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UNIVERSITY OF KANSAS PUBLICATIONS
MUSEUM OF NATURAL HISTORY
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
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
[Illustration: Look for the Union Label]
28-3030
Speciation and Evolution of the Pygmy Mice, Genus Baiomys
BY
ROBERT L. PACKARD
CONTENTS
PAGE
Introduction 583
Materials, Methods and Acknowledgments 584
Paleontology of the Genus 587
_Baiomys sawrockensis_ 588
_Baiomys rexroadi_ 589
_Baiomys kolbi_ 590
_Baiomys brachygnathus_ 590
_Baiomys minimus_ 591
Phyletic trends 592
Non-Geographic Variation 595
Variation with age 595
Secondary sexual variation 597
Individual variation 597
Pelage and molts 598
Taxonomic Characters and Relationships 600
External parts 600
Pelage 600
Skull 600
Teeth 601
Hyoid apparatus 601
Baculum 603
Auditory ossicles 605
Genus Baiomys 607
Systematic Accounts of Species and Subspecies 608
_Baiomys musculus_ 608
_Baiomys musculus brunneus_ 612
_Baiomys musculus grisescens_ 614
_Baiomys musculus handleyi_ 617
_Baiomys musculus infernatis_ 618
_Baiomys musculus musculus_ 620
_Baiomys musculus nigrescens_ 623
_Baiomys musculus pallidus_ 625
_Baiomys musculus pullus_ 628
_Baiomys taylori_ 630
_Baiomys taylori allex_ 633
_Baiomys taylori analogous_ 637
_Baiomys taylori ater_ 640
_Baiomys taylori canutus_ 643
_Baiomys taylori fuliginatus_ 645
_Baiomys taylori paulus_ 647
_Baiomys taylori subater_ 650
_Baiomys taylori taylori_ 651
Evolution and Speciation 655
Formation of the Recent Species 658
Areas of present differentiation 661
Zoogeographic position 661
Conclusions 664
Literature Cited 665
INTRODUCTION
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 Mexico. They are notably
common in central Mexico, 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, Mexico. 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) who contributed
importantly to understanding the relationships of the two living species
in central Mexico. 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.
MATERIALS, METHODS AND ACKNOWLEDGMENTS
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.
_Occipitonasal length._--From anteriormost projection of nasal bones to
posteriormost projection of supraoccipital bone. _A_ to _A'_
_Zygomatic breadth._--Greatest distance across zygomatic arches of
cranium at right angles to long axis of skull. _B_ to _B'_
_Postpalatal length._--From posterior margin of hard palate to anterior
margin of foramen magnum. _C_ to _C'_
_Least interorbital breadth._--Least distance across top of skull
between orbits. _D_ to _D'_
_Length of incisive foramina._--From anteriormost point to posteriormost
point of incisive foramina. _E_ to _E'_
_Length of rostrum._--The distance in a straight line from the notch
that lies lateral to the lacrimal to the tip of the nasal on the same
side. _F_ to _F'_
_Breadth of braincase._--Greatest distance across braincase, taken at
right angles to long axis of skull. _G_ to _G'_
_Depth of cranium._--The distance from the dorsalmost part of the
braincase to a flat plane touching tips of incisors and ventral border
of each auditory bulla. A glass slide one millimeter thick was placed on
the ventral side of the skull. One jaw of the caliper was on the lower
surface of the slide and the other jaw on the dorsalmost part of the
braincase. The depth of the slide was subtracted from the total reading.
_H_ to _H'_
_Alveolar length of maxillary tooth-row._--From anterior border of
alveolus of M1 to posterior alveolus of M3. _I_ to _I'_
[Illustration: 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, Esteli, Nicaragua.
x 1-1/3.]
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 Mexico, 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.
[Illustration: FIG. 2. Occlusal views of molars. x 13.
A. _B. taylori analogous_, subadult, female, No. 28102 KU, 4 km.
ENE Tlalmanalco, 2290 meters, Estado de Mexico. Right, upper
molars.
B. _B. musculus musculus_, subadult, male, No. 45456 USNM, Colima,
Colima, Mexico. Left, upper molars.
A'. _B. taylori analogous_, subadult, female, No. 28102 KU 4 km.
ENE Tlalmanalco, 2290 meters, Estado de Mexico. Left, lower
molars.
B'. _B. musculus musculus_, subadult, male, No. 45456 USNM, Colima,
Colima, Mexico. Right, lower molars.]
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:
American Museum of Natural History, G. G. Goodwin and R. G. VanGelder.
Carnegie Museum, J. K. Doutt.
California Academy of Sciences, Robert T. Orr.
Chicago Natural History Museum, Phillip H. Hershkovitz.
Cleveland Museum of Natural History (Collection now a part of Museum of
Zoology, University of Michigan, W. H. Burt, E. T. Hooper).
Louisiana State University, Museum of Natural History, George H. Lowery,
Jr.
Los Angeles County Museum, Charles A. McLaughlin.
United States National Museum (Biological Survey Collections), David A.
Johnson, and Viola S. Schantz.
United States National Museum, Division of Vertebrate Paleontology,
C. Lewis Gazin.
University of Arizona, E. L. Cockrum, and G. VR. Bradshaw.
University of California, Museum of Vertebrate Zoology, Seth B. Benson,
and W. Z. Lidicker.
University of Illinois, Museum of Natural History, Donald F.
Hoffmeister.
University of Michigan, Museum of Zoology, W. H. Burt, E. T. Hooper, and
Claude W. Hibbard.
University of New Mexico, James S. Findley.
University of Texas, Frank W. Blair.
Texas A & M, Cooperative Wildlife Research Collection, W. B. Davis.
The Museum, Michigan State University, Rollin H. Baker.
University of Florida Collections, James N. Layne.
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.
PALEONTOLOGY OF THE GENUS
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, 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.
=Baiomys sawrockensis= Hibbard
_Baiomys sawrockensis_ Hibbard, Papers Mich. Acad. Sci., Arts and
Letters, 38:402, April 27, 1953.
_Type._--No. 27506, Univ. Michigan; left mandibular ramus bearing m1-m3
and incisor; Saw Rock Canyon, early late Pliocene, XI member of the
Rexroad formation, sec. 36, T. 34 S, R. 31 W, Seward County, Kansas
(University of Kansas, Locality 6).
_Referred material._--Univ. Michigan, Nos. 25781, 27503-27505,
28159-28165, 29708-29715, 31015.
_Diagnosis._--Ramus of medium size to small for the genus; lower incisor
broad, moderately recurved; diastemal region broad; anterior median fold
between anterior labial conulid and anterior lingual conulid of m1 deep;
primary first fold between anteroconulid and protoconid of m2 deep;
cingular ridge (ectolophid) at entrance to posteroexternal reentrant
valley (major fold, see Figure 2) between protoconid and hypoconid of m1
and m2; average and extreme measurements of lower molar row of eight
specimens are, 2.65 (2.5-2.7).
_Comparisons._--For comparisons with _B. brachygnathus_, see account of
that species. From _B. rexroadi_, _B. sawrockensis_ differs in: anterior
median fold of m1 deeper; incisor narrower; diastemal region broader;
coronoid process broader and better developed; cingular ridges
(ectolophids and mesolophids) more pronounced in their development;
incisors less prooedont, more retrodont.
From _B. kolbi_, _B. sawrockensis_ differs in: crowns of molars
narrower; incisors less prooedont; cingular ridges (ectolophids and
mesolophids) of m1 and m2 more pronounced in their development.
From _B. minimus_, _B. sawrockensis_ differs in: incisor less
procumbent; masseteric ridge extending farther anteriorly; anterior
cingulum of m2 slightly larger.
From _B. musculus_, _B. sawrockensis_ differs in: over-all size of jaw
and molar row less; diastema more acutely curved; incisors shorter;
anterior median fold of m1 slightly deeper.
From _B. taylori_, _B. sawrockensis_ differs in: m1 and m2 smaller;
cingular ridges in m1 and m2 more pronounced; anterolingual conulid
farther forward; incisors shorter, more prooedont; molar teeth depressed,
less hypsodont; diastemal region broader, more acutely curved;
masseteric ridge not extending so far anteriorly.
_Remarks._--_B. sawrockensis_ is the oldest known pygmy mouse. The
extreme development of the anterior median fold between the
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.
=Baiomys rexroadi= Hibbard
_Baiomys rexroadi_ Hibbard, Amer. Midland Nat., 26:351, September,
1941; Hibbard, Contrib. Mus. Paleo., Univ. Michigan, 8(2):145,
June 29, 1950 (part); Hibbard, Papers Mich. Acad. Sci., Arts
and Letters, 38:403, April 27, 1953.
_Type._--No. 4670, Univ. Kansas; left mandibular ramus bearing m1-m3,
and incisor; Rexroad fauna, Locality no. 2, Upper Pliocene, Meade
County, Kansas.
_Referred material._--Univ. of Michigan Nos. 24840, 24851, 27493, 27496,
27501, 28862-28867.
_Diagnosis._--Ramus medium in size for the genus; incisors small,
prooedont; anterior median fold of m1 slight; cingulum of all molars
poorly developed; average and external measurements of lower molar row
of seven specimens are, 2.7 (2.6-3.0).
_Comparisons._--For comparisons with _B. sawrockensis_ and _B. minimus_,
see accounts of those species. From _B. kolbi_, _B. rexroadi_ differs
in: over-all size of mandibular ramus, incisors, and molars smaller;
anterior median fold of m1 present, though poorly developed.
From _B. brachygnathus_, _B. rexroadi_ differs in: over-all size of
mandibular ramus smaller; m3 larger; posterior cusps (hypoconid and
entoconid) elongated; diastema shorter, less acutely recurved; incisors
less prooedont; cingular ridges of m1 and m2 less well-developed.
From _B. musculus_, _B. rexroadi_ differs in: over-all size of
mandibular ramus less; cingular ridges of m1 and m2 less well-developed;
incisors smaller, more prooedont; molars less depressed.
From _B. taylori_, _B. rexroadi_ differs in: m3 more triangular,
posterior part narrower; mental foramen closer to anterior root of m1;
masseteric ridge closer to alveolus of m1; incisor shorter, more
prooedont; molars more depressed.
_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).
=Baiomys kolbi= Hibbard
_Baiomys kolbi_ Hibbard, Trans. Kansas Acad. Sci., 55:201, June 18,
1952; Hibbard, Papers Mich. Acad. Sci., Arts and Letters, 38:403,
April 27, 1953.
_Type._--No. 24846, Univ. Michigan; right mandibular ramus bearing m1-m3
and incisor; Fox Canyon, upper Pliocene, Rexroad formation, Rexroad
fauna, Univ. Michigan Locality K1-47, sec. 35, T. 34 S, R. 30 W, XI
Ranch, Meade County, Kansas.
_Referred material._--Univ. Michigan Nos. 24845-24848, 27494, 27497,
27499, 28566, 28861, 28878, 28880-28882, 28884, 28886.
_Diagnosis._--Ramus of medium size to large for the genus; lower incisor
short, narrow transversely, prooedont; anterior median fold of m1 reduced
or absent; cingular ridges of m1 and m2 moderately well-developed; m3
large relative to m1 and m2; average and extreme measurements of lower
molars of seven specimens are, 3.0 (3.0-3.1).
_Comparisons._--For comparisons with _B. sawrockensis_ and _B.
rexroadi_, see accounts of those species. From _B. brachygnathus_, _B.
kolbi_ differs in: molar row longer; m3 and jaw larger; diastema longer;
masseteric ridge not so far forward; molars more depressed.
From _B. minimus_, _B. kolbi_ differs in: molar row longer; m3 larger;
jaw larger; diastema not so acutely curved; incisor shorter, narrower
transversely, more prooedont.
From _B. musculus_, _B. kolbi_ differs in: anterior median fold of m1
slightly developed or absent, instead of well-developed; m3 larger (not
reduced), external reentrant valley broad and extending farther across
crown of tooth; incisor smaller, and more prooedont; cingular ridges of
m1 and m2 less well-developed.
From _B. taylori_, _B. kolbi_ differs in: molars larger, more depressed;
incisor shorter, more prooedont; m3 smaller relative to m1 and m2;
external reentrant valley of m3 broad, extending farther across crown of
tooth.
_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.
=Baiomys brachygnathus= (Gidley)
_Peromyscus brachygnathus_ Gidley, U. S. Geol. Surv. Prof. Papers,
131:124, March 15, 1922.
_Baiomys brachygnathus_, Hibbard, Amer. Midland Nat., 26:352,
September, 1941.
_P. [eromyscus] brachygnathus_, Wilson, Carnegie Inst. Washington
Publ., 473:33, May 21, 1936.
_Type._--No. 10501, U. S. Nat. Mus.; right mandibular ramus bearing
m1-m3, and incisor; 2 mi. NE Curtis Ranch house, near a line between
sec. 28 and 29, T. 18 S, R. 21 E, Mid-Pleistocene (Hibbard, 1958:25),
Cochise County, Arizona.
_Referred material._--None.
_Diagnosis._--Ramus small for the genus; m3 reduced; jaw reduced
anteroposteriorly; incisor short, slender, prooedont; cingular ridges
well-developed, posterior ectolophid continuous from protoconid to
hypoconid in m1 and m2; diastema short; length of molar row 2.8 mm.
_Comparisons._--For comparisons with _B. rexroadi_ and _B. kolbi_, see
accounts of those species. From _B. minimus_, _B. brachygnathus_ differs
in: jaw not so slender anteriorly; masseteric ridge not so far anterior;
cheek-teeth slightly broader, less depressed, therefore, more hypsodont;
incisor shorter, more prooedont.
From _B. sawrockensis_, _B. brachygnathus_ differs in: molar row
slightly longer; teeth slightly less depressed; masseteric ridge extends
farther anteriorly; incisors more prooedont.
From _B. musculus_, _B. brachygnathus_ differs in: jaw smaller; molar
row slightly shorter; molars less depressed; incisors slender, shorter,
narrower, and more prooedont.
From _B. taylori_, _B. brachygnathus_ differs in: incisor more slender,
shorter, more prooedont; diastema shorter.
_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 prooedont
incisors, which feature seems to preclude _brachygnathus_ being
ancestral to _taylori_. _B. brachygnathus_ may have been a specialized
divergence from _B. minimus_.
=Baiomys minimus= (Gidley)
_Peromyscus minimus_ Gidley, U. S. Geol. Surv. Prof. Papers,
131:124, March 15, 1922.
_Baiomys minimus_, Hibbard, Amer. Midland Nat., 26:352, September,
1941; Gazin, Prof. U. S. Nat. Mus., 92(3155):488, 1942.
_P. [eromyscus] minimus_, Wilson, Carnegie Inst. Washington Publ.,
473:33, May 21, 1936.
_Type._--No. 10500, U. S. Nat. Mus.; left mandibular ramus bearing m1-m3
and incisor; 2 mi. S Benson, sec. 22, T. 17 S, R. 20 E, Late Pliocene
(Blancan, Gazin, 1942:482), Cochise County, Arizona.
_Referred material._--None.
_Diagnosis._--Ramus small for the genus; molar teeth depressed; cingular
ridges (ectolophids) of m1 and m2 well-developed; anterior median fold
present (appearing larger owing to chip of enamel missing); external
reentrant fold of m3 progresses half way across crown of tooth; diastema
short; incisor moderately large, recurved; length of molar row, 2.6 mm.
_Comparisons._--For comparisons with _B. brachygnathus_, _B. kolbi_, and
_B. sawrockensis_, see accounts of those species. From _B. rexroadi_,
_B. minimus_ differs in: anterior median fold deeper; incisor longer,
more recurved, less prooedont; molars slightly more depressed (though
worn).
From _B. musculus_, _B. minimus_ differs in: over-all size of jaw and
molars smaller; incisors shorter; masseteric ridge more depressed.
From _B. taylori_, _B. minimus_ differs in: anterior median fold
slightly deeper; molar teeth more depressed; cingular ridges on m1 and
m2 better developed; masseteric ridge more depressed.
_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.
PHYLETIC TRENDS
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 prooedont 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 prooedont 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.
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 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.
[Illustration: 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. 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
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).
VARIATION WITH AGE
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.
_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.
TABLE 1.--Average and Extreme Measurements (in Millimeters) of
Skulls of Five Age-groups of Baiomys taylori from vic.
(see p. 595) Altamira, Tamaulipas, Mexico.
=============+===========+===========+===========+===========+===========
Age groups | Juvenile | Young | Subadult | Adult | Old adult
-------------+-----------+-----------+-----------+-----------+-----------
Number | | | | |
examined | 3 | 3 | 14 | 19 | 8
| | | | |
| | | | |
Total length | 77.0 | 92.6 | 97.6 | 99.9 | 101.6
| (74-79) | (89-96) | (91-103)| (93-105) | (98-107)
| | | | |
| | | | |
Length | 27.3 | 39.3 | 40.4 | 39.8 | 40.9
of tail | (24-29) | (37-41) | (36-43) | (35-45) | (38-45)
| | | | |
| | | | |
Length | 49.6 | 53.3 | 57.0 | 60.0 | 60.7
of body | (49-50) | (52-55) | (51-61) | (56-67) | (57-67)
| | | | |
| | | | |
Length of | 11.0 | 13.6 | 14.3 | 14.5 | 14.2
hind foot | (11) | (13-14) |(13.5-15.0)| (14-15) | (13-15)
| | | | |
| | | | |
Occipitonasal| 14.2 | 16.3 | 17.1 | 17.7 | 17.8
length |(13.6-15.2)|(15.8-16.9)|(16.7-17.6)|(17.2-18.3)|(17.6-18.1)
| | | | |
| | | | |
Zygomatic | 8.1 | 8.7 | 8.9 | 9.3 | 9.4
breadth | (7.8-8.6) | (8.6-8.8) | (8.6-9.3) | (9.0-9.6) | (9.1-9.6)
| | | | |
| | | | |
Interorbital | 3.4 | 3.4 | 3.4 | 3.6 | 3.5
breadth | (3.3-3.5) | (3.3-3.6) | (3.3-3.6) | (3.4-3.8) | (3.3-3.6)
| | | | |
| | | | |
Incisive | | | | |
foramina | 2.9 | 3.5 | 3.7 | 3.9 | 3.9
(length) | (2.8-2.9) | (3.4-3.6) | (3.6-3.9) | (3.6-4.1) | (3.5-4.0)
| | | | |
| | | | |
Depth | 5.9 | 6.5 | 6.5 | 6.7 | 6.8
of cranium | (5.6-6.2) | (6.3-6.8) | (6.2-6.8) | (6.4-7.0) | (6.5-7.1)
| | | | |
| | | | |
Alveolar | | | | |
length, | 2.7 | 2.9 | 2.9 | 3.0 | 3.0
upper molars | (2.5-2.8) | (2.9-3.0) | (2.8-3.1) | (2.9-3.2) | (3.0-3.1)
| | | | |
| | | | |
Postpalatal | 4.8 | 5.9 | 6.2 | 6.5 | 6.5
length | (4.5-5.3) | (5.8-6.0) | (5.8-6.6) | (6.2-7.2) | (6.3-6.7)
| | | | |
| | | | |
Breadth | 8.1 | 8.5 | 8.4 | 8.6 | 8.6
of braincase | (7.8-8.7) | (8.5) | (8.0-8.7) | (8.3-8.9) |(8.4-8.8)
-------------+-----------+-----------+-----------+-----------+-----------
SECONDARY SEXUAL VARIATION
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.
TABLE 2.--Analysis of Secondary Sexual Variation in Adult B. taylori
Vicinity of (see p. 595) Altamira, Tamaulipas, and Adult B.
musculus from El Salvador (see p. 595). (One Standard Deviation
on Either Side of the Mean is Given.)
==============+==========================+============================
| Baiomys taylori | Baiomys musculus
Character +------------+-------------+-------------+--------------
| 21 Males | 18 Females | 17 Males | 13 Females
--------------+------------+-------------+-------------+--------------
| | | |
Total length |98.4 +- 2.95 |100.5 +- 4.72 |112.04 +- 5.49|113.12 +- 4.23
| | | |
Length of tail|40.1 +- 2.31 | 40.3 +- 2.39 | 47.12 +- 2.95| 45.70 +- 2.92
| | | |
Length of body|57.83 +- 1.65| 60.10 +- 4.13| 66.67 +- 3.97| 67.75 +- 2.38
| | | |
Length of | | | |
hind foot |14.21 +- .53 | 14.44 +- .51 | 15.60 +- .49 | 15.38 +- .64
| | | |
Length of ear |10.00 +- .00 | 10.00 +- .00 | 11.80 +- .65 | 12.00 +- .41
| | | |
Occipitonasal | | | |
length |17.48 +- .40 | 17.47 +- .47 | 19.32 +- .35 | 19.04 +- .44
| | | |
Zygomatic | | | |
breadth | 9.17 +- .33 | 9.15 +- .30 | 9.84 +- .21 | 9.91 +- .28
| | | |
Least | | | |
interorbital | | | |
breadth | 3.53 +- .11 | 3.48 +- .11 | 3.88 +- .08 | 3.88 +- .12
| | | |
Postpalatal | | | |
length | 6.35 +- .19 | 6.38 +- .30 | 7.11 +- .15 | 6.95 +- .20
| | | |
Depth | | | |
of cranium | 6.65 +- .24 | 6.61 +- .17 | 7.10 +- .18 | 7.08 +- .18
| | | |
Incisive | | | |
foramina | | | |
(length) | 3.82 +- .15 | 3.81 +- .18 | 4.43 +- .11 | 4.35 +- .14
| | | |
Length | | | |
of rostrum | 5.87 +- .20 | 5.88 +- .21 | 6.81 +- .16 | 6.66 +- .31
| | | |
Breadth | | | |
of braincase | 8.54 +- .23 | 8.52 +- .12 | 9.84 +- .38 | 9.52 +- .20
| | | |
Alveolar | | | |
length, | | | |
upper molars | 2.98 +- .08 | 3.01 +- .08 | 3.20 +- .09 | 3.24 +- .10
--------------+------------+-------------+-------------+--------------
INDIVIDUAL VARIATION
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.
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.
TABLE 3.--Individual Variation: Coefficients of Variation for
Dimensions of External and Cranial Parts in a Population of
B. Musculus and B. Taylori.
=====================+=========================+=========================
| Baiomys taylori | Baiomys musculus
+-------------------------+-------------------------
| Vic. (see page 595) | Vic. (see page 595)
Measurement | Altamira, Tamaulipas | El Salvador
+-----------+-------------+------------+------------
| 21 Males | 18 Females | 17 Males | 13 Females
| C. V. | C. V. | C. V. | C. V.
---------------------+-----------+-------------+------------+------------
| | | |
Total length | 3.0 | 4.7 | 4.9 | 3.7
Length of tail | 5.7 | 5.9 | 6.2 | 6.4
Length of body | 2.8 | 5.0 | 5.9 | 3.5
Length of hind foot | 3.7 | 3.4 | 3.0 | 4.1
Length of ear | 0.0 | 0.0 | 5.5 | 3.3
| | | |
Occipitonasal length | 2.2 | 2.7 | 1.8 | 2.3
Zygomatic breadth | 3.6 | 3.3 | 2.2 | 2.7
Interorbital breadth | 3.2 | 3.3 | 2.2 | 2.9
Incisive foramina | | | |
(length) | 3.8 | 4.6 | 2.5 | 3.2
Depth of cranium | 3.6 | 2.5 | 2.5 | 2.5
Alveolar length, | | | |
upper molars | 2.7 | 2.5 | 2.8 | 3.2
Postpalatal length | 3.1 | 4.7 | 2.1 | 2.9
Length of rostrum | 3.3 | 3.6 | 2.4 | 4.7
Breadth of braincase | 2.7 | 1.4 | 4.0 | 4.9
---------------------+-----------+-------------+------------+------------
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.
PELAGE AND MOLTS
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 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_.
[Illustration: 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.]
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
Mexico, 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 Geronimo,
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_.
TAXONOMIC CHARACTERS AND RELATIONSHIPS
_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 the adults
examined, and in _B. musculus_ is less, or not at all, deflected.