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.

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 MÉxico, 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 RÍo 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 RÍo 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 RÍo Grande constituted a barrier. Cratogeomys castanops has not entered southeastern Texas from MÉxico, and Spermophilus spilosoma has not entered MÉxico 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 MÉxico. 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.