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 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.
FORMATION OF THE RECENT SPECIES
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 Mexico, 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 Mexico 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
Mexico 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 Mexico, and the transverse volcanic zone in central Mexico 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 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
Mexico, 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 Mexico, 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 Mexico. 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 Mexico. 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 Mexico. 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, 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_ 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.
AREAS OF PRESENT DIFFERENTIATION
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.
ZOOGEOGRAPHIC POSITION
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 in late Pliocene or early Pleistocene
times occurred also north of the present range of the genus. The belt in
west-central Mexico between nearctic and neotropical regions is the
current center of distribution of the genus and probably has been for a
considerable time.
[Illustration:
FIG. 12. Averages of the occipitonasal lengths of skulls of adults
at 19 localities of occurrence (solid symbols) of _Baiomys taylori_,
and at 17 localities of occurrence (open symbols) of _Baiomys
musculus_. Note that the occipitonasal length decreases from north
to south in each of the two species, and that in the region where
the two species occur together, west-central Mexico, _B. taylori_
is smallest and _B. musculus_ is largest. Average, extremes, number
of specimens averaged (in italic type), and name of locality, from
north to south for each species, are as follows:
_Baiomys taylori_
18.0 (17.5-18.6) _15_, 9-1/2 mi. W New Mexico state line, Ariz.
18.9 (18.2-19.4) _6_, 7 mi. S. La Belle, Jefferson Co., Texas.
18.2 (17.8-18.5) _10_, San Antonio, Bexar Co., Texas.
18.2 (18.0-18.5) _5_, 2 mi. W Minaca, Chihuahua.
18.0 (17.6-19.0) _22_, 6 mi. SW San Geronimo, Coahuila.
18.2 (18.1-18.3) _3_, Ciudad Obregon, Sonora.
18.1 (17.4-18.5) _5_, vic. (see p. 649) Durango, Durango.
18.1 (17.5-18.5) _9_, Jaumave, Tamaulipas.
18.2 (17.7-18.9) _19_, 15 mi. N Rosario Chele, Sinaloa.
17.9 (17.4-18.3) _27_, vic. (see p. 655) Altamira, Tamaulipas.
18.3 (17.9-18.7) _9_, Valparaiso, Zacatecas.
18.1 (18.1-18.2) _4_, Ciudad del Maiz, San Luis Potosi.
18.6 (18.3-18.9) _8_, Tepic, Nayarit.
18.0 (17.7-18.4) _18_, 4 mi. N, 5 mi. W Leon, Guanajuato.
18.1 (17.5-18.9) _28_, 6 mi. E Queretaro, Queretaro.
17.7 (17.1-18.1) _17_, 1 mi. SSE Ameca, Jalisco.
17.3 (16.8-17.9) _10_, 2 mi. SSE Autlan, Jalisco.
18.0 (17.5-18.6) _10_, 1 mi. S, 11 mi. W Zamora, Michoacan.
17.6 (17.4-18.2) _8_, Colima, Colima.
_Baiomys musculus_
20.2 (19.9-20.3) _6_, vic. (see p. 622) Ameca, Jalisco.
20.2 (19.9-20.3) _6_, 2 mi. SSE Autlan, Jalisco.
19.6 (19.2-20.1) _6_, Jalapa, Veracruz.
20.3 (19.7-20.9) _9_, Colima, Colima.
19.5 (19.0-20.0) _10_, Cerro Gordo, Veracruz.
19.8 (19.4-20.3) _6_, 6 mi. S Izucar de Matemores, Puebla.
20.0 (18.8-20.5) _7_, Teotitlan, Oaxaca.
20.1 (19.7-20.7) _7_, 1 km. NW Chapa, Guerrero.
19.9 (19.4-20.4) _8_, 5 mi. ESE Tecpan, Guerrero.
19.5 (19.1-20.1) _22_, 3 mi. ESE Oaxaca, Oaxaca.
19.5 (19.1-19.9) _11_, Valley of Comitan, Chiapas.
18.9 (18.2-20.1) _17_, Tehuantepec, Oaxaca.
18.9 (18.4-19.7) _15_, 6 mi. NW Tonala, Chiapas.
19.1 (18.8-20.4) _10_, 1 mi. S Rabinal, Guatemala.
19.7 (18.8-20.4) _10_, Lake Amatitlan, Guatemala.
19.2 (18.4-19.8) _26_, vic. (see p. 625) San Salvador, El Salvador.
19.3 (18.9-19.9) _24_, 8 mi. S Condega, Esteli, Nicaragua.]
CONCLUSIONS
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 prooedont 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 Mexico.
7. The two species are now sympatric in west central Mexico, 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_.
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.
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