The distributional patterns and affinities of the mammalian fauna of
Tamaulipas suggest possible routes of migration and barriers that
limited or controlled movements of the mammals.
Mammals may have reached Tamaulipas by way of a Northern route, a
Trans-plateau route, a Montane route, or a Tropical route (Fig 3).
The Northern route permitted species of mammals from the temperate
region to the north to enter the Tamaulipan Biotic Province from or via
Texas. Several came from the Great Plains, and a few came from the
eastern part of the United States. Also, a few mammals that may have
originated in the Tamaulipan Province moved northwards. Some of these,
according to Dice (1937:267) were _Liomys irroratus texensis_,
_Peromyscus leucopus texensis_, and _Lepus californicus merriami_.
Other mammals thought to have moved north by this route are _Didelphis
marsupialis_, _Dasypus novemcinctus_, _Oryzomys palustris_, _Nasua
narica_, and _Tayassu tajacu_. Some mammals that passed through
Tamaulipas into Texas have extended their geographic ranges far north
of Texas.
Mammals that came _via_ the Trans-plateau route (name proposed by
Baker, 1956:146) came no farther into Tamaulipas than the Chihuahuan
Biotic Province. They encountered the barrier formed by the Sierra
Madre Oriental. These mammals were listed in the account of the
Chihuahuan Biotic Province.
The route that Baker (1956:146) termed the "Southern Route" I here term
the Montane route because I think it was used for movement southward as
well as northward.
[Illustration: FIG. 3. Routes of movement: 1 Northern; 2 Trans-Plateau;
3 Montane; 4 Tropical.]
The Montane route was used by mammals of boreal affinities (_Microtus_
and _Neotoma_), that moved into Tamaulipas from the north; also in this
category are bats of the family Vespertilionidae. For movement from
south to north, the route was used by several species native to Mexico,
for example, _Cratogeomys castanops_. The seaward slope of the montane
area has enabled some tropical mammals to move farther north than they
have done at higher and lower elevations. _Philander opossum_ seems to
be an example.
The fourth route, the Tropical one, was used by mammals of tropical
origin. Most moved into Tamaulipas only as far as the Veracruzian
Biotic Province. The principal mammals that have used this route are
the bats and marsupials, but _Sylvilagus brasiliensis_, _Ateles
geoffroyi_, _Heterogeomys hispidus_, _Eira barbara_, and _Mazama
americana_ also can be included here. Some tropical mammals, as was
pointed out previously, not only reached Tamaulipas but have moved
through the state and far northward.
The major barriers to dispersal of mammals in Tamaulipas are three (see
Fig. 2). Two of them, the Rio Grande Barrier and the Sierra Madre
Barrier, are physiographical, but the Tropical Barrier is maintained by
a combination of environmental factors. The three barriers separate the
four Biotic Provinces in Tamaulipas. The Sierra Madre Oriental, which
forms the Potosian Biotic Province, lies between the Tamaulipan and
Chihuahuan provinces. The Tropical barrier separates the Tamaulipan and
Veracruzian biotic provinces.
The Rio Grande, as was pointed out by R. H. Baker (1956:146), has low
banks, is relatively shallow, and does not form an effective barrier
for most mammals. For only two species, insofar as I know, has the Rio
Grande constituted a barrier. _Cratogeomys castanops_ has not entered
southeastern Texas from Mexico, and _Spermophilus spilosoma_ has not
entered Mexico from southeastern Texas except on the coastal barrier
beach. Alvarez (1962:124) postulated that the beach was the route by
which _S. spilosoma_ arrived at La Pesca where the barrier beach meets
the mainland.
The Sierra Madre Barrier is a good filter for some small mammals,
especially for those that occur on the Mexican Plateau and those of
tropical origin. The mammals that occur on each side of the Sierra are
listed in accounts of the Chihuahuan (west side), Veracruzian and
Tamaulipan (east side) biotic provinces.
The Tropical Barrier is formed mainly by a climatic complex (probably a
change in temperature and rainfall) in the coastal region at or about
the latitude of Soto la Marina, where no geographic barrier is found.
In the western and central part of the Tropical Barrier, the climatic
factor is supported by a geographic factor. The Sierra Madre Oriental
is in the west and the Sierra de Tamaulipas is in the center. The
several mammals that are affected by this barrier are listed in the
accounts of the Veracruzian and Tamaulipan biotic provinces.
A peculiar pattern of distribution is that presented by _Scalopus
inflatus_ and _Geomys tropicalis_. Both are the only known species of
their genera in northeastern Mexico. Each is isolated from other
species of its genus. The nearest known record of _Scalopus_ is 45
miles northward and the nearest record of _Geomys_ is approximately 165
miles northward. A possible explanation for the distribution of these
two kinds is that each was widely distributed in one of the glacial
periods and when the glacier receded to the north these animals
remained in Tamaulipas, where they evolved and formed distinct species.
The two species, _G. tropicalis_ and _S. inflatus_, are fossorial and
for this reason probably were able to resist inhospitable climates
better than non-burrowing species.