[Transcriber's Note: Cover compiled from scanned images.]

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University of Kansas Publications

Museum of Natural History

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Volume 9, No. 23, pp. 579-670, 4 pls., 12 figs. in text

 June 16, 1960 

Speciation and Evolution of the
Pygmy Mice, Genus Baiomys

BY

ROBERT L. PACKARD

University of Kansas
Lawrence
1960

[Pg 580]

University of Kansas Publications, Museum of Natural History

Editors: E. Raymond Hall, Chairman, Henry S. Fitch,

Robert W. Wilson

Volume 9, No. 23, pp. 579-670, 4 pls., 12 figs. in text
Published June 16, 1960


University of Kansas
Lawrence, Kansas


PRINTED IN
THE STATE PRINTING PLANT
TOPEKA, KANSAS

1960


28-3030

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[Pg 583]

Pygmy mice (Genus Baiomys) are the smallest cricetine rodents in North America. They occur from Nicaragua in Central America into the southwestern United States. The principal part of the geographic range of the pygmy mice lies in the Republic of México. They are notably common in central México, but are only locally common to the north and to the south, and then only in certain seasons.

Pygmy mice were first brought to the attention of biologists in 1887 when Oldfield Thomas described a diminutive species of cricetine rodent, Hesperomys (Vesperimus) taylori. The description was based on a specimen obtained by William Taylor from San Diego, Duval County, Texas. C. Hart Merriam (1892:70) described Sitomys musculus on the basis of specimens from Colima [City of], Colima, México. Merriam (loc. cit.) mentioned that the two kinds of mice, Hesperomys taylori and Sitomys musculus, "in general appearance look almost precisely like the common house mouse (Mus musculus) but are still smaller and have shorter tails." He placed the two species in the genus Sitomys. Frederick W. True in 1894 regarded them as composing a distinct subgenus of Sitomys, Baiomys. According to True (1894:758), S. taylori and S. musculus possessed a different combination of characters (ascending ramus of mandible short and erect, condyle terminal, coronoid process well-developed, uncinate, and near the condyle, size small, tail short, plantar tubercles six, soles hairy) than either Vesperimus, or Onychomys (which had been considered as a subgenus of Hesperomys until 1889). In 1907, E. A. Mearns accorded Baiomys generic rank. Osgood (1909:252) treated Baiomys us a subgenus of Peromyscus, whereas, Miller, in 1912, regarded Baiomys as a distinct genus. Most recent students of North American mammals have followed Miller, but usually with reservations. Ellerman (1941:402) emphasized that the taxonomic position of the genus was uncertain, and wrote that Baiomys "… seems to be considerably distinct from Peromyscus, and may perhaps be a northern representative of Hesperomys or one of the small South American genera."

Only two comprehensive analyses of geographic variation and interspecific taxonomic relationships have been made; the first was by Osgood (1909) who had fewer than a fourth of the specimens of Baiomys available to me; the second was by Hooper (1952a:90-97) [Pg 584] who contributed importantly to understanding the relationships of the two living species in central México. No attempts heretofore have been made to correlate and understand the relationships of the five fossil species to one another and to the living species assigned to the genus.

Six objectives of the following report are to: (1) list characters taxonomically useful in recognizing species and subspecies; (2) record amount of variation within and between populations; (3) correlate observed variations with known biological principles; (4) show geographic ranges of the two living species; (5) indicate relationships between fossil and living species of the genus; and (6) clarify the systematic position of the genus.

This report is based on the study of approximately 3,520 museum study skins, skulls, complete skeletons, and entire animals preserved in liquid. Most specimens examined were accompanied by an attached label bearing data on locality and date of capture, name of collector, external measurements, and sex. In addition, 49 fossil specimens referable to Baiomys were studied. Nearly two-thirds of the specimens were assembled at the University of Kansas Museum of Natural History; the remainder were examined in other institutions.

Specimens studied were grouped by geographic origin, sex, age, and season of capture. Individual variation was then measured in several of the larger samples of each living species and in measurable fossil material. External measurements used were those recorded by the collectors on the labels attached to the skins. Twenty cranial measurements employed in the past in the study of Baiomys and closely related cricetine rodents were statistically analyzed. The coefficient of variation was calculated for each of the 20 measurements in order to determine which varied least. In general, measurements having the least coefficient of variation were used in comparing samples from different geographic areas. Figure 1 shows the points between which measurements were taken.

Fig. 1. Three views of the skull to show points between which measurements were taken.
Based on B. m. pullus, adult, female, No. 71611 KU, 8 mi. S Condega, Estelí, Nicaragua. × 1¹/₃.

Capitalized color-terms refer to Ridgway (1912). Color terms without initial letters capitalized do not refer to any one standard.

The names of the cusps and ridges of the teeth (see Figure 2) are those suggested by Wood and Wilson (1936:389-390). Terminology of the enamel grooves and folds is that of Hershkovitz (1944:17) and Hooper (1952b:20-21).

Because secondary sexual variation was not significant (see page 597), both males and females of like age and pelage were used in comparisons of samples designed to reveal geographic variation.

The species are arranged from less to more progressive; the subspecies are arranged alphabetically.

In the synonymy of each subspecies, the plan has been to cite: (1) the name first proposed; (2) the first usage of the name combination employed by me; (3) all other name combinations in chronological order that have been applied to the subspecies concerned.

The localities of specimens examined are listed by country from north to south. Within a country, the listing is by state, beginning with the northwesternmost state and proceeding by tiers (west to east) to the southeasternmost state. Within a state of the United States, the listing is by counties in the same geographic order as described for states. Within any county in the United States, within any state in México, and within any country in Central America, the listing of localities is from north to south. When more than one locality is on the same line of latitude, the westernmost locality is listed first. Marginal localities for each subspecies are listed in a paragraph at the end of each account. Each marginal locality is mapped by means of a circle. The circles are listed in clockwise order, beginning with the northernmost. When more than one of these localities lies on the same line of latitude, the westernmost is cited first. Localities not represented on the distribution maps, so as to avoid undue crowding of symbols, are italicized in the lists of specimens examined.

[Pg 586]

Fig. 2. Occlusal views of molars. × 13.

A.	B. taylori analogous, subadult, female, No. 28102 KU, 4 km. ENE Tlalmanalco, 2290 meters, Estado de México. Right, upper molars.
B.	B. musculus musculus, subadult, male, No. 45456 USNM, Colima, Colima, México. Left, upper molars.
A'.	B. taylori analogous, subadult, female, No. 28102 KU 4 km. ENE Tlalmanalco, 2290 meters, Estado de México. Left, lower molars.
B'.	B. musculus musculus, subadult, male, No. 45456 USNM, Colima, Colima, México. Right, lower molars.

[Pg 587]

The largest single collection of pygmy mice is in the University of Kansas Museum of Natural History, and, unless otherwise indicated, specimens cited in the taxonomic accounts beyond are there.

I am indebted to the following named institutions and persons for making specimens available for study:

I am especially grateful to Professor E. Raymond Hall who guided me in my study and gave critical assistance with the manuscript. Additional appreciated suggestions were made by Professors A. Byron Leonard, Robert W. Wilson, Henry S. Fitch, Ronald L. McGregor, and fellow graduate students. For the illustrations, I am indebted to Mrs. Lorna Cordonnier, Miss Lucy Remple and Mrs. Connie Spitz. Mr. B. J. Wilks of the University of Texas, Department of Zoology, provided a number of living pygmy mice for study in captivity. Mr. J. Raymond Alcorn and his son, Albert, collected a large share of specimens of pygmy mice now in the University of Kansas, Museum of Natural History. My wife, Patricia, aided me in secretarial work and typing of the manuscript.

For financial assistance, I am indebted to the National Science Foundation when I was a Research Assistant, to the Sigma Xi-RESA Research Fund for a Grant-in-Aid, and to the Kansas University Endowment Association through its A. Henley Aid Fund, and the Watkins Fund for out-of-state field work by the Museum of Natural History.

Five fossil species, all extinct, have been assigned to the genus and range in time from early late Pliocene (Saw Rock Canyon fauna of Hibbard, 1953:408) to Mid-Pleistocene (see Hibbard, [Pg 588] 1958:25, who assigns the Curtis Ranch fauna to late Kansan or early Yarmouth).

I examined all known fossil material and compared it with Recent material. When the antiquity of the genus is considered, the degree of difference between the oldest fossil species and the two living species is much less than might be expected.

Remarks.—B. sawrockensis is the oldest known pygmy mouse. The extreme development of the anterior median fold between the [Pg 589] anterolingual conulid and the anterolabial conulid is regarded as a primitive feature in the pygmy mice. In this character, the Recent species can be traced back in time through B. minimus to B. sawrockensis. B. sawrockensis resembles Calomys laucha of South America in general conformation of jaw and tooth structure. The molars of sawrockensis are smaller than those of C. laucha, and the anterolingual conulid of sawrockensis is farther forward.

Remarks.—Two maxillary tooth-rows and associated parts were studied. On one of these specimens, the M2 has a well-developed mesostyle; the anterior median fold of M1 is also well-developed. The other specimen possesses a low cingular ridge (enteroloph) between the protocone and the hypocone, a reduced cingular ridge (mesoloph) between the paracone and metacone of M1. On the second molar, M2, a mesostyle joins with the mesoloph somewhat in the fashion indicated by Hooper (1957:9, encircled number 2).

[Pg 590]

Remarks.—The slight development or absence of the anterior median fold in kolbi suggests that it was specialized. The anterior median fold is well-developed in all species of Baiomys save B. brachygnathus and B. taylori, in which the fold is only slightly developed or absent. B. kolbi may have paralleled B. taylori in specialization for a diet of grasses and for a life in open country.

Remarks.—The molar teeth of B. brachygnathus, although worn, resemble those of B. taylori more than those of any known fossil species. Gidley (1922:124) stated that the absence of the divided anterior lobe of the first molar (anterior median fold) in brachygnathus was one of the chief characters separating brachygnathus from taylori. In taylori, the anterior median fold characteristically is only slightly developed, and in some specimens is absent. B. brachygnathus differs from taylori chiefly in proödont incisors, which feature seems to preclude brachygnathus being ancestral to taylori. B. brachygnathus may have been a specialized divergence from B. minimus.

Remarks.—Gidley (1922:124) stated that B. minimus differed considerably from B. taylori in that the coronoid portion of the ascending ramus diverges at a wider angle from the alveolar part of the jaw. Study of large samples of lower jaws of B. taylori reveals considerable individual variation in the angle formed between the coronoid part of the jaw and the alveolar part.

B. minimus, except for its small size, is like B. musculus and is considered to be ancestral to that species.

It seems that the important trends in phyletic development in the pygmy mice have been from an ancestral stock (see Figure 3) that possessed relatively brachydont teeth having raised cingular ridges (ectolophids and mesolophids) and relatively short orthodont to proödont incisors, to species having teeth more hypsodont on which cingular ridges were reduced, stylids were isolated or completely absent, and incisors were longer and more recurved or retrodont. Baiomys sawrockensis, or an unknown stock resembling it, might have been ancestral to the other known species. Of the four remaining fossil species, B. kolbi seems least likely to have been ancestral to the two living species, owing to its proödont incisors, reduction of cingular ridges, loss of an anterior median fold in m1, and long mandibular tooth-row. B. kolbi may have been an early, specialized derivation from the ancestral stock. From his knowledge of the habitats of B. musculus, the larger species, and B. taylori, the smaller species, Hibbard (1952:203) suggests that B. kolbi, a large species, might have inhabited lowlands, and B. rexroadi, a small species, highlands. I have no evidence to dispute this suggestion except that B. musculus has more prominent cingular ridges (or at least vestiges of this lophid condition) than either B. kolbi or B. rexroadi. B. musculus (see page 610) is less of an open grassland inhabitant than is B. taylori. Therefore, both B. kolbi and B. rexroadi, because of their poorly developed cingular ridges, might be expected to have lived in a relatively open grassland habitat.

[Pg 593]

The relationship of B. rexroadi to fossil species other than B. kolbi is not clear. Superficially, the former resembles B. taylori, but, owing to the specialized development of the molars of rexroadi, it could hardly have been ancestral to either of the living species. The resemblance of B. rexroadi to B. taylori may result from each having occupied the same ecological niche in different periods. The incisors of B. rexroadi, however, are much shorter than those of B. taylori and suggest somewhat different food habits.

B. minimus seemingly is more closely related to B. sawrockensis and B. musculus than to the other described species. The development of the cingular ridges leads one to suspect that B. minimus was the ancestor of B. musculus. B. minimus may have been derived from a sawrockensis-like stock and probably gave rise to B. musculus.

Hershkovitz (1955:643-644) suggests that "… primitive brachydont, buno-mesolophodont cricetines have survived … in forested parts of the range," whereas "… the progressive branch of cricetines with mesoloph absent or vestigal, has become increasingly specialized for life in open country and a diet of grasses." Species of the genus Baiomys can be divided into two morphological groups. One group, composed of B. sawrockensis, B. minimus, and B. musculus, includes those species, the teeth of which were relatively brachydont and had prominently developed cingular ridges (ectolophids or mesolophids) or, at least, showed some development of these ridges. B. sawrockensis probably lived in semi-wooded to shrubby habitats. According to Hibbard (1953:409), "The Saw Rock Canyon fauna lived in that area at a time when conditions were comparable to the conditions at the time the Rexroad fauna lived." The conditions in which the Rexroad fauna lived are discussed by Hibbard (1941:95). Presumably, there were at least some well-wooded situations, and the climate was warm. B. sawrockensis probably inhabited denser vegetation than did B. minimus or than does B. musculus. The teeth of the second group (B. kolbi, B. rexroadi, B. brachygnathus, and B. taylori) lack cingular ridges or have them much reduced and have more hypsodont molars. The three fossil species probably inhabited relatively open grassland. This assumption is based largely on the known habitat of B. taylori (see page 632).

The suggested grouping, based on supposed similarities in niches inhabited by the extinct species, does not necessarily indicate degree of relationship. B. taylori probably was not derived from an [Pg 594] ancestor like B. rexroadi or B. kolbi, although, in certain characters, the three species resemble one another. B. kolbi and B. rexroadi were already specialized in Blancan times, probably for living on grassland. B. taylori shows only a slight advance in specialization of molar structures compared to either of the aforementioned species but is slightly smaller and does have longer and more recurved incisors. If only morphological criteria of lower jaws were considered, without recourse to other data derived from the study of many samples of populations of the living species, time alone might account for the differences among B. taylori, B. rexroadi, and B. kolbi. The available evidence (see page 658) suggests, however, that B. taylori was derived from the B. sawrockensis-B. minimus-B. musculus line.

Fig. 3. Diagram indicating probable relationships of living and extinct species of pygmy mice.

Baiomys seems to have undergone little basic evolutionary and morphological change since Late Pliocene time. According to Simpson (1945:207), hesperomine rodents as a group have undergone little basic evolution, and "The rapid evolution of new genera was more a matter of segregation of characters in a group with a great variation than of the origin of significantly new characters." Perhaps, the living southern pygmy mouse retains many basic characteristics of one of the early North American cricetine-like stocks that emigrated to South America near the end of the Pliocene epoch. [Pg 595] There is much to suggest close relationship of the pygmy mice to certain species of South American hesperomine rodents of the genus Calomys.

Non-geographic variation in pygmy mice (variation in a single population resulting from age, individual, seasonal, and secondary sexual differences) has been but little studied in the past. Mearns (1907:381) figured progressive stages of wear on the teeth of B. taylori; Osgood (1909:252) and Blair (1941:380) referred to changes in dentition, weights, and pelages.

The largest samples available for this study were 47 B. taylori from the vicinity of Altamira (6 mi. N, 6 mi. W; 5 mi. N, 5 mi. W; 1 mi. S), Tamaulipas, and 44 B. musculus from El Salvador (1 mi. S Los Planes, and 1 mi. NW San Salvador—two localities 3 miles apart).

Specimens of both species were segregated into five categories: Juveniles, young, subadults, adults, and old adults. Juvenal and young pygmy mice are readily separable from the other three categories; subadults are less easily distinguished from adults. In order to obtain an accurate understanding of geographic variation in these mice, only adults should be used in making taxonomic comparisons.

Juveniles.—Nestling mice yet unweaned; sutures in cranium incompletely closed; bony parts of skull fragile; M3 and m3 not erupted or only partly erupted and not protruding above margins of alveoli.

At birth, juveniles are pink, without pelage except for the mystacial vibrissae and a few hairs about the eye. Blair (op. cit.:381) recorded changes with age in color of the skin of new-born and suckling pygmy mice. Data obtained by me from three litters born in captivity agree with his findings. Pygmy mice are weaned when 17 to 24 days old. At that time, the mice possess a fine, but not dense, dusky-gray fur.

Young.—Weaned mice; cranium fragile; sutures between frontals and parietals, interparietal and parietals, basioccipital and basisphenoid, basisphenoid and presphenoid, premaxillaries and maxillaries widely open; M3 and m3 erupted beyond margins of their alveoli (molars erupt from anterior to posterior; M3 and m3, therefore, are last to erupt); in some specimens, molars slightly worn; pelage still dusky and relatively fine and sparse.

[Pg 596]

Subadults.—Sutures between bones of skull less widely open than in young; epiphyses of long bones incompletely coalesced to shaft; relative to length of skull, braincase higher and rostrum shorter than in adults; all cusps worn, but dentine not occlusally confluent; primary first and second folds of third upper molars present; primary first fold and major fold of lower molars visible; pelage a subtle mixture of colors of young and adult, but resembling most that of adult; molts into postjuvenal pelage between 46 and 50 days.

Adults.—Sutures of skull, and those between epiphyses and shaft of long bones obliterated except that, in some mice, sutures of skull persist between frontoparietal, and interparietal; cusps of molars so worn that dentine occlusally confluent; small island of enamel in third upper and lower molars of some specimens; relative to length of skull, cranium lower, rostrum longer, and interorbital region narrower than in subadult; cranium appears to be more flattened dorsoventrally; between subadult and adult stages, principal growth occurs in basioccipital, basisphenoid, frontals, and parietals; nasals grow less.

Although all bones of the skull grow in the subadult and early adult stages (see table 1), the above-named bones grow faster than others and thus cause the general flattening of the skull, typical of adults (similar to that reported by Hoffmeister, 1951:7). The body continues to lengthen, accounting for the increase in total length of the adult (see table 1). Hind foot, tail and ear, reach their maximum lengths by subadult stage. Adult pelage has been acquired, and the color is brighter than in either subadults or old adults.

Old Adults.—Characterized principally by well-worn molars; only thin peripheral band of enamel along with slight evidence of any primary or secondary folds on any teeth remain; all bones of skull coalesced; epiphyses and shafts of long bones ankylosed; small bony protuberances on many skulls; pelage usually ragged, tips of the hairs being worn away; white flecking and spotting not common, but occurs in some adults.

[Pg 597]

The method employed by Dice and Leraas (1936:2) was used to measure the secondary sexual differences, if there were any, in each of several age classes. As pointed out by Hooper (1952b:11), individual variation in small samples can obscure secondary sexual differences. The samples of B. taylori from the vicinity (see page 595) of Altamira, Tamaulipas, and the samples of B. musculus from El Salvador (table 2) were large enough to prevent individual variation from obscuring sexual differences. Nevertheless, no significant secondary sexual differences were found in either B. taylori or B. musculus (see table 2). Therefore, the sexes have been considered together for purposes of geographic studies.

Length of tail varied more than any other measurement used by me in taxonomic comparisons. Clark (1941:298), Hoffmeister (1951:16), and Van Gelder (1959:239) point out that external measurements generally are more variable than measurements of the cranium, probably because different techniques of measuring are employed by different collectors. As can be noted in table 3, females varied more than males.

[Pg 598]

In the 3520 specimens examined, an extra tooth was observed in only one (see Hooper, 1955:298). The left mandibular tooth-row of an adult male (USNM 71539) from Omentepec, Guerrero, is worn more than the right one. Irregularities in number of teeth and abnormalities in individual teeth seem to be rare in pygmy mice.

The posterior margin of the bony palate varies from semicircular to nearly V-shaped. The suture between the nasals and frontals varies from V-shaped to truncate to W-shaped. The maxillary part of the zygoma varies from broad to slender in dorsoventral width in both species.

There are three distinct pelages, juvenal, postjuvenal, and adult. The sequences of molt and change of pelage from the juvenal, to the postjuvenal, and from it to adult, are essentially as reported for Peromyscus by Collins (1918:78-81; 1924:58-60) and Hoffmeister (1951:5). The juvenal pelage is uniformly dusky gray throughout except for the paler gray on the venter. In most juvenal mice, the [Pg 599] yellow to ochraceous pigments of the subterminal bands are reduced or absent. Unlike Peromyscus, Baiomys has bright brownish hairs on the head as the first evidence of the postjuvenal molt (see Figure 4, part a). Blair (1941:381) reports adult pelage in pygmy mice being evident first at an age of 46 days. Two of my juveniles born in captivity began the postjuvenal molt on the 38th and 40th days. The area of new hairs on the head spreads most rapidly posteriorly. New hair appears ventrally and laterally at the end of 46 days (see Figure 4, part b). Hair replacement proceeds more slowly after the "saddle back" stage (described in Peromyscus by Collins, 1918:80) has been reached. That stage was reached in two pygmy mice at 52 days (see Figure 4, part c). Areas immediately posterior to the ears, in the scapular region, molt last. The postjuvenal pelage was seemingly complete in one captive pygmy mouse at the end of 60 days. Another captive failed to complete its growth of new pelage until two additional weeks had elapsed. Length of time required to molt in pygmy mice is about the same as that reported by Layne (1959:72) in Reithrodontomys.

Fig. 4. Diagrams showing progress of the postjuvenal molt in pygmy mice.
For explanation of a, b, and c, see text. All approximately ²/₃ natural size.

[Pg 600]

If, after the postjuvenal molt, a distinct adult pelage is acquired it is difficult to separate it from the annual replacement of pelage in adults at the beginning of the rainy season. Adults of both species have been found in molt in all months of the year. To the north, in Texas, the pelage of winter-taken specimens is denser and slightly more reddish than that of specimens taken in spring and summer. In the two last mentioned seasons, the pelage is more uniformly gray. To the south, in México, the pelage is heavy and long in most specimens taken in the rainy season. The percentage of specimens in molt immediately before the rainy season and immediately before the dry season is slightly higher than in specimens taken at other times of the year. The adult or seasonal molt (both loss of old pelage and growth of new) resembles that in Peromyscus truei gilberti, described by Hoffmeister (1951:6) as proceeding "posteriorly as a wave over the entire back." The new hair is slightly brighter than the old. Old adults are usually in ragged pelage regardless of season; possibly only one regular annual change of pelage occurs in most animals before they die. Only one case of melanism was observed among all the specimens of both species examined. It was a young male B. t. taylori, KU 35943, from 6 mi. SW San Gerónimo, Coahuila, possessing black hairs throughout. Its hairs are longer and finer than those on specimens of comparable age and sex. No albino was found, although Stickel and Stickel (1949:145) record one—an adult male of B. taylori.

External parts.—Length of body, foot, ear, and tail are useful when considered together in distinguishing species and subspecies. I found as Hooper (1952a:91) did that length of ear in combination with length of hind foot suffices to identify nearly all specimens to species, especially where the two species occur together.

Pelage.—Color in adults is of especial value in subspecific determination; the manner in which it varies geographically is described on pages 609, 630.

Skull.—Difference in occipitonasal length and zygomatic breadth, both having low coefficients of variation, are useful in separating species, especially where they are sympatric. Shape of presphenoid, nasals, interparietal, frontoparietal sutures, and length and degree of the openings of the incisive foramina are useful in delimiting subspecies. The rostrum of B. taylori, in front of the frontonasal suture, is deflected three to five degrees ventrally in 85 per cent of [Pg 601] the adults examined, and in B. musculus is less, or not at all, deflected.

Teeth.—Alveolar length of the upper and lower molar tooth-rows aids in distinguishing fossil and Recent species, and to a lesser degree in delimiting subspecies. Occlusal pattern is useful in estimating the relationship of fossil and living species. Degree of development of the mesostyle, mesostylid, mesoloph, and mesolophid have been useful in determining relationship between fossil and living species as well as useful in separating the living species. Rinker (1954:119) and Hooper (1957:48) have shown the degree of variation in dental patterns in Peromyscus, Sigmodon, and Oryzomys, mice thought to be closely related to Baiomys. In pygmy mice, however, the dental patterns are relatively constant. The lophs and styles are subject to some geographic variation but, nevertheless, are useful in estimating relationships.

Fig. 5. Ventral view of hyoid bones. × 18.

A. Baiomys musculus brunneus, adult, female, No. 30182 KU, Potrero Viejo, 1700 feet, Veracruz.

B. Baiomys taylori analogous, adult, female, No. 36761 KU, 2 mi. N Ciudad Guzmán, 5000 feet, Jalisco.

Hyoid apparatus.—Shape and, to a lesser extent, size of the hyoid apparatus differentiate nearly all specimens of B. taylori from all those of B. musculus. The hyoid of B. taylori differs from that of B. musculus principally in the shape of the basihyal. It possesses [Pg 602] an anteriorly pointed entoglossal process in B. musculus, and is not rounded to completely absent as in B. taylori (see Figure 5). The shoulders of the basihyal protrude anteriorly in B. musculus, and are not flattened as in B. taylori. The total length was measured in a sample of 55 basihyals of B. musculus, and was compared to the total length of a sample of 80 basihyals of B. taylori. The means of the two samples differ significantly at the 95 per cent level; the mean plus two standard errors of B. musculus and B. taylori, are, respectively, 2.43 ± .02; 2.18 ± .03. There is sufficient overlap of the samples (mean plus one standard deviation of B. musculus and B. taylori, respectively: 2.43 ± .15; 2.18 ± .15) to make the total length of the basihyal of only secondary importance in distinguishing species, but shape and total length of the basihyal, when considered together, serve to identify all specimens to species. When length of the basihyal is plotted against occipitonasal length (see Figure 6), all specimens studied, regardless of age or geographical origin, were separated at the level of species. The hypohyals of B. taylori seemingly remain distinct throughout life; those of B. musculus completely fuse in some adults. The ceratohyals are highly variable in shape and of little taxonomic use.

Fig. 6. Relationship of length of basihyal to occipitonasal length of skull. Black symbols, all below the curved line, represent measurements of B. taylori; open symbols, all above the curved line, represent measurements of B. musculus.

The degree of geographic variation in shape of basihyal is not great. Specimens of B. musculus pallidus from 1 km. NW Chapa, [Pg 603] Guerrero, have a small indentation on the anteriormost part of the entoglossal process. The shoulder of the basihyal is directed less forward in specimens of B. taylori taylori from 6 mi. N, 6 mi. W Altamira, Tamaulipas, than in other specimens of the species. The variations observed seemed not to be clinal.

According to White (1953:548) the hyoid, like the baculum (Burt, 1936:146), is little influenced by changes in external environment and may serve to clarify intergeneric relationships. Hyoids of both species of Baiomys are smaller than hyoids of all subgenera of Peromyscus. In shape, the hyoids of Baiomys resemble those of Ochrotomys nuttalli (as explained on page 605, Ochrotomys is here accorded generic, instead of subgeneric, rank). In size, the hyoid of both species of Baiomys resembles that in Reithrodontomys. Sprague (1941:304) reports a resemblance in shape between the ceratohyals of Baiomys and Reithrodontomys. The thyrohyals differ from those of Reithrodontomys, being less boot-shaped, and having a slight terminal expansion as in Ochrotomys (see Sprague, loc. cit.). In shape, the large basihyal of Onychomys resembles the smaller one of B. musculus. The basihyal of Oryzomys lacks the entoglossal process present in Baiomys. On the basis of shape of hyoid, Baiomys seems to be most closely related to Ochrotomys.

Fig. 7. Dorsal view of bacula. × 16.

A. B. musculus brunneus, adult, No. 24336 KU, 3 kms. W Boca del Río, 10 feet, Veracruz.

B. B. taylori taylori, adult, No. 35937 KU, 6 mi. SW San Gerónimo, Coahuila.

Baculum.—Of Baiomys, 166 bacula were processed, using the method of White (1951:125), and studied. They provide characters of taxonomic worth at the level of species and aid in evaluating generic relationships.

The baculum of B. taylori differs from that of B. musculus in: shaft narrow; wings anterior to base projecting dorsolaterally instead [Pg 604] of anteriorly; anterior part knob-shaped having indentation at tip, instead of anterior part spatulate-shaped (in some) to knob-shaped (see Figure 7), without indentation; significantly shorter (see Table 4).

In each of the two species, individual and geographic variation in the baculum is slight; its length varies insignificantly according to age. Excluding juveniles contained in Table 4, but including young and subadults, only three bacula of B. taylori were longer than 3 mm., and only one baculum of B. musculus (a young) was shorter than 3 mm. The total length of the baculum, considered together with its shape, serves to identify to species all specimens examined by me.

The bacula of both species of Baiomys were compared with bacula of Akodon, Scotinomys, Holochilus, Oryzomys, Zygodontomys, Reithrodontomys, Thaptomys, and Calomys and illustrations of bacula by Blair (1942:197, 200) of Peromyscus (subgenera Peromyscus, Haplomylomys, Podomys), Ochrotomys, and material at the University of Kansas Museum of Natural History of Megadontomys. Shape of baculum most resembled that of Ochrotomys and Calomys. The bacula of Baiomys, as pointed out by Blair (op cit.:203), differ as much from those of the genus Peromyscus as do the bacula of Reithrodontomys and Onychomys. In size of baculum, Baiomys resembles Ochrotomys. Blair (op. cit.:202) pointed out that the length of the baculum of B. taylori subater was contained in the length of the animal's body 20.3 times, and 24.2 times in the length of that of Ochrotomys nuttalli. The length of the baculum of B. musculus (average of 58 specimens without regard to subspecies) is contained in the length of the body (of specimens from which the bacula were removed) 22.7 times, a figure approaching that in Ochrotomys. When bacula of both species of Baiomys were compared to those of O. nuttalli, bacula of B. musculus were found to most closely resemble those of O. nuttalli. The baculum of a single [Pg 605] specimen of Calomys (C. laucha) was contained in the length of the body 15.5 times. In general shape, as well as in possession of an anterior knob and the position of the expanded posterior wings, the baculum of C. laucha resembles the baculum of Ochrotomys and Baiomys musculus.

Blair (op. cit.:201) considers generic versus subgeneric rank for Ochrotomys, and on the basis of studies of the phallus Hooper (1958:23) stated that "it is clear that nuttalli should be removed from Peromyscus and should be listed as Ochrotomys nuttalli (Harlan)." I agree with Hooper (loc. cit.) and point out that on the basis of the baculum, there is less of a hiatus between Baiomys on the one hand, and Ochrotomys and Calomys on the other hand, than there is between any one of those three genera and Peromyscus.

White (1953:631) reported that the baculum of chipmunks might indicate relationships more clearly than do skulls and skins. He thought that skulls might more quickly than bacula reflect the habitus of the animal. The resemblance in cranial morphology between Peromyscus and Baiomys is judged to be the result of such a convergence of habitus and the baculum in Baiomys is thought to reflect relationships more accurately than does the skull.

Auditory ossicles.—Examination of a number of auditory ossicles of Baiomys reveals constant interspecific differences in the malleus and incus. There is only slight individual variation, slight variation with age, and no secondary sexual variation. In Baiomys taylori the orbicular apophysis of the malleus (see Figure 8, A) is rounded to nearly ovoid; the anterior process is pointed, and the neck is short, being slightly recurved. The body of the incus is round and the short process is elongate. The sides of the long limb of the incus are nearly parallel. The lenticular process is relatively large. The posterior and anterior crus of the stapes are bowed, and the muscular process is either absent or much reduced.

In Baiomys musculus, the orbicular apophysis of the malleus (see Figure 8, B) is round to oblong, and less ovoid than in B. taylori; the anterior process is less acutely pointed than in B. taylori, and the neck is long, less recurved than in B. taylori. The body of the incus, though tending to be round, is more flattened, and the short process is knob-shaped, not elongated. The sides of the long limb of the incus are not parallel. The lenticular process is, relative to the size of the incus, small. The posterior and anterior crus of the stapes are more nearly straight than in taylori. A prominent muscular process occurs on the posterior crus.

[Pg 606]

The auditory ossicles of representative species of all the subgenera of Peromyscus were studied as were the ossicles of Onychomys, Ochrotomys, Oryzomys, Akodon, Thaptomys, Zygodontomys, Calomys, Reithrodontomys, and Holochilus.

Fig. 8. Lateral views of auditory ossicles. × 20.

A. B. taylori analogous, adult, female, No. 28104 KU, 4 kms. ENE Tlalmanalco, 2290 meters, Estado de México.

B. B. musculus pallidus, adult, male, No. 28346 KU, Cahuilotal, Sacacoyuca, 960 meters, Guerrero.

The general plan of structure of the auditory ossicles in Baiomys resembles that in Calomys, Akodon, and Thaptomys. The ossicles of Calomys and Thaptomys, in particular, closely resemble the auditory ossicles of Baiomys musculus. The short process of the incus is knoblike in Calomys and Thaptomys, and the general conformation of malleus and stapes in those two genera is nearly identical to that in B. musculus. In Akodon, the anterior and posterior crus of the stapes is more rounded than in B. musculus, resembling that in B. taylori.

[Pg 607]

Reithrodontomys differ from Baiomys in having a more elongate orbicular apophysis on the body of the malleus, an elongated short limb on the incus, and a stapes having anterior and posterior crura bowed as in mice of the genus Peromyscus.

In Ochrotomys, the orbicular apophysis of the malleus resembles the orbicular apophysis of B. musculus, but the short process of the incus is longer, resembling the short process of B. taylori. In general conformation of the malleus, incus, and stapes, Ochrotomys shows closer resemblance to B. taylori than to B. musculus.

In Holochilus the anterior crus and posterior crus of the stapes are similar to those in B. musculus, but in shape and size of malleus and incus, Holochilus differs considerably from B. musculus and B. taylori.

In Zygodontomys, size and shape of the ossicles differ greatly from those of Baiomys.

In the genus Peromyscus, only Peromyscus floridanus (subgenus Podomys) possesses a knoblike short process on the incus similar to that in B. musculus; representatives of the other subgenera examined possess an elongated short limb on the incus. The conformation of the ossicles of both Onychomys and Oryzomys appears to be more nearly like that in Peromyscus than that of Baiomys.

On the basis of shape and size of auditory ossicles, Baiomys resembles South American hesperomines (Calomys and Thaptomys) rather than North American hesperomines.

Diagnosis.—Size small (total length in adults, 93-135); tail shorter than head and body; hind foot in adults 12-17; ears small (8-12) and rounded; upper parts blackish sepia to ochraceous-buff; underparts slaty gray to white or pale buffy; eyes small; hind feet having six plantar pads, soles nearly naked except for some hairs on anterior parts of soles and anteriorly to base of toes and between toes; occipitonasal length of skull in adults, 17.0-21.5; zygomatic breadth, 9.0-11.5; coronoid process of mandible well developed, strongly recurved; ascending ramus of mandible short and erect; anterior palatine foramina (incisive foramina) long, usually terminating posterior to plane of the front of first molars; posterior palatine foramina nearly opposite middle of M2; interorbital space wide relative to widest part of frontals; nasals projecting only slightly over incisors; condyle terminal; upper incisors relatively heavy; primary first fold of M3 obliterated at an early stage of wear; major cusps of upper and lower anteriormost two molars alternating, more so in m1-m2 than in M1-M2, dental formula I/i, 1/1; C/c, 0/0; P/p, M/m, 3/3 = 16.

For distribution of the genus, see Figure 9.

[Pg 608]

Fig. 9. Geographic distribution of the genus Baiomys. Black area shows where the two species occur together. Black dot (Acultzingo, Veracruz) shows locality where Baiomys taylori occurs within the range of B. musculus, but B. musculus is not known to occur at that locality.

Geographic variation.—Eight subspecies are here recognized (see Figure 10). Features that vary geographically are external size, color of pelage, certain cranial dimensions (occipitonasal length, zygomatic breadth, least interorbital breadth, length of rostrum, length of incisive foramina, depth and breadth of cranium, and alveolar length of upper molar tooth-row).

External and cranial size (except for B. m. handleyi) is less in the southernmost subspecies, B. m. pullus, B. m. grisescens, B. m. nigrescens, and more in the northernmost subspecies, B. m. musculus, B. m. brunneus, and B. m. infernatis. Increase in size from south to north is in keeping with Bergman's Rule that within a species, smaller individuals occur in warmer parts of its geographic range. Southern pygmy mice at high altitudes average larger than those from low elevations, except where the two species are sympatric. There the Southern Pygmy Mouse is uniformly larger, regardless of altitude.

Osgood (1909:257, 259) suggested that degree of relative humidity might in some way control color of pelage in both B. taylori and B. musculus. In B. musculus, the darker subspecies, B. m. brunneus, B. m. nigrescens, and B. m. pullus, occur in zones of rather constant high relative humidity, whereas the paler subspecies infernatis, musculus, handleyi, and to a less extent grisescens and pallidus, occur in zones of lower relative humidity. This is in keeping with Gloger's Rule, which states that melanins increase in the warm and humid parts of the range of a species, and reddish or yellowish-brown phaeomelanins prevail in arid climates. B. m. musculus ranges into areas where relative humidity is such that darker pelages might be expected, but this is in the area where the two species are sympatric, and color of pelage may be an important character of recognition.

[Pg 610]

Fig. 10. Distribution of Baiomys musculus. Known localities of occurrence are represented by circles and black dots; the former denote localities that are peripheral (marginal) for the subspecies concerned.

1. B. m. brunneus 2. B. m. grisescens 3. B. m. handleyi 4. B. m. infernatis

5. B. m. musculus 6. B. m. nigrescens 7. B. m. pallidus 8. B. m. pullus

Habitat and numbers.—In Veracruz, Dalquest obtained the southern pygmy mouse in stands of tall grass (Spartina?) in sandy loam soil bordering, and in, dense vegetation; Davis (1944:394) found the species living in dense stands of grasses and seemingly utilizing underground burrows. Near Chilpancingo, Guerrero, rocky situations seemed to be the preferred habitat. Davis (loc. cit.) believed [Pg 611] that the species has a wide tolerance to kinds of habitats. In Morelos, Davis and Russell (1954:75) found these mice to be abundant along rock fences separating cultivated fields, and in arid lowlands. In Colima, Hooper (1955b:13) obtained specimens from an open thorn forest in sparse grass and rocky hillside bounding a stream and in litter below shrubs on the floor of a nut-palm forest; in Michoacán, these mice were taken in cane grass, shrubs, and mesquite near an irrigation ditch. From Guatemala, Goodwin (1934:39, 40) records specimens from Sacapulas, a hot, dry, sandy area where cactus and sparse grasses are present, and from La Primavera, on the edges of pine-oak-alder forests. Felten (1958:137) has taken musculus from bushy areas in El Salvadore. In 1955, I obtained the southern pygmy mouse 6 mi. SW Izucár de Matemores, Puebla, along a stream in heavy grass bordered by cypress, willow, fig, bamboo, and in rocky grazed area near sugar cane fields.

The southern pygmy mouse seems to be locally abundant in certain parts of its geographic range, and in other parts, scarce. For example, Dalquest (in. litt.) recorded the pygmy mouse as common at a place 2 km. N Paraje Nuevo, 1700 feet, Veracruz, where, by means of 50 traps, he took 14 of these mice in one night. The species was scarcer, although the habitat seemed suitable, 3 km. N Presidio, 1500 feet, Veracruz, where he caught only two pygmy mice in several days of trapping. Six miles southwest of Izucár de Matemores, the pygmy mouse was the most common rodent. I have trapped for it in Oaxaca and Veracruz in habitats that seemed almost identical to those mentioned by Dalquest, and also that at Izucár de Matemores, Puebla, with almost no success. The reason for the seeming disparity in numbers at different localities having nearly the same kind of habitat is unknown to me and bears further investigation.

Behavior.—Little is recorded concerning the behavior of this species. David and Russell (op. cit.:76) found that of small mammals B. musculus was the first to appear at night. I caught mice of this species by hand in the afternoon in Puebla. They seemed to be active from noon until dark. Albert Alcorn wrote in his field notes that specimens were taken near noon at a place 9 mi. NNW Estelí, Nicaragua. My impression is that musculus is diurnal to crepuscular.

Enemies and food.—Owl pellets (thought to be those of a barn owl, Tyto alba) from within the geographic range of B. musculus, from 6 mi. SW Izucár de Matemores, yielded mandibular tooth-rows belonging to musculus. Presumably, most of the carnivorous mammals [Pg 612] and raptorial birds within the range of the southern pygmy mouse could be listed as enemies. Diurnal to crepuscular habits of this mouse may protect it from some of the nocturnal carnivorous mammals and raptorial birds.

Food of the southern pygmy mouse includes nuts, bark, grass seeds, and leaves. Dalquest (MS) writes that bits of banana proved to be useful bait in trapping these mice in Veracruz.

Reproduction.—Notations concerning lactation and embryos on specimen labels of females suggest that the southern pygmy mouse breeds in all months. I have records of pregnant or lactating females in every month, save January, April, May, and June. The average of 26 counts of embryos or young per litter is 2.92 (1-4).

Remarks.—Specimens from Chichicaxtle, Puente Nacional, 3 km. W Boca del Río, 1 km. E. Mecayucan, and Río Blanco (20 km. WNW Piedras Negras), are all paler than the paratypical series and other specimens from within the assigned range of B. m. brunneus. All these specimens from the coastal plain average considerably paler than those from the front range and slopes of the mountains. Specimens from Puente Nacional are intermediate in color between paler, grayish brown, specimens from the coastal plains and the darker, brown, specimens from the mountains. When Allen and Chapman (1897:203) described brunneus, they did so on the basis of the darker brown mice from the higher altitudes. The name, brunneus, sensu stricto, could be restricted to those mice from the [Pg 614] higher altitudes of central Veracruz. However, when the mice of intermediate color from Puente Nacional are considered, it seems best to include the material from the coastal plain with brunneus. Crania from the higher altitudes are slightly larger than, but not significantly different from, crania of specimens from the coastal plains. Specimens examined from the coastal plains resemble the darker series of B. m. pallidus to the west in central México. But there is no evidence of gene flow between the paler coastal specimens and B. m. pallidus to the west. In fact, these paler brown mice on the coastal plain grade in color into the darker brown mice from the mountains. The paler mice from the coast may be an incipient subspecies.

The type and paratypes seem to have faded somewhat since they were described by Allen and Chapman (loc. cit.) and by Osgood (1909:259). However, the color of the paratypes and other specimens herein assigned is the feature most useful for distinguishing brunneus from all other subspecies of B. musculus.

Remarks.—Goodwin (1942:160) mentioned that a specimen from the type locality of grisescens was as dark as specimens of B. m. nigrescens from Guatemala. However, all specimens from Guatemala, other than those from Sacapulas, were referred by Goodwin (1934:40) to B. m. nigrescens. My studies reveal a grayish-brown population in central Honduras near to and including the type locality. This population appears to grade into a slightly paler, particularly as concerns color of hind foot and tail, group of Guatemalan mice from 1 mi. S Rabinal, from ¹/₂ mi. N, 1 mi. E Salama, and from Lake Atescatempa. Specimens from western Guatemala at Nentón and Jacaltenango, on the other hand, are darker brownish-black, more nearly like the paratypical series of nigrescens from the Valley of Comitán, Chiapas, Republic of México. This darker brownish-black color of the back persists in specimens from southern [Pg 616] Guatemala and El Salvador (see specimens examined of B. m. nigrescens for localities), and they are best referred to nigrescens. B. m. grisescens, in color and certain cranial characters, therefore, seems to grade into two different subspecies: (1) B. m. handleyi, pale mice in the Río Negro valley in central Guatemala, and (2) B. m. nigrescens, dark mice from southern Guatemala, and parts of El Salvador.

Felten (1958:136) referred all B. musculus from El Salvador to B. m. grisescens. Although I have not examined the specimens reported on by Felten (loc. cit.), I have examined specimens from Lake Atescatempa, Guatemala (which I refer to grisescens), not too distant from Cerro Blanco, and Finca Las Canarias, Department of Ahuachapan, and Laguna de Guija, Department of Santa Ana (localities listed by Felten). It would seem that specimens from these localities might indeed be grisescens. However, specimens that I examined from 1 mi. S Los Planes, and 1 mi. NW San Salvador were considerably darker than paratypes of grisescens and were nearly intermediate in color between nigrescens and pullus. I refer the specimens from 1 mi. NW San Salvador, and 1 mi. S Los Planes to nigrescens rather than to grisescens.

There is no positive evidence that B. m. grisescens intergrades with B. m. pullus to the south in Nicaragua. But, there is a suggestion that intergradation occurs between these subspecies in a series of 76 skins from La Piedra de Jesús Sabana Grande, Honduras, referable to grisescens. A total of 16 of 76 skins from this locality (21 per cent) possess the mid-ventral white stripe found in 18 of 20 skins (90 per cent), from the type locality of pullus in Nicaragua. Further collection in areas between central Honduras and western Nicaragua may yield specimens of B. musculus that are intermediate in characters between grisescens and pullus.

Remarks.—B. m. handleyi seems to be restricted to the valley of the Río Negro, in the region of Sacapulas, Guatemala. Stuart (1954:7) points out that the Río Negro drops down into a gorge at a place near Sacapulas and flows northward through a deep canyon [Pg 618] for approximately 60 kilometers. The Río Negro, then, flows onto the lowlands of the Yucatán Peninsula. The habitat is xerophytic in the valley of the Río Negro near Sacapulas. Stuart (op. cit.:10) suggests that this xerophytic habitat may be continuous to a place to the north of Chixoy, Chiapas, where the vegetation then becomes more mesic. The mesic conditions to the north in Tabasco and Yucatán probably have restricted the movement of pygmy mice to the north. No specimens of this mouse are known from the Yucatán Peninsula or from the State of Tabasco, México. B. m. handleyi intergrades with B. m. grisescens to the south. Specimens from 1 mi. S Rabinal, and those from a second locality ¹/₂ mi. N and 1 mi. E Salama, Guatemala, are intermediate in color of pelage between handleyi and grisescens. Stuart (op. cit.:5) mentions the continuity of habitat and tributaries from the Salama Basin into the valley of the Río Negro. Absence of physiographic and biotic barriers in the corridor between these two basins probably allows for some gene flow between handleyi and grisescens, and results in populations intermediate in color. To the north and northwest of Sacapulas, the Sierra de los Cuchumatanes rises abruptly and separates the known geographic range of handleyi from that of nigrescens to the north, while to the west the cactus-mesquite habitat of handleyi gives way to the oak-pine timber that, so far as known, does not support Baiomys. The difference in elevation and flora seems to restrict gene flow between handleyi and the more northern nigrescens. The only evidence of integration between these two subspecies is provided by one specimen from Chanquejelve, Guatemala. That specimen is intermediate in color between the pale handleyi and blackish-brown nigrescens.

The subspecies closest, geographically, to B. m. handleyi is B. m. nigrescens, from which B. m. handleyi differs more in color than from any of the other named subspecies, except B. m. pullus. There is a close correlation of pallor of mice and the xeric Río Negro Valley, and the darkness (melanistic color) of mice and the mesic mountains and valleys to the north.

[Pg 619]

Remarks.—B. m. infernatis resembles B. m. handleyi more than any other subspecies in color of pelage and in external and cranial dimensions. The resemblance in color between B. m. pallidus, in certain parts of its range, and B. m. handleyi may have resulted from nearly parallel selective forces that gave rise to two subspecies, widely separated geographically. The same relation obtains between B. m. infernatis and B. m. handleyi. Both inhabit arid river basins. In them, pale soil and low relative humidity are important passive factors of selection that give adaptive value to the pale colors of pelage of both infernatis and handleyi.

Specimens from 6¹/₂ mi. SW Izucár de Matemores, and 1 mi. SSW Tilapa, Puebla, are intergrades between B. m. infernatis and B. m. [Pg 620] pallidus. These specimens are intermediate in color and cranial characters between the aforementioned subspecies but possess more of the pale brown overtones seen in paratypes of pallidus, and are best referred to that subspecies.

Remarks.—Merriam (1892:170) described Sitomys [= Baiomys] musculus on the basis of 23 specimens (from Colima City, Colima; Armeria, Colima; Plantinar, and Zapotlán, Jalisco). According to the original description, B. musculus resembled a small house mouse and was smaller than any known species of Sitomys except S. taylori [= Baiomys taylori]. From taylori, musculus differed in being larger [in size of body], and in having longer ears and tail, and larger hind feet. When Allen and Chapman (1897:203) described Peromyscus [= Baiomys] musculus brunneus from Jalapa, Veracruz, the specimens described by Merriam from Colima and Jalisco became representative of the nominal subspecies B. m. musculus. Osgood (1909:258) assigned specimens from Colima, Guerrero, Jalisco, Michoacán, Morelos, Oaxaca, Puebla, Sinaloa, Veracruz, and Zacatecas to the subspecies musculus. Subsequently, Russell (1952:21) named the subspecies pallidus from the arid lowlands of Morelos; Hooper (1952:96) described the subspecies infernatis [Pg 622] from northern Oaxaca and southeastern Puebla; and Goodwin (1959:1) described a new subspecies nebulosus from the Oaxaca highlands. Each of the subspecies mentioned immediately above was described from within the geographic range assigned to B. m. musculus by Osgood (loc. cit.). Hall and Kelson (1959:661) mapped the range of B. m. musculus so as to include Colima, parts of Jalisco, Michoacán, Guerrero, Oaxaca, and Veracruz. Lukens (1955:159), in a study of the mammals of Guerrero, has shown that the characters attributed to B. m. pallidus are not significantly different from those of pygmy mice studied from Guerrero. He (loc. cit.) concluded that: (1) if the specimens of pygmy mice from central Guerrero were typical of the subspecies musculus, then pallidus did not deserve subspecific recognition, or; (2) the name B. m. musculus should be restricted to the larger pygmy mice inhabiting the lowlands immediately adjacent to the Pacific Coast and the area to the north. My data (see Figure 12) show pygmy mice from southwestern Nayarit, northwestern and central Jalisco, Colima, and parts of Michoacán to be significantly larger in certain cranial and external measurements than pygmy mice from Guerrero, Oaxaca, Morelos, and parts of Puebla. This finding essentially corroborates Hooper's (1952a:96) findings. It seems advisable, therefore, to restrict the range of B. musculus musculus to the large mice inhabiting west-central México and the coastal lowlands of Colima and Michoacán. The name pallidus is applicable to the smaller mice occupying Morelos, southwestern Puebla, Guerrero, Oaxaca, and southwestern Chiapas.

B. m. musculus intergrades with B. m. pallidus in eastern Michoacán and central and western Guerrero. Specimens from San José Prura and 12 mi. S Tzitzio, Michoacán, though referable to B. m. musculus because of slightly larger size of crania are intermediate in size and color between the smaller and slightly darker pallidus to the south and east and the larger, slightly paler musculus to the northwest.

Remarks.—Hooper (1952a:93-94) reported specimens from the coastal strip of southern Chiapas as the most intensely pigmented, whereas, specimens from central and western Chiapas were distinctly paler. Crania of specimens from the coastal region of southern Chiapas were smaller than crania from the central highlands and mountains of Chiapas. My studies essentially corroborate the findings of Hooper. The gradation of color between the pale brown pallidus to the north in Oaxaca, and the brownish-black nigrescens to the south in Chiapas is extremely gradual. Specimens from the central and western parts of Chiapas (see Figure 10 for localities) are difficult to assign to either pallidus or nigrescens. Equal justification exists for assignment to either subspecies. I have assigned the specimens to nigrescens because they are geographically closer to the type locality of nigrescens. Specimens from Reforma, Oaxaca (assigned by Hooper, 1952a:93-94, to nigrescens), are nearly identical in size and color to paratypes of pallidus. I assign the Reforma specimens to pallidus.

The darkest of all the specimens examined and assigned to nigrescens are from 1 mi. NW San Salvador and 1 mi. S Los Planes, El Salvador. The variations in color in this subspecies closely correspond to degree of relative humidity; the palest samples are from areas of low relative humidity and the darkest are from areas [Pg 625] of high relative humidity. In view of the present state of differentiation of specimens from the southern coastal areas of Chiapas and mountainous areas of El Salvador, it would seem that populations there might be incipient subspecies.

[Pg 626]

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.

[Pg 627]

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 altitudes 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 Miahuatlán; 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.

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 passes 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.

[Pg 630]

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

There is close correlation in B. taylori, as also in B. musculus, of darker pelages with zones of high relative humidity. The subspecies having dark pelages are: analogous, fuliginatus, and subater. The two first-mentioned subspecies occur at high altitudes, and [Pg 631] the other, subater, occurs in the humid coastal region of Texas. The paler subspecies, taylori, canutus, and allex, occur at lower altitudes. Two subspecies that occur at relatively high altitudes, ater and paulus, are reddish-brown. The color of pelage in these subspecies resembles the color of soil upon which they live. Blair and Blossom (1948:5) demonstrated close correlation of color of soil with color of pelage in B. t. ater by use of an Ives tint photometer.

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.

1. B. t. allex 2. B. t. analogous 3. B. t. ater 4. B. t. canutus

5. B. t. fuliginatus 6. B. t. paulus 7. B. t. subater 8. B. t. taylori

[Pg 632]

Habitat and numbers.—The habitat occupied by the northern pygmy mouse ranges from sparse grassy areas along rock walls in central México (see Davis, 1944:394), and mesquite-cactus associations in southern Texas (Blair, 1952:242) to heavy stands of grasses such as Bouteloua sp., Andropogon sp., Hilaria sp., and sacaton grass intermixed with Yucca glauca in New Mexico, Arizona (see Hoffmeister 1956:281), and Chihuahua. Baker (1951:213) reports the species from 2 km. W El Carrizo, Tamaulipas, in dense grass and weeds at the edge of a cornfield. Hooper (1953:7) recorded the northern pygmy mouse in a cultivated field overgrown with herbaceous vegetation at Pano Ayuctle, Tamaulipas. In the State of Sinaloa, Hooper (1955b:13) obtained specimens in grass and among shrubs and vines bordering a fallow field. The northern pygmy mouse, in general, lives in situations more xerophytic and more grassy than does the southern pygmy mouse.

The northern pygmy mouse, as the southern pygmy mouse, is locally abundant in its geographic range. Stickel and Stickel (op. cit.: 145) pointed out that on the third night of live-trapping in Bexar County, Texas, there was a sudden increase in unmarked pygmy mice trapped. This increase in numbers, after the resident population was seemingly marked, followed a one-half inch rainfall. Collectors from the University of Kansas, myself included, have had similar experiences in trapping these mice. In the Mexican states of Guanajuato, Querétaro, and Jalisco, B. taylori is one of the commonest small mammals. In New Mexico and Arizona and the Mexican states of Sonora and Sinaloa, nevertheless, these mice are rare.

Stickel and Stickel (loc. cit.) thought that the home range normal for B. taylori in a grassy habitat was less than 100 square feet, but Blair (1953:10) thought that a complete home range had not been recorded by Stickel and Stickel.

Behavior.—The northern pygmy mouse is crepuscular to nocturnal and where I trapped in northern Mexico was one of the first small rodents to appear in my traps in the evening. Hall and Villa-R (1949:460) recorded this habit in Michoacán. 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 [Pg 633] litter born in captivity in the course of my study, the female killed the male when the young were four days old. In another instance, the female and two eight-day-old young were killed by the male. Until that time, the male, female, and young had lived together peacefully. In other litters born in captivity, adult males did not harm the other mice.

I have noted, as Blair (loc. cit.) did, that B. taylori utters high-pitched squeals in a "singing" posture resembling that of the coyote, yet remains silent when being handled.

The northern pygmy mouse makes runways in the grass, in miniature resembling those of Microtus, and often uses runways constructed by Sigmodon. A small firm nest of finely shredded plant material (mostly grasses) is constructed in burrows or under logs, rocks, or fallen cactus plants. Thomas (1888:447) recorded nests of fine curly grass and cornsilk. Secondary refuge nests are not uncommon. Thomas (loc. cit.) states, "If other mice live in the same place, the individuals of Baiomys watch till others disappear, then suddenly steal part of the other nest and run to their own with it."

Enemies and food.—Little is recorded of the animals that prey upon the northern pygmy mouse. Twente and Baker (1951:120) found remains of B. taylori in 16 per cent of barn owl pellets (Tyto alba pratincola) collected 21 mi. SW Guadalajara, Jalisco. Presumably most of the crepuscular and early nocturnal raptorial birds and carnivorous mammals feed on these mice.

Food of B. taylori consists in part of grass seeds and leaves, prickly pear (Opuntia sp.) and the softer exposed parts of roots of vegetation among which the mice reside.

Reproduction.—The northern pygmy mouse breeds throughout the year. The only months in which I have not recorded pregnant females or females with young are June and October. Forty-one records of embryos or young per litter average 2.48 (less than in B. musculus), and range from as few as one to as many as four per litter.

[Pg 634]

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

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

A series of 39 specimens from 1 mi. SSE Ameca, 4000 ft., Jalisco, are uniformly grayish-brown. This series averages grayer than paratypes of allex. There is little, if any, difference between the series from 1 mi. SSE Ameca and paratypes of allex in external size of body, hind foot, length of ear, and size and conformation of the cranium. Populations from Ameca and vicinity might be expected to average considerably larger inasmuch as they occur at higher altitudes (see Bergman's Rule, p. 609) then the material from the lower coastal plains to the south in Colima and Michoacán, and at lower elevations in the west in Jalisco and Nayarit. The means of external [Pg 636] and cranial measurements are not significantly different between the specimens from the highlands and those from the lowlands. In the area of Ameca where the two species B. musculus and B. taylori occur together, interspecific competition seems to have limited, perhaps even reduced, size of external and cranial parts of taylori (see p. 660).

In color, specimens from the northern part of the valley of the Río Tepalcatepec (10 mi. S, 1 mi. W Apatzingán) in Michoacán resemble paratypes of allex. Intergradation probably occurs to the north with analogous.

In the eight specimens from 13 mi. E and 1 mi. N Talpa de Allendé, the skull, as reflected in occipitonasal length and zygomatic breadth relative to length of body, is larger than in other specimens here assigned 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.

Remarks.—The pelage of analogous becomes paler with wear as pointed out by Osgood (1909:257). A paratype, U. S. Nat. Mus. 120260, and several specimens from 1 mi. S, 11 mi. W Zamora, Michoacán, are grayish rather than brownish-black. All of these are old adults having the terminal black parts of the hairs on the dorsum nearly worn away. Excluding such grayish individuals, B. t. analogous, like B. t. subater and B. t. fuliginatus, is uniformly brownish-black. Both analogous and fuliginatus occur in relatively high mountainous country on dark soils or pedregals, and all three of the aforementioned subspecies occur in zones of high relative humidity.

B. t. analogous intergrades with B. t. paulus (see account of that subspecies) and B. t. allex south and west of Lago de Chapala in Jalisco. Additional specimens are needed from Querétaro and San Luis Potosí in order to ascertain whether or not B. t. analogous intergrades with B. t. fuliginatus or B. t. taylori. Specimens from western Jalisco, in the past referred to B. t. analogous, are referable to B. t. allex (see account of that subspecies). Specimens obtained west of, and bordering, the Río del Naranjo in Jalisco show a mixture of characters of both B. t. allex and B. t. analogous. For example, specimens from 2 mi. N Ciudad Guzmán resemble analogous on the dorsum, whereas, on the belly, the individual hairs are white-tipped, pale gray at the base, and in over-all appearance are whitish-gray, unlike typical analogous (being like allex instead). The dorsal surface of the forefeet are sooty to light brownish (as in analogous), [Pg 639] whereas, the hind feet are flesh-colored (as in allex). Another series of specimens from 4 mi. W León, Guanajuato, are intergrades between B. t. analogous and B. t. paulus. These specimens are grayish to brownish on the dorsum, have sooty forefeet and hind feet (more nearly as in analogous than in paulus), are grayish-white on the venter, and have a distinctly bicolored tail (resembling that of paulus more than that of analogous). When the average of cranial characters is considered, both series are best referred to analogous.

Hooper (1947:50) pointed out that specimens from the pedregal San Gerónimo, Distrito Federal, were more nearly black than topotypes and generally showed less brownish hues typical of analogous. I have examined this series and several others from this area (see Specimens examined, p. 640) and am convinced that these populations average darker. Actually, the dorsum is more nearly black and the venter is more buffy than in typical analogous. The hairs of these individuals average longer than in other populations of analogous. Skulls of the specimens from the pedregal are indistinguishable from those of paratypes of analogous. The populations from the Distrito Federal seem to be incipient subspecies.

Remarks.—Blossom and Burt (1942:1) described B. t. ater as the darkest of the known subspecies. It is dark, but specimens from some parts of the ranges of B. t. analogous, B. t. fuliginatus, and B. t. subater exceed in melanins the darkest individuals of ater. Blair and Blossom (1948:5) also concluded by the use of an Ives tint photometer that B. t. subater was significantly darker than B. t. ater.

When paratypes of ater and specimens of B. t. paulus are compared, the darkest individuals of ater exceed but slightly the darkest of paulus. The darkest specimens of paulus occur in southern Zacatecas, and northern Jalisco, and the palest of the series are in northern Durango and southern Chihuahua. When paratypes of ater and paulus are compared, the difference in color is readily distinguishable. Specimens from 1¹/₂ mi. N San Francisco, in northern Chihuahua, appear to be intermediate in color between ater and paulus except for a faint tinge of buff ventrally. In characters of the crania, these specimens resemble ater and are referred to that subspecies. A slightly different pattern of color is present in pygmy mice from the Peloncillo Mountains and the Animas Valley of New Mexico; the upper parts resemble those of paratypes of ater, but the venter has only the faintest suggestion of the [Pg 642] buffy wash. Crania of these specimens from New Mexico are inseparable from those of paratypes of ater, and the specimens are, therefore, referred to ater.

When specimens are arranged by localities from Arizona east into southern New Mexico, thence south into Chihuahua and Durango, gradual intergradation in color is evident from dark in the north to pale browns in the south, whereas, size and shape of interparietal and size and shape of coronoid process of the lower jaw divide quite distinctly into two morphological types in central Chihuahua.

Cranial variation in size and proportion among adults is slight throughout the range of ater compared to variation detected in other subspecies of Baiomys taylori. Perhaps such a relatively stable pattern of characters of the crania reflects the homogeneity of the gene pool, with respect to these characters, of the populations sampled. The fact that the color of the pelage of this subspecies varies considerable throughout its known range and that the crania do not is perhaps a clue to the mode of inheritance of characters in these mice. Seemingly, color of pelage is inherited independently of characters of the cranium. The relative lack of variability in the crania of ater may result from uniform environmental conditions, which have served to select for uniform characters in the populations. All of the other wide-ranging subspecies of B. taylori occupy more diverse habitats than ater. Secondly, the rather abrupt change in the cline of measured characters of the crania between ater and paulus in central Chihuahua suggests a secondary zone of intergradation. The probable cessation of gene flow in the past between these two subspecies, allowing ater to be isolated for a time, may also, in part, account for the relative lack of variability in the crania of ater.

Remarks.—Burt (1938:54) reluctantly assigned specimens from Ciudad Obregón to B. t. paulus, probably being influenced by the resemblance in size. He suggested that, perhaps, a distinct subspecies occurs in the State of Sonora. Study of larger series of specimens than were available to Burt reveals that populations of pygmy mice inhabiting the northwest coastal plains of México are indeed distinct.

The darkest of the material assigned to canutus is from Nayarit (for specific localities see specimens examined). According to Tamayo (1949:Carta de Suelos), color of soil changes from chestnut in northern Sinaloa to black in southern Sinaloa and northern Nayarit. There seems, therefore, to be a close correlation between color of pelage and color of soil in this area. In Nayarit, particularly in the central and southern parts, the mice are intermediate in color between the paler, grayer population to the north and the more brownish samples, representative of allex to the south. The coastal vegetation changes from the arid tropical thorn forests of the north and central parts of Sinaloa to a savannah in Nayarit, thence to a tropical deciduous forest farther south (see Leopold, 1950:508).

In size and color, specimens from 3 mi. SE Tepic and 2 mi. SW Rosa Morada are intermediate between the larger, grayer canutus and the smaller, light-brownish allex. In size of cranium, these specimens are more nearly like canutus, and are referred to that subspecies. Mice from the western coastal plain are relatively homogeneous as regards size of body and skull, except that those from 13.5 mi. S Acaponéta, Nayarit, average somewhat larger.

B. t. canutus, like B. t. subater, is predominantly a lowland or coastal subspecies. The pallor of the former, that lives on generally paler soils, presumably is of adaptive value.

[Pg 645]

Pygmy mice are seemingly rare in the northern part of the range of this subspecies. J. Raymond Alcorn and Albert Alcorn were successful in collecting only two specimens from the type locality after three successive nights of trapping with 100 traps set each night. Only six specimens are known from Sonora. These were obtained in the irrigated regions of Ciudad, Obregón, and Navajoa. Charles Sibley obtained one specimen 10.6 mi. SE Ciudad Obregón in a "maguey field." I obtained one specimen 1 mi. NNW Navajoa in a sparse grassway, 20 feet wide, bordering an open sewer, which coursed northward into the Río Mayo. Irrigated wheat fields bordered the grassway and ditch.

Remarks.—Dalquest (1953:155-157) and Booth (1957:15) assigned all of the pygmy mice that they examined from the state of San Luis Potosí to B. t. taylori. Examination of all of the material that was available to Dalquest, plus additional specimens at the University of Kansas Museum of Natural History, reveals that there are three subspecies in San Luis Potosí. B. t. taylori occurs in the eastern part of the State at lower altitudes; B. t. analogous occurs to the southeast at higher altitudes; B. t. fuliginatus occurs in the northeastern part of the State in the Sierra Madre Oriental.

Specimens obtained from Ebano, Pujal, and Tamuín, representative of B. t. taylori, are much paler on the belly and on the ventral surface of the forefeet and hind feet than are specimens from Ciudad del Maíz, representative of B. t. fuliginatus. The tail in B. t. taylori is nearly unicolored and less hairy than in the paratypical series of fuliginatus. Specimens from 4 km. NE Ciudad Valles are nearly intermediate in color of the belly, dorsum, forefeet and hind [Pg 647] feet, and tail, between the palest mice from the coastal plain and the darker mice in the mountains of the northeastern part of the State (specimens from El Salto average paler, however, than the type and paratypes). These specimens seem to be intergrades between B. t. taylori to the east on the coastal plain and fuliginatus to the northwest in the mountains. It seems best to refer the mice from 4 km. N Ciudad Valles to B. t. taylori on the basis of the average of external and cranial characters. Specimens from 6 mi. SW San Gerónimo, Coahuila, also referred to B. t. taylori, resemble in color the mice from 4 km. N Ciudad Valles. When more specimens are obtained from the front range of the Sierra Madre Oriental, at lower altitudes, the manner in which these two subspecies intergrade with one another will be better understood. At present, populations from higher altitudes in the mountains seem to represent a dark subspecies; populations from the coastal plain represent a pale subspecies, and those from the lower slopes and high valleys seemingly are intergrades. B. t. fuliginatus occurs in a somewhat limited strip of chernozem soil (or suelos negros of Tamayo, 1949: Carta de Suelos). The populations occurring at lower altitudes on the coastal plain are on generally paler soils.

[Pg 648]

Remarks.—J. A. Allen (1903:599) correctly pointed out that young specimens, in first pelage, were gray brown; young adults were darker and more varied with some blackish; adults and old adults were buffy to grayish. The change in color of pelage with increasing age is more pronounced in paulus than in other subspecies of B. taylori. Of two males collected on April 12, 1949, one, an adult, is buffy brown, and the other, an old adult with worn pelage, is grayish-brown. In mice in the earlier stages of adulthood, [Pg 649] underfur of the dorsum is buffy at the tips and gray basally. With increased wear, the buffy tip is lost. Consequently, mice in the later stages of adulthood are grayish.

B. t. paulus intergrades with ater to the north in Chihuahua (see account of that subspecies), with analogous to the south in Jalisco, and with allex (see account of that subspecies) to the southwest in Nayarit and Jalisco. The zone of intergradation between paulus and analogous in Jalisco approximately borders the Río Grande de Santiago from the western part of the State to the northwest shore of Lago de Chapala. Nineteen specimens from 2 mi. WNW Lagos de Moreno in northwest Jalisco seem to be intermediate between paulus and analogous in color, averaging slightly grayer than typical paulus. The series of 19 is referable to paulus on the basis of cranial characters.

A series of 34 specimens from 3 mi. W La Venta, Jalisco (referable to paulus), is indistinguishable in color of pelage from two series of paulus from 5 mi. N Durango, and from 8 mi. NE of Durango, except that the antiplantar surfaces of the hind feet are sooty as in analogous. Seemingly, features of color mentioned above as diagnostic of the two subspecies are either present or absent and there is no tendency toward intermediacy in color in the population from 3 mi. W La Venta.

The Río Grande de Santiago may have acted in the past as a physical barrier reducing gene flow between allex and paulus and in separating completely the two populations for limited periods.

[Pg 651]

Remarks.—This subspecies retains its chief diagnostic character, blackish mid-dorsal region, throughout nearly all parts of its range. Specimens from the general area of Matagorda Bay and Lavaca County grade into taylori in characters of color and crania. The Colorado and Brazos rivers seemingly serve as barriers reducing gene flow between taylori and subater. These rivers may well have been important factors in the origin and the limitation of these two seemingly closely-related subspecies.

Baiomys taylori subater is not differentiated in color of pelage and characters of crania from B. t. taylori to the same degree that B. t. paulus is differentiated from B. t. analogous, or that B. t. taylori is differentiated from several of the other subspecies of Baiomys taylori. B. t. subater probably is a more recent occupant of the area in which it now lives than is the case with any other one of the subspecies of taylori. Sufficient time probably has not elapsed to allow for formation of more distinctive phenotypic patterns.

Remarks.—The geographic range of taylori is 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) from B. t. subater with which taylori might 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 to taylori.

On December 27, 1958, a specimen, KU 81552, was obtained 3 mi. N Bowie, Montague County, Texas. This record station extends the known range of B. taylori 65 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 either B. t. taylori or B. t. subater. They may represent an incipient subspecies. I tentatively assign them to B. t. taylori because of the pale rather than dark (like B. t. subater) pelage. Additional specimens are needed from these areas and from the hiatus between the ranges of B. t. taylori and B. t. subater the better to understand the manner in which these two subspecies intergrade.

Among named subspecies of Baiomys taylori, B. t. taylori most closely resembles B. t. subater to the north in Texas. Nine specimens examined from Yoakum are intergrades between taylori and subater. These specimens have the sooty dorsal color of subater, but ventrally are inseparable from topotypes of taylori. In length of body and [Pg 654] tail, specimens from Yoakum are like subater, but in length of hind foot, they are intermediate between the two subspecies. Cranially, they are like subater. When all characters are considered, the specimens are best referred to subater. Bailey (1905:103) suggested that specimens from the southern part of the range, which he ascribed to subater, tended to a more grayish color than topotypes of subater, therefore, grading into taylori. 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 of Baiomys taylori in an area where life-zones might seem less important than in the mountainous west.

In the southern part of the range of taylori, intergradation occurs between B. t. taylori in western Tamaulipas and B. t. fuliginatus in the mountains of San Luis Potosí.

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

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. Baiomys seems 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 in [Pg 656] a 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. taylori seems to be morphologically more specialized for life in an arid grassland than was B. sawrockensis.

The geographic range of ancestral pygmy mice possibly extended farther south in late Pliocene time than the range of B. musculus does now. Anyhow, B. sawrockensis of the early late Pliocene dwelt in a more mesic type of habitat than B. musculus does, 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 from Baiomys, 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 from Baiomys in 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 in Calomys now living in South America. The present resemblance in internal morphological features between it and Baiomys, 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 distinguishing Baiomys musculus from Calomys laucha are as follows: posterior lacerate foramina between second, rather than first, upper molars; parapterygoid fossa shallower; mesopterygoid fossa as wide or [Pg 657] wider, 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 between Calomys and Baiomys may reflect convergence, but the total of internal and external morphological characters shared, I think reflects true relationships.

Peromyscus has a large number of living and extinct species and exhibits a wide range of morphological variation, whereas Baiomys has 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 of Baiomys today 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 that B. taylori was 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 of Baiomys may 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 habitat [Pg 658] still 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 extinct B. kolbi and B. rexroadi. The extinct B. brachygnathus, presumably a dweller of a xerophytic grassland, may have had its origin from a B. minimus-like stock in the manner outlined.

The morphological difference between the extinct B. minimus and the living B. musculus is not great, and musculus seems to be the product of the B. sawrockensis-B. minimus line of development. Morphological characters of the parental stock of the two living species, musculus and taylori, may have been intermediate between those of B. minimus and those of B. musculus. The principal part of the range of Baiomys today 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 Terra et 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 climates [Pg 659] or pluvial periods. Such climates probably altered habitat formerly suitable for Baiomys. There is no record of Baiomys known 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 (see Figure 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 analogous is a large subspecies; it lives allopatrically in the southern part of the range of the species. B. musculus pallidus is 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. taylori is smaller than elsewhere and B. musculus is larger than elsewhere. B. t. analogous lives 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 with B. musculus musculus at lower elevations in west-central México. The small size of allex could 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 of musculus to the south at higher elevations. B. m. musculus lives at low elevations along the coast of west-central México. Unlike B. m. pallidus, B. m. musculus is large at lower elevations. It occurs sympatrically with B. 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, musculus and taylori, made contact at lower elevations where individuals of taylori may have been smallest, but individuals of musculus were not the largest of the species. The differences, therefore, between the two species in their initial contact probably were slight. Hybrids, [Pg 660] 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 (see Figure 12). The two species could be confused if a sample of adults of taylori from 7 mi. S La Belle, Jefferson County, Texas, were compared to a sample of adults of musculus from Tehuantepec, Oaxaca (see Figure 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 (see Figure 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 both B. musculus and B. taylori in a single trap line. A series of pygmy mice collected from San Gabriel, Jalisco, contained one taylori and 33 musculus; another sample from La Resolana, Jalisco, had a ratio of 25 taylori to 6 musculus. 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 of B. musculus (typical of arid tropical lowlands) meets that of B. taylori (typical of arid temperate highlands), the two geographic ranges interdigitate with parts of the range of musculus extending into the highlands and parts of the range of taylori extending into the lowlands. In the lowlands, musculus may be better adapted to environmental conditions and, therefore, more successful in competition with taylori for available habitat. The reverse situation may exist in the highlands. Also, the fact that musculus is more of a diurnal animal than is taylori [Pg 661] may 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 many musculus. 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. In Baiomys, (c) seems to apply. Natural selection probably would favor a continuation of diurnal activity in musculus and nocturnal activity in taylori, thereby preventing frequent meeting of the two species.

In both species of Baiomys, the most distinct subspecies, B. t. allex and B. 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 of B. musculus in Guatemala, El Salvador and Honduras. Three subspecies occur in this area (grisescens, handleyi and nigrescens) and incipient subspeciation is in evidence there.

Hooper (1949:25) regards Baiomys as a member of the rodent fauna of the arid, western Sonoran region, whereas Hershkovitz (1958:609) suggests that Baiomys is 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 of Baiomys to certain hesperomine mice of South America, it is postulated that Baiomys, in more primitive form than now, occurred farther south in past times than it does now. Fossils show that primitive stocks of the genus [Pg 662] in 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.

[Pg 663]

[Pg 664]

1. Two Recent species, each polytypic with eight subspecies, and five fossil species are recognized.

2. The phyletic trends in the genus Baiomys have been from an ancestral stock that possessed relatively brachydont teeth having raised cingular ridges and orthodont to proödont incisors, to species having hypsodont teeth with reduced cingular ridges and retrodont incisors.

3. Reduction of cingular ridges in pygmy mice is associated with an existence in open grassland (more xeric than mesic), whereas, the presence of cingular ridges is associated with an existence in a savannah habitat (more mesic than xeric).

4. Shifts of geographical range of populations of pygmy mice at and near the periphery of their geographic range may account for the differentiation of the extinct species.

5. The two living species, B. musculus and B. taylori, are seemingly derived from a common ancestor that in morphological structure was intermediate between B. minimus and B. musculus.

6. The living species of pygmy mice resulted from a geographic separation, perhaps occurring in the Iowan glacial period (See De Terra, 1949:51) in the transverse volcanic zone of central México.

7. The two species are now sympatric in west central México, where morphological characters (size and shape of body and length of skull) differ most. Where the two species are allopatric, these same morphological characters differ least.

8. This is a documented instance of character displacement in mammals.

9. On the basis of internal morphological characters studied (auditory ossicles, hyoid apparatus, and baculum), Baiomys seems to be more closely related to a South American hesperomine, perhaps Calomys, than to any North American cricetine.

10. Pygmy mice were more widely distributed in the past than they are at present. Part of the ancestral stock of the pygmy mice may have emigrated from North America into South America in a brief period in the Pliocene; if so, it is easy to understand why certain South American hesperomines resemble Baiomys.

[Pg 665]

11. The combination of morphological and behavioral characters in the living pygmy mice warrants generic status for them. If Baiomys were treated as a subgenus of the genus Peromyscus, there would be adequate justification for including in the genus Peromyscus a number of other genera, some of them occurring in South America. Such lumping of genera would reduce our understanding of the natural relationships among this group of cricetine rodents.

[Pg 666]

[Pg 667]

[Pg 668]

[Pg 669]

[Pg 670]

Transmitted March 4, 1960.

28-3030

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