.a.logous_, Miller, Bull. U. S. Nat. Mus., 79:137, December 31, 1912; Miller, Bull. U. S. Nat. Mus., 128:318, April 29, 1924; Ellerman, The Families and Genera of Living Rodents, British Mus. Nat. Hist., 2:402, March 21, 1941; Poole and Schantz, Bull.

U. S. Nat. Mus., 178:259, March 6, 1942; Davis, Jour. Mamm., 25:394, December 12, 1944; Hooper, Jour. Mamm., 28:50, February 15, 1947; Hall and Villa-R., Univ. Kansas Publs., Mus. Nat. Hist., 1:460, December 27, 1949; Hall and Villa-R., a.n.a.l. del Inst. Biol., 21:196, September 28, 1950; Goldman, Smith. Miscl. Coll., 114:373, July 31, 1951 (part); Hall and Kelson, Univ. Kansas Publs., Mus. Nat. Hist., 5:367, December 15, 1952 (part); Villa-R., a.n.a.l. del Inst. Biol., 23:435, May 20, 1953; Miller and Kellogg, Bull. U. S. Nat. Mus., 205:512, March 3, 1955; Hooper, Occas. Papers Mus. Zool. Univ.

Michigan, 565:13, March 31, 1955; Packard, Proc. Biol. Soc.

Was.h.i.+ngton, 71:17, April 11, 1958.

_Peromyscus musculus brunneus_, Elliot, Field Columb. Mus. Publ., 115(8):203, 1907 (part).

_Peromyscus musculus_ [_musculus_], Osgood, N. Amer. Fauna, 28:258, April 17, 1909 (part).

_Baiomys musculus musculus_, Hall and Villa-R., Univ. Kansas Publs., Mus. Nat. Hist., 1:460, December 27, 1949 (part); Hall and Villa-R., a.n.a.l. del Inst. Biol., 21:196, September 28, 1950 (part).