Chapter 8

[55]Texas A & M, Cooperative Wildlife Research Collection.[56]Carnegie Museum.[57]U. S. Nat. Museum (Biol. Surv. Coll.).[58]Los Angeles County Museum.[59]American Museum of Natural History.Baiomys taylori taylori(Thomas)Hesperomys(Vesperimus)tayloriThomas, Ann. Mag. Nat. Hist., ser. 5, 19:66, January, 1887.Baiomys taylori[taylori], Mearns, Bull. U. S. Nat. Mus., 56:381, April 13, 1907; Stickel and Stickel, Jour. Mamm., 30:141, May 23, 1949.Baiomys taylori taylori, Miller, Bull. U. S. Nat. Mus., 79:136, December 31, 1912; Miller, Bull. U. S. Nat. Mus., 128:317, April 29, 1924; Anthony, Field Book of North American Mammals, p. 327, 1928; Ellerman, The Families and Genera of Living Rodents, 2:402, March 21, 1941; Taylor and Davis, Texas Game, Fish and Oyster Comm. Bull., 27:56, August, 1947 (part); Blair, Texas Jour. Sci., 2:104, March 31, 1950; Goldman,[Pg 652]Smith. Miscl. Coll., 115:373, 426, July 31, 1951; Baker, Univ. Kansas Publs., Mus. Nat. Hist., 5:212, December 15, 1951; Blair, Texas Jour. Sci., 4:242, June 30, 1952; Hooper, Occas. Papers, Univ. Michigan, Mus. Zool., 544:7, March 25, 1953; Dalquest, Louisiana State Univ. Studies (Biol. Sci. Ser.), 1:155, December 28, 1953 (part); Blair, Adv. in Genetics, 5:10, January 27, 1954; Miller and Kellogg, Bull. U. S. Nat. Mus., 205:511, March 3, 1955; Baker, Univ. Kansas Publs., Mus. Nat. Hist., 9:273, June 15, 1956; Packard, Proc. Biol. Soc. Washington, 71:17, April 11, 1958; Hall and Kelson, The Mammals of North America, 2:659, March 31, 1959 (part).Cricetus(Vesperimus)taylori, Thomas, Proc. Zool. Soc. London, 68:446, November 20, 1888.Sitomys taylori, Merriam, Proc. Biol. Soc. Washington, 7:170, September 29, 1892.Sitomys(Baiomys)taylori, True, Proc. U. S. Nat. Mus., 16(972):758, February 7, 1894; J. A. Allen, Bull. Amer. Mus. Nat. Hist., 6:181, May 31, 1894.S.[itomys]taylori, Rhoads, Proc. Acad. Nat. Sci. Philadelphia, 46:256, October, 1894.Peromyscus(Baiomys)taylori, J. A. Allen, Bull. Amer. Mus. Nat. Hist., 8:65, April 22, 1896.[Peromyscus]taylori, Trouessart, Cat. Mamm., 1:517, 1898.Peromyscus taylori[taylori], Elliot, Field Columb. Mus. Publ., 105(4):135, July 1, 1905; V. Bailey, N. Amer. Fauna, 25:101, October 24, 1905; Elliot, Field Columb. Mus. Publ., 115(8):203, 1907; Osgood, N. Amer. Fauna, 28:253, April 17, 1909.Type.—Adult male, skin and skull; No. 87.11.24.1, British Museum, Natural History; San Diego, Duval County, Texas; obtained by William Taylor.Range.—North-central to southeastern Texas, excluding the coastal plain north of the region of Matagorda Bay, thence south into the southern part of Tamaulipas and west into Coahuila and Nuevo León, seeFigure 11. Occurs from near sea level in Texas up to 1500 feet in Coahuila. Zonal range: mostly Lower Austral (in México and southeastern half of Texas, the Tamaulipas Biotic Province of Goldman and Moore, 1945:349, and Blair, 1952:230).Diagnosis.—Size medium for the species; dorsum grayish in freshly taken specimens to Hair Brown in preserved specimens; individual guard hairs of dorsum black-tipped, grayish basally, underfur black-tipped with a subterminal band of olive-buff; sides of body pale-grayish near venter, individual hairs buffy proximally, grayish basally; belly pale grayish, individual hairs white-tipped, Pale Neutral Gray basally; throat and chin colored as is belly; forefeet and hind feet sooty-gray dorsally, sparsely-haired ventrally, thus appearing flesh-colored; tail unicolored gray to sooty-gray. Average and extreme cranial measurements of 22 adults from 6 mi. SW San Gerónimo, Coahuila, are as follows: occipitonasal length, 18.0 (17.4-19.0); zygomatic breadth, 9.6 (9.2-10.2); postpalatal length, 6.5 (5.9-7.1); least interorbital breadth, 3.6 (3.3-3.8); length of incisive foramina, 4.0 (3.6-4.3); length of rostrum, 6.1 (5.7-6.7); breadth of brain case, 8.8 (8.5-9.1); depth of cranium, 6.5 (6.0-7.0); alveolar length of maxillary tooth-row, 3.1 (3.0-3.3). Average and extreme external measurements of 19 adults from 6 mi. SW San Gerónimo are as follows: total length, 102.2 (95-115); length of tail vertebrae, 39.4 (21-46); length of body, 62.8 (53-76); length of hind foot, 14.0 (12-15); length of ear from notch, 10.7 (10-12); for photographs of skull, seePlate 2g, andPlate 4h.[Pg 653]Comparisons.—For comparisons withB. t. subater,B. t. analogous, andB. t. fuliginatus, see accounts of those subspecies. FromB. t. paulus, found to the southwest,B. t. tayloridiffers as follows: dorsum grayish rather than fawn-colored; hairs on dorsal parts of forefeet and hind feet sooty-gray (not white to white-brown); venter gray to Light Drab-Gray, rather than whitish with gray overtones; tail unicolored instead of bicolored; skull averaging slightly larger over-all; maxillary part of zygoma forms right angle with rostrum rather than obtuse angle; incisive foramina extending posteriorly to anterior plane of first upper molars instead of to a transverse plane at middle of right and left first upper molars; bullae less inflated; interorbital region broader relative to length of skull; rostrum sloping gently from frontonasal suture to anterior tip of nasals rather than declining abruptly from frontonasal suture to anterior tip of nasals.Remarks.—The geographic range oftayloriis relatively large, and the subspecies is locally variable. Nevertheless, none of the external and cranial measurements of specimens assigned to this subspecies differs significantly from the corresponding measurements of material from the type locality and adjacent areas in southeastern Texas. In southeastern Texas, south of the Guadalupe River, south to the coastal plain of Tamaulipas, this subspecies differs in color (being paler) fromB. t. subaterwith whichtaylorimight be confused. The foothills of the Sierra Madre Oriental in western Tamaulipas, north through Nuevo León and Coahuila, seem to mark the southwestern limit of the range assignable totaylori.On December 27, 1958, a specimen, KU 81552, was obtained 3 mi. N Bowie, Montague County, Texas. This record station extends the known range ofB. taylori65 miles northward from the previous northernmost locality, listed by Hunsaker, Raun, and Swindells (1959:447). Two specimens, KU 81553 and 81554, were collected by the author 2 mi. NE Cedar Hill, Dallas County, Texas, on October 31, 1958. These two specimens, plus the single specimen from Bowie County are all paler with more buffy bellies than eitherB. t. tayloriorB. t. subater. They may represent an incipient subspecies. I tentatively assign them toB. t. tayloribecause of the pale rather than dark (likeB. t. subater) pelage. Additional specimens are needed from these areas and from the hiatus between the ranges ofB. t. tayloriandB. t. subaterthe better to understand the manner in which these two subspecies intergrade.Among named subspecies ofBaiomys taylori,B. t. taylorimost closely resemblesB. t. subaterto the north in Texas. Nine specimens examined from Yoakum are intergrades betweentayloriandsubater. These specimens have the sooty dorsal color ofsubater, but ventrally are inseparable from topotypes oftaylori. In length of body andtail, specimens from Yoakum are likesubater, but in length of hind foot, they are intermediate between the two subspecies. Cranially, they are likesubater. When all characters are considered, the specimens are best referred tosubater. Bailey (1905:103) suggested that specimens from the southern part of the range, which he ascribed tosubater, tended to a more grayish color than topotypes ofsubater, therefore, grading intotaylori. The zone of intergradation runs from Matagorda Bay northwest through Lavaca County, thence north to the Colorado River, and closely follows the boundary between the Lower Austral and Humid Division of Lower Austral Life-zone as plotted by Bailey (loc. cit.). Findley (1955:44) pointed out that where two life-zones meet, the resulting populations of shrews are mostly intergrades. Such is the case between these two subspecies ofBaiomys tayloriin an area where life-zones might seem less important than in the mountainous west.In the southern part of the range oftaylori, intergradation occurs betweenB. t. tayloriin western Tamaulipas andB. t. fuliginatusin the mountains of San Luis Potosí.Dalquest (1953:156) found no indication of intergradation between the two species,B. tayloriandB. musculus, in San Luis Potosí. After examination of specimens from San Luis Potosí, I am in agreement that they are all referable to the speciestaylori.Specimens examined.—Total 435.Texas:Montague County: 3 mi. N Bowie, 1.Dallas County: 2 mi. NE Cedar Hill, 2.Travis County: 8 mi. NW Austin, 2[60];Austin, 2[60];4 mi. E Austin, 4[60];5 mi. E Austin, 3[60];6 mi. E Austin, 16[60], 1;7 mi. E Austin, 1[60];15 mi. E Austin, 1[60];4 mi. S Austin, 1[60].Bastrop County: 25 mi. E Austin, 2.Kendall County: Boerne, 1[61].Bexar County:1 mi. N Randolph Field, 3[64];5 mi. ENE(on U. S. Highway 81)San Antonio, 1;3 mi. NE San Antonio, 1; San Antonio, 26[61], 11[62], 1[63];5 mi. E San Antonio, 11;41/2mi. E Sayers, 3.Gonzales County: 7 mi. S Luling, 2[60].Wilson County: 4 mi. W LaVernia, 3; 12 mi. W Floresville, 1.Atascosa County: 9 mi. SW Somerset, 1.Goliad County: 8 mi. NE Goliad, 1[60].Bee County: Beeville, 1[61].Aransas County: Aransas (Wildlife) Refuge, 1[65];5 mi. E Copana Bay, 1[65];4.6 mi. NE Rockport, 5[60];4.5 mi. NW Rockport, 2[60]; 3 mi. N, 2 mi. E Rockport, 4;Rockport, 1[60], 1[61], 1[63];11/2mi. SW Rockport, 1[60];2 mi. SW Rockport, 2[60];13.4 mi. SW Rockport, 1[60];14 mi. SW Rockport, 1.San Patricio County: Welder Wildlife Refuge, 7.Duval County: type locality, 2[61], 1[66].Nueces County: Corpus Christi (south Nueces Bay), 1[64](Cleveland Mus. Coll.).Kleberg County:2 mi. S Riviera, 3[65].Brooks County: 3 mi. S Falfurrias, 2[65].Hidalgo County: 6 mi. S McAllen, 17[60].Willacy County: 28 mi. E Raymondville, 10[65].Cameron County: Brownsville, 31[61], 23[62], 5[64].Coahuila: 6 mi. SW San Gerónimo, 32.Nuevo León: Santa Catarina, 1[61]; 14 mi. N Monterrey, 1950 ft., 2[67]; Monterrey, 1[61]; 20 km. N General Terán, 3[64].Tamaulipas:Near Headwaters Río Sabinas, 8 km. W, 10 km. N El Encino, 400 ft., 1; Camargo, 5[61]; Charco Escondido, 20 mi. S Reynosa, 3[67]; Matomoras, 5[61];Ejido Santa Isabel, 2 km. W Inter-American Highway, 2000 ft., 7; Hidaglo, 7[61];Hda. Station Engracia, 4[63]; 4 mi. N La Pesca, 1; 29 mi. N Ciudad Victoria, 1[67]; Ciudad Victoria, 6[61], 3; Jaumavé, 2400 ft., 6[64], 10; Sierra de Tamaulipas, 3[64];25 mi. N El Manté, 3 km. W Inter-American Highway(on Rancho Pano Ayuctle), 300 ft., 4;6 mi. N Gomez Farias(on Rancho Pano Ayuctle), 1;5 mi. NE Gomez Farias, 12[64], 1[62]; 70 km. (by highway)S Ciudad Victoria, 2 km. W El Carrizo, 5[62], 2; Antigua Morelos, 5[64];6 mi. N, 6 mi. W Altamira, 31;5 mi. N, 5 mi. W Altamira, 4;Alta Mira(Altamira), 2[61]; 1 mi. S Altamira, 6;10 mi. NW Tampico, 1.San Luis Potosí: Ebano, 5[68];4 km. NE Ciudad Valles, 1; Ciudad Valles, 1;3 km. W Tamuín, 1[68];Tamuín, 6[68];Pujal, 300 m., 1[64].Veracruz: Tampico Alto, 50 ft., 1; Potrero Llano, 350 ft., 1; Ozulama, 2; Cerro Azul, 350 ft., 1.Marginal Records.—Texas: 3 mi. N Bowie; 2 mi. NE Cedar Hill; 25 mi. E Austin; 7 mi. S Luling; 8 mi. NE Goliad; Aransas (Wildlife) Refuge; 3 mi. N, 2 mi. E Rockport; Corpus Christi (South Nueces Bay); 2 mi. S Riviera; 28 mi. E Raymondville; Brownsville.Tamaulipas: Matomores; 4 mi. N La Pesca; 1 mi. S Altamira.Veracruz: Tampico Alto; Ozulama; Cerro Azul; Potrero Llano.San Luis Potosí: Ciudad Valles.Tamaulipas: Antigua Morelos; 70 km. S Ciudad Victoria, 2 km. W El Carrizo; Jaumavé; Hidalgo.Nuevo León: 20 km. N General Terán; Santa Catarina.Coahuila: 6 mi. SW San Gerónimo.Texas: 9 mi. SW Somerset; Boerne; 8 mi. NW Austin.[60]Coll. University of Texas.[61]U. S. Nat. Museum (Biol. Surv. Coll.).[62]American Museum of Natural History.[63]Chicago Natural History Museum.[64]Univ. Michigan, Museum of Zoology.[65]Texas A & M Coop. Wildlife Res. Coll.[66]Carnegie Museum.[67]Univ. California, Mus. Vert. Zool.[68]Museum of Natural History, Louisiana State University.EVOLUTION AND SPECIATIONThe history of the genus dates back to the early late Pliocene, but morphological change since then has been slight insofar as can be judged from lower jaws.Baiomysseems to have been relatively conservative also in types of habitat occupied.According to Wilson (1937:59), the late Pliocene was a time of decided expansion of myomorph rodents, more particularly cricetines. Furthermore, at this time, the climate in the interior basin of southwestern North America presumably was becoming arid, if we can judge from the spread of elements of the Madro-Tertiary flora. Axelrod (1950:266) points out that the drier, continental climate initiated in the early Tertiary probably had its culmination in middle Pliocene time. Some floras of early late Pliocene of the southwestern United States reflect a climate slightly cooler and more moist than the climates of the middle Pliocene. However, late Pliocene times reflect an arid climate. The flora of the southwestern interior basin of North America in early late to late Pliocene was intermediate between the previous grassland floras of the middle Pliocene and the savannah flora of upper Pliocene. Axelrod (loc. cit.) suggests that this intermediate flora of the interior basin of southwestern North America resulted from the folding of the Cascades and uplifting of the Sierra Nevada and Peninsular ranges to the south. The development of these mountains produced greater aridity to the lee of the mountains, thus accounting for the grassland-savannah flora. Pygmy mice probably originated in that time, I judge in México, and moved northward and southward ina grassland-savannah habitat that seemingly existed as far north as what is now Meade County, Kansas (where the Sawrock fauna lived). Further evidence for occupancy of a grassland-savannah habitat by ancestral pygmy mice stems from the distribution of the living species,B. taylori, that at present occupies territory adjacent to parts of the Sonoran and Chihuahuan deserts.B. tayloriseems to be morphologically more specialized for life in an arid grassland than wasB. sawrockensis.The geographic range of ancestral pygmy mice possibly extended farther south in late Pliocene time than the range ofB. musculusdoes now. Anyhow,B. sawrockensisof the early late Pliocene dwelt in a more mesic type of habitat thanB. musculusdoes, and such habitat may have existed from the Pacific lowlands of Central America to the Caribbean lowlands of northern South America (see Duellman, 1958:136, and Dunn, 1940:156) during late Pliocene times. An ancestral stock of hesperomine mice, not greatly different fromBaiomys, may have emigrated from the North American continent into South America across the continuous land connection, which Simpson (1950:395) suggests was formed in the Chapadmalalan age (= Blancan age of North American terminology). The length of time of interchange of genes between northern and southern populations of mice across the Central American land connection probably was brief. Duellman (op. cit.:129) pointed out that once the Panamanian portal was closed, the warm counter equatorial current, El Niño, combined with the uplifting of the Andes, began to produce heavy rain forests in Central America and northern South America in late Pliocene or early Pleistocene times. These forests presumably isolated the stock in North America from that in South America where the latter probably evolved rapidly into kinds that differed from one another and fromBaiomysin shape of body, type of pelage, and shape of skull. Internal structures such as hyoid apparatus, auditory ossicles, and baculum remained almost unchanged, as for example inCalomysnow living in South America. The present resemblance in internal morphological features between it andBaiomys, I judge, reflects taxonomic relationships more accurately than do shape and conformation of body and skull that seem to respond more rapidly to external environmental changes. The cranial characters distinguishingBaiomys musculusfromCalomys lauchaare as follows: posterior lacerate foramina between second, rather than first, upper molars; parapterygoid fossa shallower; mesopterygoid fossa as wide orwider, instead of narrower, than parapterygoid processes; burr for attachment of superficial masseter muscle hypertrophied instead of well-developed. In other cranial characters studied, the two genera closely resemble each other. Such similarities of crania betweenCalomysandBaiomysmay reflect convergence, but the total of internal and external morphological characters shared, I think reflects true relationships.Peromyscushas a large number of living and extinct species and exhibits a wide range of morphological variation, whereasBaiomyshas a small number (7) of species and exhibits a narrow range of morphological variation. The small number of known species of pygmy mice suggests their conservatism in elaboration of morphological characters. Possibly this is because the habitat, or even the ecological niche, occupied in geological time by these mice was restricted, geographically and in kind. If the habitat of the pygmy mice oscillated between savannah and arid grassland, then an hypothesis can be made possibly accounting for the origin of species of these mice. My idea is that the geographical distribution ofBaiomystoday reflects a predilection on the part of these mice for a relatively uniform warm climate. Therefore, in the past, in times of warmer continental climate, these mice moved toward favorable habitat northward from an area in central and northern México. In cooler periods, the mice moved southward as habitats to the north became unfavorable.Dr. W. B. Davis (in. litt.) informs me thatB. tayloriwas uncommon in Brazos County, Texas, approximately 15 years ago, and suggests that the abundance there now of this mouse and my taking it in 1958 northward nearly to the southern border of Oklahoma reflects a definite movement northward. Movement in the same direction in late years has been suggested for the nine-banded armadillo and the hispid cotton rat (Hall, 1959:373) that are associated with warm climates to the south. These movements possibly reflect only minor fluctuations of climate, but in a long period of warmth movements northward would be expected to be pronounced and extensive.Extinct species ofBaiomysmay have originated as a result of extension northward of the geographical range and subsequent retreat southward of the northern populations, as follows: (1) the range of the genus moved northward in a warm period; (2) in cooler times, most of the mice in the north disappeared and only isolated colonies remained in small patches of remaining habitatstill favorable to the mice; (3) the small populations of isolated pygmy mice after a time changed through mutations, recombinations and subsequent selection to a degree that prevented crossbreeding once populations from the south again moved northward and came in contact with previously isolated stocks; (4) then competition caused further divergence in morphological characters. Such an hypothesis would account for the morphological differences between the extinctB. kolbiandB. rexroadi. The extinctB. brachygnathus, presumably a dweller of a xerophytic grassland, may have had its origin from aB. minimus-like stock in the manner outlined.FORMATION OF THE RECENT SPECIESThe morphological difference between the extinctB. minimusand the livingB. musculusis not great, and musculus seems to be the product of theB. sawrockensis-B. minimusline of development. Morphological characters of the parental stock of the two living species,musculusandtaylori, may have been intermediate between those ofB. minimusand those ofB. musculus. The principal part of the range ofBaiomystoday is in México, and probably was there through much of Pleistocene time. Extension northward of the species and retreat southward of those northern populations of pygmy mice would not only have left isolated populations in the north, but would have allowed the mice that retreated south to share a common gene pool. Therefore, populations of pygmy mice occurring to the south in central México might be expected to maintain a relatively high degree of heterozygosity in morphological and behavioral characters. The occurrence of any physical or biotic barrier that would have separated this homogeneous group would be conducive to speciation. There is evidence that a barrier occurred in the Pleistocene in central México sufficient to separate the supposed interbreeding, relatively homogeneous populations of pygmy mice. According to Sears (1955:529) and De Terraet al. (1949:51), parts of the higher regions in the Valley of México, and the transverse volcanic zone in central México were glaciated. On the mountain Ixtaccihuatl, De Terra (op. cit.:52) found evidence of four marked advances of ice, from oldest to youngest, as follows: Salto, ice advanced to 3100 meters; Xopano, ice at 3200-3300 meters; Trancas, ice to 3400 meters; Ayolotepito, ice to 4350 meters. The Salto advance is correlated by De Terra (loc. cit.) with the Iowan glacial period. The advance of ice down the mountain sides in the transverse volcanic zone was accompanied by cool moist climatesor pluvial periods. Such climates probably altered habitat formerly suitable forBaiomys. There is no record ofBaiomysknown to me exceeding 8000 feet in elevation, although the lower edge of the ice on Ixtaccihuatl is at approximately 15,300 feet (4600 meters, Sears,loc. cit.). Presumably, the advance of ice down the mountains forced the pygmy mice to move to lower altitudes. Pluvial conditions possibly rendered the habitat even at lower altitudes uninhabitable for the mice, with the result that none continued to live in the transverse volcanic zone, but only north and south thereof. Long-continued separation of these northern and southern segments allowed species formation to occur. As climatic and habitat conditions became more favorable in central México, the two species moved back toward each other, and eventually their geographic ranges overlapped.An analysis of external and cranial characters of pygmy mice (seeFigure 12) reveals that both species are essentially largest to the north and smallest to the south. There are exceptions to this cline in both species. For example,B. taylori analogousis a large subspecies; it lives allopatrically in the southern part of the range of the species.B. musculus pallidusis not the largest subspecies; it lives allopatrically in the northern part of the range of the species. In west-central México, where the two species are sympatric,B. tayloriis smaller than elsewhere andB. musculusis larger than elsewhere.B. t. analogouslives in the mountains of the transverse volcanic zone in central México. Its large size may be a result of the cooler climate in the mountains.B. t. allex, the smallest subspecies, lives sympatrically withB. musculus musculusat lower elevations in west-central México. The small size ofallexcould be a result of the warmer climate of the lower elevations.B. m. pallidus, at lower elevations in southern Oaxaca, is smaller than other subspecies ofmusculusto the south at higher elevations.B. m. musculuslives at low elevations along the coast of west-central México. UnlikeB. m. pallidus,B. m. musculusis large at lower elevations. It occurs sympatrically withB. t. allex. It is my idea that during the period of separation, when the two species were evolving, larger subspecies evolved to the north or at higher altitudes where climates were cooler; smaller subspecies evolved to the south or at lower elevations; the two cognate species,musculusandtaylori, made contact at lower elevations where individuals oftaylorimay have been smallest, but individuals ofmusculuswere not the largest of the species. The differences, therefore, between the two species in their initial contact probably were slight. Hybrids,if they occurred, were probably inviable, sterile, or ill-suited for occupancy of the habitat of either of the parental stocks. The occurrence of hybrids, therefore, would result in what geneticists call "gamete wastage," and any further divergence in the parental stock, either in external characters (size and shape of body and head), or behavior, useful in recognition of species, would be favored by natural selection (see Dobzhansky, 1951:225; and Koopman, 1950:147). The two species seem to have diverged more in external characters where they occur together than in areas where they live separately (seeFigure 12). The two species could be confused if a sample of adults oftaylorifrom 7 mi. S La Belle, Jefferson County, Texas, were compared to a sample of adults ofmusculusfrom Tehuantepec, Oaxaca (seeFigure 12). No confusion in species identity would arise, however, if a sample of adults was taken from the area where the two species live together (seeFigure 12). Brown and Wilson (1956:49) pointed out that where two closely related species occur together, characters (morphological, ecological, physiological, or behavioral) of each species are easily distinguished. However, where the two species are allopatric, the two closely related species so resemble one another that the species are not easily distinguished. This phenomenon has been called "character displacement" by Brown and Wilson (loc. cit.).In the area where the two species of pygmy mice occur together, there seems to be a disparity in numbers between them. Hooper (1952a:91) has recorded the collection of bothB. musculusandB. tayloriin a single trap line. A series of pygmy mice collected from San Gabriel, Jalisco, contained onetayloriand 33musculus; another sample from La Resolana, Jalisco, had a ratio of 25taylorito 6musculus. The disparity in numbers where the two species occur together has been further substantiated by collections of the University of Kansas. Possibly this disparity in numbers is a result of interspecific competition. Hooper (op. cit.:90) pointed out that where the range ofB. musculus(typical of arid tropical lowlands) meets that ofB. taylori(typical of arid temperate highlands), the two geographic ranges interdigitate with parts of the range ofmusculusextending into the highlands and parts of the range oftayloriextending into the lowlands. In the lowlands,musculusmay be better adapted to environmental conditions and, therefore, more successful in competition withtaylorifor available habitat. The reverse situation may exist in the highlands. Also, the fact thatmusculusis more of a diurnal animal than istaylorimay account for the difference in numbers of individuals of the two species taken in trap lines. Many collectors set their traps in late afternoon or evening and retrieve them in early morning. Such a schedule might not yield manymusculus. If interspecific competition does occur in the area where the two species occur, any change in habits or microhabitat by either species that would reduce this competition would be favored by natural selection (see Mayr, 1949:518; Lack, 1944:262-263; and Brown, 1958:154-155). Brown (op. cit.:154), as I understand him, pointed out (taking account of Gause's principle) that when two species having similar ecological valences move into the same niche in the same locality, one of three things must eventually happen: (a) the two species occupy different geographic ranges; (b) they compete and one is eventually eliminated; (c) the two species, because of differentiation or specialization, exploit different aspects of the niche. InBaiomys, (c) seems to apply. Natural selection probably would favor a continuation of diurnal activity inmusculusand nocturnal activity intaylori, thereby preventing frequent meeting of the two species.AREAS OF PRESENT DIFFERENTIATIONIn both species ofBaiomys, the most distinct subspecies,B. t. allexandB. m. musculus, occur in the area where the two species are sympatric. Seven subspecies, or 44 per cent, occur either in or adjacent to the transverse volcanic zone. This area is the major area of active differentiation. Incipient subspecies are also evident in these areas. A secondary area of differentiation is indicated within the range ofB. musculusin Guatemala, El Salvador and Honduras. Three subspecies occur in this area (grisescens, handleyiandnigrescens) and incipient subspeciation is in evidence there.ZOOGEOGRAPHIC POSITIONHooper (1949:25) regardsBaiomysas a member of the rodent fauna of the arid, western Sonoran region, whereas Hershkovitz (1958:609) suggests thatBaiomysis a nearctic-neotropical varicant (a kind that occurs in contiguous zoogeographic regions without our knowing in which region the taxon originated). The findings from my study do not contradict either of the above suggestions. Because of the close resemblance ofBaiomysto certain hesperomine mice of South America, it is postulated thatBaiomys, in more primitive form than now, occurred farther south in past times than it does now. Fossils show that primitive stocks of the genusin late Pliocene or early Pleistocene times occurred also north of the present range of the genus. The belt in west-central México between nearctic and neotropical regions is the current center of distribution of the genus and probably has been for a considerable time.Fig. 12.Averages of the occipitonasal lengths of skulls of adults at 19 localities of occurrence (solid symbols) ofBaiomys taylori, and at 17 localities of occurrence (open symbols) ofBaiomys musculus. Note that the occipitonasal length decreases from north to south in each of the two species, and that in the region where the two species occur together, west-central México,B. tayloriis smallest andB. musculusis largest. Average, extremes, number of specimens averaged (in italic type), and name of locality, from north to south for each species, are as follows:Baiomys taylori18.0 (17.5-18.6)15, 91/2mi. W New Mexico state line, Ariz.18.9 (18.2-19.4)6, 7 mi. S. La Belle, Jefferson Co., Texas.18.2 (17.8-18.5)10, San Antonio, Bexar Co., Texas.18.2 (18.0-18.5)5, 2 mi. W Miñaca, Chihuahua.18.0 (17.6-19.0)22, 6 mi. SW San Gerónimo, Coahuila.18.2 (18.1-18.3)3, Ciudad Obregón, Sonora.18.1 (17.4-18.5)5, vic. (seep. 649) Durango, Durango.18.1 (17.5-18.5)9, Jaumavé, Tamaulipas.18.2 (17.7-18.9)19, 15 mi. N Rosario Chelé, Sinaloa.17.9 (17.4-18.3)27, vic. (seep. 655) Altamira, Tamaulipas.18.3 (17.9-18.7)9, Valparaíso, Zacatecas.18.1 (18.1-18.2)4, Ciudad del Maíz, San Luis Potosí.18.6 (18.3-18.9)8, Tepic, Nayarit.18.0 (17.7-18.4)18, 4 mi. N, 5 mi. W León, Guanajuato.18.1 (17.5-18.9)28, 6 mi. E Querétaro, Querétaro.17.7 (17.1-18.1)17, 1 mi. SSE Ameca, Jalisco.17.3 (16.8-17.9)10, 2 mi. SSE Autlán, Jalisco.18.0 (17.5-18.6)10, 1 mi. S, 11 mi. W Zamora, Michoacán.17.6 (17.4-18.2)8, Colima, Colima.Baiomys musculus20.2 (19.9-20.3)6, vic. (seep. 622) Ameca, Jalisco.20.2 (19.9-20.3)6, 2 mi. SSE Autlán, Jalisco.19.6 (19.2-20.1)6, Jalapa, Veracruz.20.3 (19.7-20.9)9, Colima, Colima.19.5 (19.0-20.0)10, Cerro Gordo, Veracruz.19.8 (19.4-20.3)6, 6 mi. S Izucár de Matemores, Puebla.20.0 (18.8-20.5)7, Teotitlán, Oaxaca.20.1 (19.7-20.7)7, 1 km. NW Chapa, Guerrero.19.9 (19.4-20.4)8, 5 mi. ESE Tecpán, Guerrero.19.5 (19.1-20.1)22, 3 mi. ESE Oaxaca, Oaxaca.19.5 (19.1-19.9)11, Valley of Comitán, Chiapas.18.9 (18.2-20.1)17, Tehuantepec, Oaxaca.18.9 (18.4-19.7)15, 6 mi. NW Tonalá, Chiapas.19.1 (18.8-20.4)10, 1 mi. S Rabinal, Guatemala.19.7 (18.8-20.4)10, Lake Amatitlán, Guatemala.19.2 (18.4-19.8)26, vic. (seep. 625) San Salvador, El Salvador.19.3 (18.9-19.9)24, 8 mi. S Condega, Estelí, Nicaragua.

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

[56]Carnegie Museum.

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

[58]Los Angeles County Museum.

[59]American Museum of Natural History.

Baiomys taylori taylori(Thomas)

Hesperomys(Vesperimus)tayloriThomas, Ann. Mag. Nat. Hist., ser. 5, 19:66, January, 1887.

Baiomys taylori[taylori], Mearns, Bull. U. S. Nat. Mus., 56:381, April 13, 1907; Stickel and Stickel, Jour. Mamm., 30:141, May 23, 1949.

Baiomys taylori taylori, Miller, Bull. U. S. Nat. Mus., 79:136, December 31, 1912; Miller, Bull. U. S. Nat. Mus., 128:317, April 29, 1924; Anthony, Field Book of North American Mammals, p. 327, 1928; Ellerman, The Families and Genera of Living Rodents, 2:402, March 21, 1941; Taylor and Davis, Texas Game, Fish and Oyster Comm. Bull., 27:56, August, 1947 (part); Blair, Texas Jour. Sci., 2:104, March 31, 1950; Goldman,[Pg 652]Smith. Miscl. Coll., 115:373, 426, July 31, 1951; Baker, Univ. Kansas Publs., Mus. Nat. Hist., 5:212, December 15, 1951; Blair, Texas Jour. Sci., 4:242, June 30, 1952; Hooper, Occas. Papers, Univ. Michigan, Mus. Zool., 544:7, March 25, 1953; Dalquest, Louisiana State Univ. Studies (Biol. Sci. Ser.), 1:155, December 28, 1953 (part); Blair, Adv. in Genetics, 5:10, January 27, 1954; Miller and Kellogg, Bull. U. S. Nat. Mus., 205:511, March 3, 1955; Baker, Univ. Kansas Publs., Mus. Nat. Hist., 9:273, June 15, 1956; Packard, Proc. Biol. Soc. Washington, 71:17, April 11, 1958; Hall and Kelson, The Mammals of North America, 2:659, March 31, 1959 (part).

Cricetus(Vesperimus)taylori, Thomas, Proc. Zool. Soc. London, 68:446, November 20, 1888.

Sitomys taylori, Merriam, Proc. Biol. Soc. Washington, 7:170, September 29, 1892.

Sitomys(Baiomys)taylori, True, Proc. U. S. Nat. Mus., 16(972):758, February 7, 1894; J. A. Allen, Bull. Amer. Mus. Nat. Hist., 6:181, May 31, 1894.

S.[itomys]taylori, Rhoads, Proc. Acad. Nat. Sci. Philadelphia, 46:256, October, 1894.

Peromyscus(Baiomys)taylori, J. A. Allen, Bull. Amer. Mus. Nat. Hist., 8:65, April 22, 1896.

[Peromyscus]taylori, Trouessart, Cat. Mamm., 1:517, 1898.

Peromyscus taylori[taylori], Elliot, Field Columb. Mus. Publ., 105(4):135, July 1, 1905; V. Bailey, N. Amer. Fauna, 25:101, October 24, 1905; Elliot, Field Columb. Mus. Publ., 115(8):203, 1907; Osgood, N. Amer. Fauna, 28:253, April 17, 1909.

Type.—Adult male, skin and skull; No. 87.11.24.1, British Museum, Natural History; San Diego, Duval County, Texas; obtained by William Taylor.Range.—North-central to southeastern Texas, excluding the coastal plain north of the region of Matagorda Bay, thence south into the southern part of Tamaulipas and west into Coahuila and Nuevo León, seeFigure 11. Occurs from near sea level in Texas up to 1500 feet in Coahuila. Zonal range: mostly Lower Austral (in México and southeastern half of Texas, the Tamaulipas Biotic Province of Goldman and Moore, 1945:349, and Blair, 1952:230).Diagnosis.—Size medium for the species; dorsum grayish in freshly taken specimens to Hair Brown in preserved specimens; individual guard hairs of dorsum black-tipped, grayish basally, underfur black-tipped with a subterminal band of olive-buff; sides of body pale-grayish near venter, individual hairs buffy proximally, grayish basally; belly pale grayish, individual hairs white-tipped, Pale Neutral Gray basally; throat and chin colored as is belly; forefeet and hind feet sooty-gray dorsally, sparsely-haired ventrally, thus appearing flesh-colored; tail unicolored gray to sooty-gray. Average and extreme cranial measurements of 22 adults from 6 mi. SW San Gerónimo, Coahuila, are as follows: occipitonasal length, 18.0 (17.4-19.0); zygomatic breadth, 9.6 (9.2-10.2); postpalatal length, 6.5 (5.9-7.1); least interorbital breadth, 3.6 (3.3-3.8); length of incisive foramina, 4.0 (3.6-4.3); length of rostrum, 6.1 (5.7-6.7); breadth of brain case, 8.8 (8.5-9.1); depth of cranium, 6.5 (6.0-7.0); alveolar length of maxillary tooth-row, 3.1 (3.0-3.3). Average and extreme external measurements of 19 adults from 6 mi. SW San Gerónimo are as follows: total length, 102.2 (95-115); length of tail vertebrae, 39.4 (21-46); length of body, 62.8 (53-76); length of hind foot, 14.0 (12-15); length of ear from notch, 10.7 (10-12); for photographs of skull, seePlate 2g, andPlate 4h.[Pg 653]Comparisons.—For comparisons withB. t. subater,B. t. analogous, andB. t. fuliginatus, see accounts of those subspecies. FromB. t. paulus, found to the southwest,B. t. tayloridiffers as follows: dorsum grayish rather than fawn-colored; hairs on dorsal parts of forefeet and hind feet sooty-gray (not white to white-brown); venter gray to Light Drab-Gray, rather than whitish with gray overtones; tail unicolored instead of bicolored; skull averaging slightly larger over-all; maxillary part of zygoma forms right angle with rostrum rather than obtuse angle; incisive foramina extending posteriorly to anterior plane of first upper molars instead of to a transverse plane at middle of right and left first upper molars; bullae less inflated; interorbital region broader relative to length of skull; rostrum sloping gently from frontonasal suture to anterior tip of nasals rather than declining abruptly from frontonasal suture to anterior tip of nasals.

Type.—Adult male, skin and skull; No. 87.11.24.1, British Museum, Natural History; San Diego, Duval County, Texas; obtained by William Taylor.

Range.—North-central to southeastern Texas, excluding the coastal plain north of the region of Matagorda Bay, thence south into the southern part of Tamaulipas and west into Coahuila and Nuevo León, seeFigure 11. Occurs from near sea level in Texas up to 1500 feet in Coahuila. Zonal range: mostly Lower Austral (in México and southeastern half of Texas, the Tamaulipas Biotic Province of Goldman and Moore, 1945:349, and Blair, 1952:230).

Diagnosis.—Size medium for the species; dorsum grayish in freshly taken specimens to Hair Brown in preserved specimens; individual guard hairs of dorsum black-tipped, grayish basally, underfur black-tipped with a subterminal band of olive-buff; sides of body pale-grayish near venter, individual hairs buffy proximally, grayish basally; belly pale grayish, individual hairs white-tipped, Pale Neutral Gray basally; throat and chin colored as is belly; forefeet and hind feet sooty-gray dorsally, sparsely-haired ventrally, thus appearing flesh-colored; tail unicolored gray to sooty-gray. Average and extreme cranial measurements of 22 adults from 6 mi. SW San Gerónimo, Coahuila, are as follows: occipitonasal length, 18.0 (17.4-19.0); zygomatic breadth, 9.6 (9.2-10.2); postpalatal length, 6.5 (5.9-7.1); least interorbital breadth, 3.6 (3.3-3.8); length of incisive foramina, 4.0 (3.6-4.3); length of rostrum, 6.1 (5.7-6.7); breadth of brain case, 8.8 (8.5-9.1); depth of cranium, 6.5 (6.0-7.0); alveolar length of maxillary tooth-row, 3.1 (3.0-3.3). Average and extreme external measurements of 19 adults from 6 mi. SW San Gerónimo are as follows: total length, 102.2 (95-115); length of tail vertebrae, 39.4 (21-46); length of body, 62.8 (53-76); length of hind foot, 14.0 (12-15); length of ear from notch, 10.7 (10-12); for photographs of skull, seePlate 2g, andPlate 4h.

[Pg 653]

Comparisons.—For comparisons withB. t. subater,B. t. analogous, andB. t. fuliginatus, see accounts of those subspecies. FromB. t. paulus, found to the southwest,B. t. tayloridiffers as follows: dorsum grayish rather than fawn-colored; hairs on dorsal parts of forefeet and hind feet sooty-gray (not white to white-brown); venter gray to Light Drab-Gray, rather than whitish with gray overtones; tail unicolored instead of bicolored; skull averaging slightly larger over-all; maxillary part of zygoma forms right angle with rostrum rather than obtuse angle; incisive foramina extending posteriorly to anterior plane of first upper molars instead of to a transverse plane at middle of right and left first upper molars; bullae less inflated; interorbital region broader relative to length of skull; rostrum sloping gently from frontonasal suture to anterior tip of nasals rather than declining abruptly from frontonasal suture to anterior tip of nasals.

Remarks.—The geographic range oftayloriis relatively large, and the subspecies is locally variable. Nevertheless, none of the external and cranial measurements of specimens assigned to this subspecies differs significantly from the corresponding measurements of material from the type locality and adjacent areas in southeastern Texas. In southeastern Texas, south of the Guadalupe River, south to the coastal plain of Tamaulipas, this subspecies differs in color (being paler) fromB. t. subaterwith whichtaylorimight be confused. The foothills of the Sierra Madre Oriental in western Tamaulipas, north through Nuevo León and Coahuila, seem to mark the southwestern limit of the range assignable totaylori.

On December 27, 1958, a specimen, KU 81552, was obtained 3 mi. N Bowie, Montague County, Texas. This record station extends the known range ofB. taylori65 miles northward from the previous northernmost locality, listed by Hunsaker, Raun, and Swindells (1959:447). Two specimens, KU 81553 and 81554, were collected by the author 2 mi. NE Cedar Hill, Dallas County, Texas, on October 31, 1958. These two specimens, plus the single specimen from Bowie County are all paler with more buffy bellies than eitherB. t. tayloriorB. t. subater. They may represent an incipient subspecies. I tentatively assign them toB. t. tayloribecause of the pale rather than dark (likeB. t. subater) pelage. Additional specimens are needed from these areas and from the hiatus between the ranges ofB. t. tayloriandB. t. subaterthe better to understand the manner in which these two subspecies intergrade.

Among named subspecies ofBaiomys taylori,B. t. taylorimost closely resemblesB. t. subaterto the north in Texas. Nine specimens examined from Yoakum are intergrades betweentayloriandsubater. These specimens have the sooty dorsal color ofsubater, but ventrally are inseparable from topotypes oftaylori. In length of body andtail, specimens from Yoakum are likesubater, but in length of hind foot, they are intermediate between the two subspecies. Cranially, they are likesubater. When all characters are considered, the specimens are best referred tosubater. Bailey (1905:103) suggested that specimens from the southern part of the range, which he ascribed tosubater, tended to a more grayish color than topotypes ofsubater, therefore, grading intotaylori. The zone of intergradation runs from Matagorda Bay northwest through Lavaca County, thence north to the Colorado River, and closely follows the boundary between the Lower Austral and Humid Division of Lower Austral Life-zone as plotted by Bailey (loc. cit.). Findley (1955:44) pointed out that where two life-zones meet, the resulting populations of shrews are mostly intergrades. Such is the case between these two subspecies ofBaiomys tayloriin an area where life-zones might seem less important than in the mountainous west.

In the southern part of the range oftaylori, intergradation occurs betweenB. t. tayloriin western Tamaulipas andB. t. fuliginatusin the mountains of San Luis Potosí.

Dalquest (1953:156) found no indication of intergradation between the two species,B. tayloriandB. musculus, in San Luis Potosí. After examination of specimens from San Luis Potosí, I am in agreement that they are all referable to the speciestaylori.

Specimens examined.—Total 435.Texas:Montague County: 3 mi. N Bowie, 1.Dallas County: 2 mi. NE Cedar Hill, 2.Travis County: 8 mi. NW Austin, 2[60];Austin, 2[60];4 mi. E Austin, 4[60];5 mi. E Austin, 3[60];6 mi. E Austin, 16[60], 1;7 mi. E Austin, 1[60];15 mi. E Austin, 1[60];4 mi. S Austin, 1[60].Bastrop County: 25 mi. E Austin, 2.Kendall County: Boerne, 1[61].Bexar County:1 mi. N Randolph Field, 3[64];5 mi. ENE(on U. S. Highway 81)San Antonio, 1;3 mi. NE San Antonio, 1; San Antonio, 26[61], 11[62], 1[63];5 mi. E San Antonio, 11;41/2mi. E Sayers, 3.Gonzales County: 7 mi. S Luling, 2[60].Wilson County: 4 mi. W LaVernia, 3; 12 mi. W Floresville, 1.Atascosa County: 9 mi. SW Somerset, 1.Goliad County: 8 mi. NE Goliad, 1[60].Bee County: Beeville, 1[61].Aransas County: Aransas (Wildlife) Refuge, 1[65];5 mi. E Copana Bay, 1[65];4.6 mi. NE Rockport, 5[60];4.5 mi. NW Rockport, 2[60]; 3 mi. N, 2 mi. E Rockport, 4;Rockport, 1[60], 1[61], 1[63];11/2mi. SW Rockport, 1[60];2 mi. SW Rockport, 2[60];13.4 mi. SW Rockport, 1[60];14 mi. SW Rockport, 1.San Patricio County: Welder Wildlife Refuge, 7.Duval County: type locality, 2[61], 1[66].Nueces County: Corpus Christi (south Nueces Bay), 1[64](Cleveland Mus. Coll.).Kleberg County:2 mi. S Riviera, 3[65].Brooks County: 3 mi. S Falfurrias, 2[65].Hidalgo County: 6 mi. S McAllen, 17[60].Willacy County: 28 mi. E Raymondville, 10[65].Cameron County: Brownsville, 31[61], 23[62], 5[64].Coahuila: 6 mi. SW San Gerónimo, 32.Nuevo León: Santa Catarina, 1[61]; 14 mi. N Monterrey, 1950 ft., 2[67]; Monterrey, 1[61]; 20 km. N General Terán, 3[64].Tamaulipas:Near Headwaters Río Sabinas, 8 km. W, 10 km. N El Encino, 400 ft., 1; Camargo, 5[61]; Charco Escondido, 20 mi. S Reynosa, 3[67]; Matomoras, 5[61];Ejido Santa Isabel, 2 km. W Inter-American Highway, 2000 ft., 7; Hidaglo, 7[61];Hda. Station Engracia, 4[63]; 4 mi. N La Pesca, 1; 29 mi. N Ciudad Victoria, 1[67]; Ciudad Victoria, 6[61], 3; Jaumavé, 2400 ft., 6[64], 10; Sierra de Tamaulipas, 3[64];25 mi. N El Manté, 3 km. W Inter-American Highway(on Rancho Pano Ayuctle), 300 ft., 4;6 mi. N Gomez Farias(on Rancho Pano Ayuctle), 1;5 mi. NE Gomez Farias, 12[64], 1[62]; 70 km. (by highway)S Ciudad Victoria, 2 km. W El Carrizo, 5[62], 2; Antigua Morelos, 5[64];6 mi. N, 6 mi. W Altamira, 31;5 mi. N, 5 mi. W Altamira, 4;Alta Mira(Altamira), 2[61]; 1 mi. S Altamira, 6;10 mi. NW Tampico, 1.San Luis Potosí: Ebano, 5[68];4 km. NE Ciudad Valles, 1; Ciudad Valles, 1;3 km. W Tamuín, 1[68];Tamuín, 6[68];Pujal, 300 m., 1[64].Veracruz: Tampico Alto, 50 ft., 1; Potrero Llano, 350 ft., 1; Ozulama, 2; Cerro Azul, 350 ft., 1.Marginal Records.—Texas: 3 mi. N Bowie; 2 mi. NE Cedar Hill; 25 mi. E Austin; 7 mi. S Luling; 8 mi. NE Goliad; Aransas (Wildlife) Refuge; 3 mi. N, 2 mi. E Rockport; Corpus Christi (South Nueces Bay); 2 mi. S Riviera; 28 mi. E Raymondville; Brownsville.Tamaulipas: Matomores; 4 mi. N La Pesca; 1 mi. S Altamira.Veracruz: Tampico Alto; Ozulama; Cerro Azul; Potrero Llano.San Luis Potosí: Ciudad Valles.Tamaulipas: Antigua Morelos; 70 km. S Ciudad Victoria, 2 km. W El Carrizo; Jaumavé; Hidalgo.Nuevo León: 20 km. N General Terán; Santa Catarina.Coahuila: 6 mi. SW San Gerónimo.Texas: 9 mi. SW Somerset; Boerne; 8 mi. NW Austin.

Marginal Records.—Texas: 3 mi. N Bowie; 2 mi. NE Cedar Hill; 25 mi. E Austin; 7 mi. S Luling; 8 mi. NE Goliad; Aransas (Wildlife) Refuge; 3 mi. N, 2 mi. E Rockport; Corpus Christi (South Nueces Bay); 2 mi. S Riviera; 28 mi. E Raymondville; Brownsville.Tamaulipas: Matomores; 4 mi. N La Pesca; 1 mi. S Altamira.Veracruz: Tampico Alto; Ozulama; Cerro Azul; Potrero Llano.San Luis Potosí: Ciudad Valles.Tamaulipas: Antigua Morelos; 70 km. S Ciudad Victoria, 2 km. W El Carrizo; Jaumavé; Hidalgo.Nuevo León: 20 km. N General Terán; Santa Catarina.Coahuila: 6 mi. SW San Gerónimo.Texas: 9 mi. SW Somerset; Boerne; 8 mi. NW Austin.

[60]Coll. University of Texas.

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

[62]American Museum of Natural History.

[63]Chicago Natural History Museum.

[64]Univ. Michigan, Museum of Zoology.

[65]Texas A & M Coop. Wildlife Res. Coll

[66]Carnegie Museum.

[67]Univ. California, Mus. Vert. Zool.

[68]Museum of Natural History, Louisiana State University.

EVOLUTION AND SPECIATION

The history of the genus dates back to the early late Pliocene, but morphological change since then has been slight insofar as can be judged from lower jaws.Baiomysseems to have been relatively conservative also in types of habitat occupied.

According to Wilson (1937:59), the late Pliocene was a time of decided expansion of myomorph rodents, more particularly cricetines. Furthermore, at this time, the climate in the interior basin of southwestern North America presumably was becoming arid, if we can judge from the spread of elements of the Madro-Tertiary flora. Axelrod (1950:266) points out that the drier, continental climate initiated in the early Tertiary probably had its culmination in middle Pliocene time. Some floras of early late Pliocene of the southwestern United States reflect a climate slightly cooler and more moist than the climates of the middle Pliocene. However, late Pliocene times reflect an arid climate. The flora of the southwestern interior basin of North America in early late to late Pliocene was intermediate between the previous grassland floras of the middle Pliocene and the savannah flora of upper Pliocene. Axelrod (loc. cit.) suggests that this intermediate flora of the interior basin of southwestern North America resulted from the folding of the Cascades and uplifting of the Sierra Nevada and Peninsular ranges to the south. The development of these mountains produced greater aridity to the lee of the mountains, thus accounting for the grassland-savannah flora. Pygmy mice probably originated in that time, I judge in México, and moved northward and southward ina grassland-savannah habitat that seemingly existed as far north as what is now Meade County, Kansas (where the Sawrock fauna lived). Further evidence for occupancy of a grassland-savannah habitat by ancestral pygmy mice stems from the distribution of the living species,B. taylori, that at present occupies territory adjacent to parts of the Sonoran and Chihuahuan deserts.B. tayloriseems to be morphologically more specialized for life in an arid grassland than wasB. sawrockensis.

The geographic range of ancestral pygmy mice possibly extended farther south in late Pliocene time than the range ofB. musculusdoes now. Anyhow,B. sawrockensisof the early late Pliocene dwelt in a more mesic type of habitat thanB. musculusdoes, and such habitat may have existed from the Pacific lowlands of Central America to the Caribbean lowlands of northern South America (see Duellman, 1958:136, and Dunn, 1940:156) during late Pliocene times. An ancestral stock of hesperomine mice, not greatly different fromBaiomys, may have emigrated from the North American continent into South America across the continuous land connection, which Simpson (1950:395) suggests was formed in the Chapadmalalan age (= Blancan age of North American terminology). The length of time of interchange of genes between northern and southern populations of mice across the Central American land connection probably was brief. Duellman (op. cit.:129) pointed out that once the Panamanian portal was closed, the warm counter equatorial current, El Niño, combined with the uplifting of the Andes, began to produce heavy rain forests in Central America and northern South America in late Pliocene or early Pleistocene times. These forests presumably isolated the stock in North America from that in South America where the latter probably evolved rapidly into kinds that differed from one another and fromBaiomysin shape of body, type of pelage, and shape of skull. Internal structures such as hyoid apparatus, auditory ossicles, and baculum remained almost unchanged, as for example inCalomysnow living in South America. The present resemblance in internal morphological features between it andBaiomys, I judge, reflects taxonomic relationships more accurately than do shape and conformation of body and skull that seem to respond more rapidly to external environmental changes. The cranial characters distinguishingBaiomys musculusfromCalomys lauchaare as follows: posterior lacerate foramina between second, rather than first, upper molars; parapterygoid fossa shallower; mesopterygoid fossa as wide orwider, instead of narrower, than parapterygoid processes; burr for attachment of superficial masseter muscle hypertrophied instead of well-developed. In other cranial characters studied, the two genera closely resemble each other. Such similarities of crania betweenCalomysandBaiomysmay reflect convergence, but the total of internal and external morphological characters shared, I think reflects true relationships.

Peromyscushas a large number of living and extinct species and exhibits a wide range of morphological variation, whereasBaiomyshas a small number (7) of species and exhibits a narrow range of morphological variation. The small number of known species of pygmy mice suggests their conservatism in elaboration of morphological characters. Possibly this is because the habitat, or even the ecological niche, occupied in geological time by these mice was restricted, geographically and in kind. If the habitat of the pygmy mice oscillated between savannah and arid grassland, then an hypothesis can be made possibly accounting for the origin of species of these mice. My idea is that the geographical distribution ofBaiomystoday reflects a predilection on the part of these mice for a relatively uniform warm climate. Therefore, in the past, in times of warmer continental climate, these mice moved toward favorable habitat northward from an area in central and northern México. In cooler periods, the mice moved southward as habitats to the north became unfavorable.

Dr. W. B. Davis (in. litt.) informs me thatB. tayloriwas uncommon in Brazos County, Texas, approximately 15 years ago, and suggests that the abundance there now of this mouse and my taking it in 1958 northward nearly to the southern border of Oklahoma reflects a definite movement northward. Movement in the same direction in late years has been suggested for the nine-banded armadillo and the hispid cotton rat (Hall, 1959:373) that are associated with warm climates to the south. These movements possibly reflect only minor fluctuations of climate, but in a long period of warmth movements northward would be expected to be pronounced and extensive.

Extinct species ofBaiomysmay have originated as a result of extension northward of the geographical range and subsequent retreat southward of the northern populations, as follows: (1) the range of the genus moved northward in a warm period; (2) in cooler times, most of the mice in the north disappeared and only isolated colonies remained in small patches of remaining habitatstill favorable to the mice; (3) the small populations of isolated pygmy mice after a time changed through mutations, recombinations and subsequent selection to a degree that prevented crossbreeding once populations from the south again moved northward and came in contact with previously isolated stocks; (4) then competition caused further divergence in morphological characters. Such an hypothesis would account for the morphological differences between the extinctB. kolbiandB. rexroadi. The extinctB. brachygnathus, presumably a dweller of a xerophytic grassland, may have had its origin from aB. minimus-like stock in the manner outlined.

FORMATION OF THE RECENT SPECIES

The morphological difference between the extinctB. minimusand the livingB. musculusis not great, and musculus seems to be the product of theB. sawrockensis-B. minimusline of development. Morphological characters of the parental stock of the two living species,musculusandtaylori, may have been intermediate between those ofB. minimusand those ofB. musculus. The principal part of the range ofBaiomystoday is in México, and probably was there through much of Pleistocene time. Extension northward of the species and retreat southward of those northern populations of pygmy mice would not only have left isolated populations in the north, but would have allowed the mice that retreated south to share a common gene pool. Therefore, populations of pygmy mice occurring to the south in central México might be expected to maintain a relatively high degree of heterozygosity in morphological and behavioral characters. The occurrence of any physical or biotic barrier that would have separated this homogeneous group would be conducive to speciation. There is evidence that a barrier occurred in the Pleistocene in central México sufficient to separate the supposed interbreeding, relatively homogeneous populations of pygmy mice. According to Sears (1955:529) and De Terraet al. (1949:51), parts of the higher regions in the Valley of México, and the transverse volcanic zone in central México were glaciated. On the mountain Ixtaccihuatl, De Terra (op. cit.:52) found evidence of four marked advances of ice, from oldest to youngest, as follows: Salto, ice advanced to 3100 meters; Xopano, ice at 3200-3300 meters; Trancas, ice to 3400 meters; Ayolotepito, ice to 4350 meters. The Salto advance is correlated by De Terra (loc. cit.) with the Iowan glacial period. The advance of ice down the mountain sides in the transverse volcanic zone was accompanied by cool moist climatesor pluvial periods. Such climates probably altered habitat formerly suitable forBaiomys. There is no record ofBaiomysknown to me exceeding 8000 feet in elevation, although the lower edge of the ice on Ixtaccihuatl is at approximately 15,300 feet (4600 meters, Sears,loc. cit.). Presumably, the advance of ice down the mountains forced the pygmy mice to move to lower altitudes. Pluvial conditions possibly rendered the habitat even at lower altitudes uninhabitable for the mice, with the result that none continued to live in the transverse volcanic zone, but only north and south thereof. Long-continued separation of these northern and southern segments allowed species formation to occur. As climatic and habitat conditions became more favorable in central México, the two species moved back toward each other, and eventually their geographic ranges overlapped.

An analysis of external and cranial characters of pygmy mice (seeFigure 12) reveals that both species are essentially largest to the north and smallest to the south. There are exceptions to this cline in both species. For example,B. taylori analogousis a large subspecies; it lives allopatrically in the southern part of the range of the species.B. musculus pallidusis not the largest subspecies; it lives allopatrically in the northern part of the range of the species. In west-central México, where the two species are sympatric,B. tayloriis smaller than elsewhere andB. musculusis larger than elsewhere.B. t. analogouslives in the mountains of the transverse volcanic zone in central México. Its large size may be a result of the cooler climate in the mountains.B. t. allex, the smallest subspecies, lives sympatrically withB. musculus musculusat lower elevations in west-central México. The small size ofallexcould be a result of the warmer climate of the lower elevations.B. m. pallidus, at lower elevations in southern Oaxaca, is smaller than other subspecies ofmusculusto the south at higher elevations.B. m. musculuslives at low elevations along the coast of west-central México. UnlikeB. m. pallidus,B. m. musculusis large at lower elevations. It occurs sympatrically withB. t. allex. It is my idea that during the period of separation, when the two species were evolving, larger subspecies evolved to the north or at higher altitudes where climates were cooler; smaller subspecies evolved to the south or at lower elevations; the two cognate species,musculusandtaylori, made contact at lower elevations where individuals oftaylorimay have been smallest, but individuals ofmusculuswere not the largest of the species. The differences, therefore, between the two species in their initial contact probably were slight. Hybrids,if they occurred, were probably inviable, sterile, or ill-suited for occupancy of the habitat of either of the parental stocks. The occurrence of hybrids, therefore, would result in what geneticists call "gamete wastage," and any further divergence in the parental stock, either in external characters (size and shape of body and head), or behavior, useful in recognition of species, would be favored by natural selection (see Dobzhansky, 1951:225; and Koopman, 1950:147). The two species seem to have diverged more in external characters where they occur together than in areas where they live separately (seeFigure 12). The two species could be confused if a sample of adults oftaylorifrom 7 mi. S La Belle, Jefferson County, Texas, were compared to a sample of adults ofmusculusfrom Tehuantepec, Oaxaca (seeFigure 12). No confusion in species identity would arise, however, if a sample of adults was taken from the area where the two species live together (seeFigure 12). Brown and Wilson (1956:49) pointed out that where two closely related species occur together, characters (morphological, ecological, physiological, or behavioral) of each species are easily distinguished. However, where the two species are allopatric, the two closely related species so resemble one another that the species are not easily distinguished. This phenomenon has been called "character displacement" by Brown and Wilson (loc. cit.).

In the area where the two species of pygmy mice occur together, there seems to be a disparity in numbers between them. Hooper (1952a:91) has recorded the collection of bothB. musculusandB. tayloriin a single trap line. A series of pygmy mice collected from San Gabriel, Jalisco, contained onetayloriand 33musculus; another sample from La Resolana, Jalisco, had a ratio of 25taylorito 6musculus. The disparity in numbers where the two species occur together has been further substantiated by collections of the University of Kansas. Possibly this disparity in numbers is a result of interspecific competition. Hooper (op. cit.:90) pointed out that where the range ofB. musculus(typical of arid tropical lowlands) meets that ofB. taylori(typical of arid temperate highlands), the two geographic ranges interdigitate with parts of the range ofmusculusextending into the highlands and parts of the range oftayloriextending into the lowlands. In the lowlands,musculusmay be better adapted to environmental conditions and, therefore, more successful in competition withtaylorifor available habitat. The reverse situation may exist in the highlands. Also, the fact thatmusculusis more of a diurnal animal than istaylorimay account for the difference in numbers of individuals of the two species taken in trap lines. Many collectors set their traps in late afternoon or evening and retrieve them in early morning. Such a schedule might not yield manymusculus. If interspecific competition does occur in the area where the two species occur, any change in habits or microhabitat by either species that would reduce this competition would be favored by natural selection (see Mayr, 1949:518; Lack, 1944:262-263; and Brown, 1958:154-155). Brown (op. cit.:154), as I understand him, pointed out (taking account of Gause's principle) that when two species having similar ecological valences move into the same niche in the same locality, one of three things must eventually happen: (a) the two species occupy different geographic ranges; (b) they compete and one is eventually eliminated; (c) the two species, because of differentiation or specialization, exploit different aspects of the niche. InBaiomys, (c) seems to apply. Natural selection probably would favor a continuation of diurnal activity inmusculusand nocturnal activity intaylori, thereby preventing frequent meeting of the two species.

AREAS OF PRESENT DIFFERENTIATION

In both species ofBaiomys, the most distinct subspecies,B. t. allexandB. m. musculus, occur in the area where the two species are sympatric. Seven subspecies, or 44 per cent, occur either in or adjacent to the transverse volcanic zone. This area is the major area of active differentiation. Incipient subspecies are also evident in these areas. A secondary area of differentiation is indicated within the range ofB. musculusin Guatemala, El Salvador and Honduras. Three subspecies occur in this area (grisescens, handleyiandnigrescens) and incipient subspeciation is in evidence there.

ZOOGEOGRAPHIC POSITION

Hooper (1949:25) regardsBaiomysas a member of the rodent fauna of the arid, western Sonoran region, whereas Hershkovitz (1958:609) suggests thatBaiomysis a nearctic-neotropical varicant (a kind that occurs in contiguous zoogeographic regions without our knowing in which region the taxon originated). The findings from my study do not contradict either of the above suggestions. Because of the close resemblance ofBaiomysto certain hesperomine mice of South America, it is postulated thatBaiomys, in more primitive form than now, occurred farther south in past times than it does now. Fossils show that primitive stocks of the genusin late Pliocene or early Pleistocene times occurred also north of the present range of the genus. The belt in west-central México between nearctic and neotropical regions is the current center of distribution of the genus and probably has been for a considerable time.

Fig. 12.Averages of the occipitonasal lengths of skulls of adults at 19 localities of occurrence (solid symbols) ofBaiomys taylori, and at 17 localities of occurrence (open symbols) ofBaiomys musculus. Note that the occipitonasal length decreases from north to south in each of the two species, and that in the region where the two species occur together, west-central México,B. tayloriis smallest andB. musculusis largest. Average, extremes, number of specimens averaged (in italic type), and name of locality, from north to south for each species, are as follows:

Baiomys taylori18.0 (17.5-18.6)15, 91/2mi. W New Mexico state line, Ariz.18.9 (18.2-19.4)6, 7 mi. S. La Belle, Jefferson Co., Texas.18.2 (17.8-18.5)10, San Antonio, Bexar Co., Texas.18.2 (18.0-18.5)5, 2 mi. W Miñaca, Chihuahua.18.0 (17.6-19.0)22, 6 mi. SW San Gerónimo, Coahuila.18.2 (18.1-18.3)3, Ciudad Obregón, Sonora.18.1 (17.4-18.5)5, vic. (seep. 649) Durango, Durango.18.1 (17.5-18.5)9, Jaumavé, Tamaulipas.18.2 (17.7-18.9)19, 15 mi. N Rosario Chelé, Sinaloa.17.9 (17.4-18.3)27, vic. (seep. 655) Altamira, Tamaulipas.18.3 (17.9-18.7)9, Valparaíso, Zacatecas.18.1 (18.1-18.2)4, Ciudad del Maíz, San Luis Potosí.18.6 (18.3-18.9)8, Tepic, Nayarit.18.0 (17.7-18.4)18, 4 mi. N, 5 mi. W León, Guanajuato.18.1 (17.5-18.9)28, 6 mi. E Querétaro, Querétaro.17.7 (17.1-18.1)17, 1 mi. SSE Ameca, Jalisco.17.3 (16.8-17.9)10, 2 mi. SSE Autlán, Jalisco.18.0 (17.5-18.6)10, 1 mi. S, 11 mi. W Zamora, Michoacán.17.6 (17.4-18.2)8, Colima, Colima.

Baiomys musculus20.2 (19.9-20.3)6, vic. (seep. 622) Ameca, Jalisco.20.2 (19.9-20.3)6, 2 mi. SSE Autlán, Jalisco.19.6 (19.2-20.1)6, Jalapa, Veracruz.20.3 (19.7-20.9)9, Colima, Colima.19.5 (19.0-20.0)10, Cerro Gordo, Veracruz.19.8 (19.4-20.3)6, 6 mi. S Izucár de Matemores, Puebla.20.0 (18.8-20.5)7, Teotitlán, Oaxaca.20.1 (19.7-20.7)7, 1 km. NW Chapa, Guerrero.19.9 (19.4-20.4)8, 5 mi. ESE Tecpán, Guerrero.19.5 (19.1-20.1)22, 3 mi. ESE Oaxaca, Oaxaca.19.5 (19.1-19.9)11, Valley of Comitán, Chiapas.18.9 (18.2-20.1)17, Tehuantepec, Oaxaca.18.9 (18.4-19.7)15, 6 mi. NW Tonalá, Chiapas.19.1 (18.8-20.4)10, 1 mi. S Rabinal, Guatemala.19.7 (18.8-20.4)10, Lake Amatitlán, Guatemala.19.2 (18.4-19.8)26, vic. (seep. 625) San Salvador, El Salvador.19.3 (18.9-19.9)24, 8 mi. S Condega, Estelí, Nicaragua.

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