Speciation and Evolution of the Pygmy Mice, Genus Baiomys Part 8

_Diagnosis._--Size medium for the species; dorsum Buffy Brown in palest series to Olive-Brown in darkest series, individual hairs Warm Buff, Neutral Gray basally, some with black tips and a subterminal band of Warm Buff, guard hairs of dorsum black-tipped, gray basally; hairs on sides creamy-buff, gray basally; face same color as back fading to white on throat; vibrissae white-tipped, pale brown basally; venter, whitish with tinges of buff on lower throat, individual hairs having tips white to buffy-white, light gray basally; dorsal surface of forefeet and hind feet whitish to flesh-color; tail indistinctly bicolored, brownish above, grayish brown below; zygoma bowed as in _B. m. grisescens_; tail short; average and extreme external and cranial measurements for 17 adults from Tehuantepec, Oaxaca, are: total length, 117.3 (110-126); length of tail vertebrae, 46.9 (41-51); length of body, 70.4 (65-76); length of hind foot, 15.8 (15-16); occipitonasal length, 18.9 (18.2-20.1); zygomatic breadth, 10.1 (9.7-10.6); postpalatal length, 6.9 (6.6-7.5); least interorbital breadth, 3.8 (3.6-3.9); length of incisive foramina, 4.4 (4.2-4.7); length of rostrum, 6.7 (6.3-7.2); breadth of braincase, 9.3 (8.7-9.7); depth of cranium, 6.6 (6.4-6.8); alveolar length of maxillary tooth-row, 3.2 (3.1-3.4); for photographs of skull, see Plate 1g, and Plate 3g.

_Comparisons._--For comparisons with _B. m. brunneus_ and _B. m.

infernatis_, see accounts of those subspecies.

From _B. m. musculus_, _B. m. pallidus_ differs in: dorsum more olive-gray and brown, less ochraceous on either side of mid-dorsal region; face below eye grayish, not buffy; sides gray with buffy overtone, not creamy with light yellow overtones; venter grayish-white rather than an olive-buff; zygomata more tapering anteriorly; maxillary part of zygoma narrower when viewed from above; external and cranial dimensions smaller.

From _B. m. nigrescens_, _B. m. pallidus_ differs in: dorsum paler, fewer black hairs medially; face paler, less sooty; vibrissae brownish with white tips rather than black with brownish tips; venter paler; dorsal surface of forefeet and hind feet whitish to flesh-colored rather than sooty to dusky-white; tail paler; nasals slightly more attenuated; averaging slightly larger in external and cranial measurements.

_Remarks._--Russell (1952:21) described _pallidus_, on the basis of specimens from the arid Balsas Basin, of Morelos, as pale gray dorsally.

After examining the original material from Morelos, I find the dorsal color of _pallidus_ to be much closer to a buffy brown than a pale grayish. Even so, smaller size differentiates _pallidus_ from _musculus_. _B. m. infernatis_, not _B. m. pallidus_, is the most pallid of all named subspecies of _B. musculus_.

_B. m. pallidus_ intergrades to the northwest with _B. m. musculus_, to the northeast with _B. m. infernatis_, and to the southeast with _B. m.

nigrescens_.

According to Goodwin (1959:2), _B. m. nebulosus_ (named on the basis of one specimen) differs from _B. m. musculus_ [= _pallidus_] from southern Oaxaca in: darker and longer pelage; larger skull; interorbital region broader and less constricted posteriorly. From _B. m. nigrescens_ and _B. m. brunneus_, _B. m. nebulosus_ differs as follows: pelage longer and softer; skull larger.

Study of specimens of _B. musculus_ from Oaxaca reveals considerable variation in external and cranial measurements as well as color, corresponding to that reported by Goodwin (_loc. cit._). Specimens from higher alt.i.tudes average somewhat darker and larger in external and cranial size than those at lower elevations. These differences seem to be microgeographic and not of subspecific rank. Among specimens that I have studied in Oaxaca are several from different localities (KU 63052, an adult male, from 3 mi. W Miahuatlan; KU 68964, an adult male from 3 mi. W Mitla, 6000 ft.; KU 63055, an adult female from 3 mi. S Candelario, 1200 ft.) that, according to Goodwin (_in. litt._) match _nebulosus_ in reported color, size of body and skull (except for the region of the rostrum).

Two of the three specimens (KU 63052 and 63055) are the darkest of a series in which the palest are inseparable from _B. m. pallidus_.

Goodwin, who kindly compared the three specimens with the type of _nebulosus_, mentioned (_in. litt._) that the skull of the type has a slenderer rostrum. Included in the series of skulls of _B. m. pallidus_ from 3 mi. W Mitla are several adults (not seen by Goodwin) with slender rostra. _B. m. nebulosus_ is judged to be a synonym of _B. m. pallidus_.

Populations of pygmy mice occurring in partially isolated areas of highland in Oaxaca seem to me to be incipient subspecies.

_Specimens examined._--Total 824 all from the Republic of Mexico and distributed as follows: PUEBLA: 2 mi. S Atlixco, 5800 ft., 1; _1 mi. SSW Tilapa_, 5800 ft., 2; _6 mi. SW Izucar de Matemores_, 7; _Piaxtla_, 3900 ft., 4[18]; Acatlan, 4100 ft., 1. MORELOS: 5 mi. W Tepoztlan, 6000 ft., 7[19]; _1 mi. W Tepoztlan_, 6000 ft., 9[19]; _2 mi. SW Tepoztlan_, 7000 ft., 1[20]; _Cuernvaca_, 9[19]; _6 mi. W Yautepec_, 4500 ft., 1[20]; _Yautepec_, 12[19]; _3 mi. N Alpuyeca_, 4000 ft., 2[20]; _Puente de Ixtla_, 2[19]; _Tetecala_, 4[21]; _2 km. S Jonacatepec_, 4500 ft., 6[20]; _type locality_, 6 (including the type). GUERRERO: _Yerbabuena_, 1800 m., 1; _Cueva de tia Juana_ [= _1.5 km. SSW Yerbabuena_], 1; _Laguna Honda_, 1840 m. [= _1.5 km. S Yerbabuena_], 3; 9 mi. SE Taxco, 3800 ft., 1[22]; _17 km. S Taxco_, 4000 ft., 2[20]; _Iguala_, 5[19]; _3.2 km. SSE Iguala_, 970 m., 1; 1 km. SSE Texcaizintla, 1600 m., 2; _Teloloapan_, 20[19], 5[24]; _1 km. N Chapa_, 1470 m., 6; _Chapa_, 1470 m., 5; El Limon, 3[18]; 2-1/2 mi. W Mexcala, 2100 ft., 1[20]; _Rio Balsas_, 1[18]; Ayusinaha [= Ayotzinapa], 1[18]; _Tlapa_, 3900 ft, 1[18]; _2.5 mi. S Almolonga_, 5600 ft., 13[20]; _1 km. N Zihuatanejo_, 1; Zihuatanejo Bay, 4[19]; _Las Gatas_ [= _2 km. S. Zihuatanejo_], 2; _2 km. SSE Zihuatanejo_, 9; _4 mi.

W Chilpancingo_, 5800 ft., 3[20]; _Chilpancingo_, 4800 ft., 14[18], 21[19], 45[21]; _2 mi. N Tixtla_, 4400 ft., 3[20]; _3.2 km. S Chilpancingo_, 4; _Cd. Chamilpa_ [= _12 km. ESE Chilpancingo_], 5; _Tlalixtaquilla_, 4200 ft., 2[18]; _15 km. S. Chilpancingo_, 4300 ft., 10[20]; _1 mi. SW Colotlipa_, 2700 ft., 16[20]; _2 mi. SW Colotlipa_, 2700 ft., 1[20]; _Achuitzotla_, 2800 ft., 7[20]; _8 mi. SW Colotlipa_, 1[20]; _5 mi. S Rincon_, 2600 ft., 2[20]; _8 mi. SW Tierra Colorado_, 600 ft., 1[20]; Rio Aguacatillo, _30 km. N Acapulco_, 1000 ft., 3[20]; 5 mi. ESE Tecpan, 50 ft., 9; _Ejido Viejo_, _12 km. NNW Acapulco_, 1; _2 mi. NNW Acapulco_, 7; Acapulco, 3[18], 3[21]; Omentepec, 200 ft., 7[18]. OAXACA: _4 mi. E Huajuapam_, 5000 ft., 1; 2 mi. NW Tamazulapan, 6550 ft., 1; Yalalag, 3000 ft., 5[18]; _11 mi. NW Oaxaca_ [_City_], 1; _Yaganiza_, 3900 ft., 1[18]; Oaxaca [City], 5000 ft., 15, 7[21], 7[19], 5[24]; _3 mi. ESE Oaxaca_ [_City_], 30; _4 mi. ESE Oaxaca_ [_City_], 5050 ft., 1; _10 mi. SE Oaxaca_ [City], 1[22]; _Cerro Ocotepec_, 1[23]; Tepantepec, 9[23]; _1 mi. E Tlacolula_, 5500 ft., 53[19]; _3 mi. W Mitla_, 11; Jalapa, El Campanario, 1[23]; _2 mi. SE Matalan_, 5950 ft., 14; _Lachiguiri_, 2[23]; _Tres Cruces_, 10[23]; _Agua Blanca_, 11[23]; _San Jose_, 1[23]; Reforma, 30[19], 7[21], 10[23], 6[24] _Totolapa_, 1[18]; _Nej.a.pa_, _85 km. WNW Tehuantepec_, 500 m., 12[19], 6[24]; _Chicapa_, 2[18]; _Gueladu_ [= _Jalapa_], 6[23]; _Juchitan_, _Laguna Superior_; Manteca, 8[23], 1[23]; San Bartolo, 3000 ft., 1[18]; _Ejutla_, 1400 m., 21[19]; _El Bambita_, _Tequisitlan_ 4[23]; _Mixtequilla_, 2[23]; _Guiencola_, 5[23]; _Tehuantepec_, 200 ft., 26[18], 11[19]; _Sola de la Vega_, 26[19], 3[24]; Huilotepec, 13[18], 3[23]; _Santa Lucia_, 24[23]; _Cerro de Paste_, _Tenango_, 7[23]; _Sta. C. Quieri_, 3[23]; _Santa Marie Ecatepec_, _Zarzamora_, 13[23]; _Rincon Bamba_, 11[23]; _3 mi. W Miahuatlan_, 5300 ft., 1; _Miahuatlan_, 12[19], 1[23], 6[24]; _San Juan Acaltepec_, 5[23]; _Zapot.i.tlan_, 1[23]; _Llano Grande_, 3[18]; Pinotepa, 700 ft., 2[18]; Juquila, 8[18]; _Arroyo_, _San Juan_, _north of Cerro Otate_, 1[23]; Cerro Otate, 3[23]; 3 mi. S Candelaria, 1.

_Marginal records._--MORELOS: 5 mi, W Tepoztlan, 6000 ft. PUEBLA: 2 mi.

S Atlixco, 5800 ft.; Acatlan, 4100 ft. OAXACA: 2 mi. NW Tamazulapan, 6550 ft; Tepantepec; Oaxaca [City], 5000 ft; Yalalag, 3000 ft; Jalapa, El Campanario; Reforma; Huilotepec; 3 mi. S Candelaria; Cerro Otate; Pinotepa, 700 ft. GUERRERO: Acapulco; Zihuatanejo Bay; El Limon; 9 mi.

SE Taxco, 3800 ft.

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

[19] Univ. Michigan, Museum of Zoology.

[20] Texas A & M, Cooperative Wildlife Research Collection.

[21] Chicago Natural History Museum.

[22] California Academy of Sciences.

[23] American Museum of Natural History.

[24] University of Florida Collections.

=Baiomys musculus pullus= Packard

_Baiomys musculus pullus_ Packard, Univ. Kansas Publs., Mus. Nat.

Hist., 9:401, December 19, 1958.

_Baiomys musculus grisescens_, Goodwin, Bull. Amer. Mus. Nat. Hist., 79(2):161, May 29, 1942 (part); Miller and Kellogg, Bull. U. S. Nat.

Mus., 205:513, March 3, 1955 (part); Hall and Kelson, The Mammals of North America, 2:661, March 31, 1959 (part).

_Type._--Adult female, skin and skull; No. 71605 University of Kansas Museum of Natural History; 8 mi. S Condega, Esteli, Nicaragua, obtained on July 15, 1956, by A. A. Alcorn, original number 4218.

_Range._--West-central Nicaragua, from Matagalpa northwest into the valley of the Rio Esteli, east as far as Jinotega, see Figure 10. Zonal range: Upper Tropical Life-zone.

_Diagnosis._--Size medium to small for the species; dorsum Fuscous-Black, individual hairs black-tipped with a subterminal band of Ochraceous-Buff, Neutral Gray at base; some hairs on dorsum all black to Neutral Gray at base; hair on sides Neutral Gray tinged with blackish; face blackish, becoming buffy on sides of head, and white on throat; vibrissae black; tail unicolored Chaetura Black; forefeet and hind feet sooty to dusky-white; mid-ventral region of venter white, hairs white to base; in region of a.n.u.s and throat, hairs white-tipped, Neutral Gray at base; average and extreme external and cranial measurements of the type and 16 paratypes are as follows: total length, 117.3 (111-121); length of tail vertebrae, 47.2 (44-50); length of body, 70.4 (66-74); length of hind foot, 15.5 (14-17); length of ear from notch, 11.9 (10-13); occipitonasal length, 19.3 (18.9-19.8); zygomatic breadth, 10.2 (9.7-10.6); postpalatal length, 7.0 (6.8-7.3); least interorbital breadth, 3.9 (3.8-4.1); length of incisive foramina, 4.3 (4.0-4.6); length of rostrum, 7.0 (6.5-7.4); breadth of braincase, 9.6 (9.3-10.0); depth of cranium, 7.0 (6.8-7.3); alveolar length of maxillary tooth-row, 3.1 (3.0-3.2); for photographs of skull, see Plate 1_h_, and Plate 3_h_.

_Comparisons._--From _B. m. grisescens_, _B. m. pullus_ differs in: dorsum and tail darker; sides and lateral parts of venter grayish instead of buffy-brown, thus forming distinct mid-ventral white stripe; average length of body and tail significantly longer, thus total length greater; maxillary tooth-row significantly shorter; slightly larger in other cranial and external dimensions.

From _B. m. nigrescens_, _B. m. pullus_ differs in: dorsum slightly darker; face grayish, not sooty; mid-ventral white stripe (absent in most specimens of _nigrescens_) present and becoming grayish laterally; tail darker, less hairy, and averaging significantly longer; smaller in most external and cranial dimensions.

_Remarks._--_B. m. pullus_ resembles _B. m. nigrescens_ in size and color but can readily be distinguished from _nigrescens_ by the shorter tail. _B. m. pullus_ intergrades with _nigrescens_ as shown by specimens, referable to _B. m. nigrescens_, from 1 mi. NW San Salvador and from 1 mi. S Los Planes, El Salvador. In color of the dorsum, specimens from these localities are intermediate between _nigrescens_ and _pullus_.

The mid-ventral white stripe characteristic of _pullus_ is present in three of 28 adults from El Salvador. Goodwin (1942:160) reported white hairs on the pectoral region of several topotypes of _B. m. grisescens_.

The areas of white hairs on the venter of _grisescens_ occur in approximately 10 per cent of the specimens examined, whereas in _pullus_, the frequency of occurrence is 90 per cent. The areas of white hairs in _grisescens_ are in broad patches on the pectoral region, while in _pullus_, a white stripe pa.s.ses from the pectoral region to the inguinal region in both males and females. I know of no selective advantage that the presence of this white stripe would confer on the mice.

_Specimens examined._--Total 46, all from NICARAGUA, and distributed as follows: Type locality, 32 (including the type); _9 mi. NNW Esteli_, 8; _8 mi. NNW Esteli_, 3; San Rafael Del Norte, 1 (Amer. Mus. Nat. Hist.); _1 mi. NW Jinotega_, 1; Matagalpa, 1 (Amer. Mus. Nat. Hist.).

_Marginal records._--NICARAGUA: San Rafael Del Norte; Matagalpa; type locality.

=Baiomys taylori=

Northern Pygmy Mouse

(Synonymy under subspecies)

_Type._--_Hesperomys_ (_Vesperimus_) _taylori_ Thomas, Ann. Mag. Nat.

Hist., Ser. 5, 19:66, January, 1887.

_Range._--Southeastern Arizona and southwestern New Mexico, south into Chihuahua and Durango, just east of the Sierra Madre Occidental, thence southeast through Zacatecas, Aquascalientes, Jalisco, Queretaro, and Guanajuato; two fingerlike projections extend northward, one on the west along the coast of Sinaloa into southern Sonora, and the other on the east covering eastern San Luis Potosi, Tamaulipas, eastern Coahuila, Nuevo Leon, into south, southeast, and north-central Texas. Southern margin of range in central Mexico approximates the 19th degree of lat.i.tude (see Figure 11). Arid lower and arid upper subdivisions of the Tropical Life-zone in south; princ.i.p.ally Lower Sonoran and Lower Austral life-zones in north.

_Characters for ready recognition._--Unless otherwise noted, characters are usable for the age-categories of adult and old adult. Differs from _B. musculus_ in: hind foot less than 16 millimeters; occipitonasal length less than 19 millimeters; zygomatic breadth less than 10 millimeters; rostrum deflected ventrally at frontoparietal suture rather than curving gradually toward anteriormost point of nasals; cingular ridges and secondary cusps on teeth reduced or absent; basihyal having entoglossal process much reduced or absent, shoulders of basihyal not protruding anteriorly, but more flattened (characteristic of all age categories); baculum having narrower shaft, k.n.o.b-shaped tip, wings at base projecting laterally, baculum less than 3 millimeters long; short process of incus attenuate; muscular process of posterior crus of stapes reduced.

_Characters of the species._--Size small (extremes in external measurements of adults: total length, 87-123; length of tail vertebrae, 34-53; length of hind foot, 12-15; length of ear, 9-12). Upper parts pale drab or reddish-brown to almost black; underparts grayish to cream-buff.

_Geographic variation._--Eight subspecies are here recognized (see Figure 11). Features that vary geographically are mostly the same as those that do so in _B. musculus_ (see page 609).

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. a.n.a.logous_. Size is largest in subspecies that occur at higher alt.i.tudes. Those subspecies are _B. t. a.n.a.logous_ 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: _a.n.a.logous_, _fuliginatus_, and _subater_. The two first-mentioned subspecies occur at high alt.i.tudes, and the other, _subater_, occurs in the humid coastal region of Texas. The paler subspecies, _taylori_, _canutus_, and _allex_, occur at lower alt.i.tudes.

Two subspecies that occur at relatively high alt.i.tudes, _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.

[Ill.u.s.tration: FIG. 11. Distribution of _Baiomys taylori_. Known localities of occurrence are represented by circles and black dots; the former denote localities that are peripheral (marginal) for the subspecies concerned.