Owing to the differences in climate from one region to another, the
flora and fauna also differ, especially in the southern part of the
state as compared with the northern part.
For expressing the taxonomic resemblance of mammalian faunas
having nearly equal numbers of taxa, Burt (1959:139)
recommended the following formula: C x 100/(N_{1} + N_{2} - C)
(where C is the number of taxa common to the two faunas,
N_{1} is the number of taxa in the smaller fauna, and N_{2}
is the number of taxa in the larger fauna). For non-flying
mammals the resemblance of the Tamaulipan fauna to that of
Texas, adjacent to the north, and Veracruz, adjacent to the
south, is as follows:
_Genera._--Texas 65 per cent, Veracruz 60 per cent.
_Species._--Texas 45 per cent, Veracruz 39 per cent.
For bats the resemblance of the Tamaulipan fauna to those of
Texas and Veracruz is as follows:
_Genera._--Texas 40 per cent, Veracruz 51 per cent.
_Species._--Texas 24, Veracruz 39.
TABLE 1.--NUMBER OF GENERA AND SPECIES OF NON-INTRODUCED LAND MAMMALS
IN THREE STATES.
==========+===========================+===========================
| Number of taxa | Number of taxa in common
+-------------+-------------+-------------+-------------
| genera | species | genera | species
----------+--------+----+--------+----+--------+----+--------+----
States |non-bats|bats|non-bats|bats|non-bats|bats|non-bats|bats
----------+--------+----+--------+----+--------+----+--------+----
Texas | 51 | 12 | 103 | 25 | 39 | 10 | 58 | 12
Tamaulipas| 48 | 23 | 83 | 36 | .. | .. | .. | ..
Veracruz | 53 | 36 | 94 | 60 | 38 | 20 | 50 | 27
----------+--------+----+--------+----+--------+----+--------+----
For all of the land mammals of Tamaulipas, the resemblance
is as follows:
_Genera._--Texas 58, Veracruz 57.
_Species._--Texas 40, Veracruz 39.
On the whole, the fauna of Tamaulipas resembles faunas of both the
Brazilian Subregion and the North American part of the Nearctic
Subregion (see Hershkovitz, 1958:611). Considering the 48 genera of
non-flying land mammals of Tamaulipas, 24 genera occur in habitats from
the North American part through habitats of northern Mexico into the
Brazilian Subregion. Of the remaining 24 genera, 16 occur in the North
American part of the Nearctic Subregion or in it and the part of
northern Mexico north of the Brazilian boundary, whereas eight occur in
the Brazilian Subregion or in it and the northern part of Mexico. None
occurs only in Tamaulipas or only in northern Mexico.
The non-flying fauna of the coastal plain east of the Sierra Madre
Oriental and south of the Sierra de Tamaulipas and Soto la Marina is
mainly tropical in affinities; only 27 per cent of that fauna (at the
subspecific level) resembles the fauna north of Soto la Marina, which
is Nearctic in its affinities. The fauna of the Sierra de Tamaulipas
has a greater taxonomic resemblance (20.4 per cent at subspecific
level) to that of the Sierra Madre Oriental, than does the fauna of the
Sierra San Carlos (17.6 per cent). Taxonomic resemblance between the
faunas from the Sierra San Carlos and the Sierra de Tamaulipas amounts
to only 16.1 per cent. Therefore, the faunas of these two Sierras (both
are included in the same zoogeographic unit) resemble each other less
than either resembles the fauna of the Sierra Madre Oriental (in
another zoogeographic unit). Of the three sierran faunas, those of the
Sierra Madre Oriental and the Sierra de Tamaulipas have most in common.
Migration from one to the other in relative recent time may account for
the resemblance. The Sierra San Carlos may have been isolated for a
long time and interchange between its fauna and those of the other two
sierras, therefore, may have been slight.
Study of the taxonomic resemblance shows that the dividing line, in
eastern Mexico, between Nearctic and Neotropical faunas is along the
eastern base of the Sierra Madre Oriental, the southern base of the
Sierra de Tamaulipas and thence to the coast at or near Soto la Marina.